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Recent Advances in Structural Modifications of Hyperbranched Polymers and Their Applications Shahidul Islam Bhat, Younes Ahmadi, and Sharif Ahmad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01969 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Industrial & Engineering Chemistry Research

Recent Advances in Structural Modifications of Hyperbranched Polymers and Their Applications Shahidul Islam Bhat,a Younes Ahmadi,a Sharif Ahmad*a a

Materials Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi-

110025, India. Authors Detail Shahidul Islam Bhat Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India Mobile: +91-9650205540 E-mail address: [email protected]; [email protected] Orcid Id: 0000-0002-1672-5060 Younes Ahmadi Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India Mobile: +91-7291083933 E-mail address:[email protected] Orcid Id: 0000-0003-0607-1259 Sharif Ahmad* Materials Research Laboratory Department of Chemistry, Jamia Millia Islamia (A Central University) New Delhi-110025, India Tel no. +91 11 26827508 Fax: +91 11 26840229 E-mail address: [email protected] Orcid Id: 0000-0001-5799-7348 *Corresponding Author

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Hyperbranched polymers (HPs) are subclass of dendritic polymers, having globular and highly branched structure, containing number of functional groups emerging from a core. The core contains more than two functional sites where the growing branches are connected, resulting in the formation of a 3D macromolecule comprising large number of peripheral groups. Their versatile properties and facile structural modifications have attracted considerable attention of researchers. HPs have been potentially used in various applications such as coatings, drug delivery, nanotechnology, additives, sensors, solar cells, etc. Thus, present review emphasizes on recent structural modifications of HPs that result in enhancement of existing or emerging new properties. In addition, these modifications have broadened the use of HPs in various advanced technologies such as biological applications, storage devices, energy convertors, catalysis etc., which have not been covered in earlier reviews. Further, this article discusses the limitations associated with their fabrication and application in various fields. KEYWORDS: Hyperbranched polymers; bio-imaging; Drug delivery; Flame-retardant; Energy storage.


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1. INTRODUCTION Nature always acts as the best source of inspiration for scientists and technologists to create various innovations. For instance, the idea of branching in polymers was derived from trees, river basins, nerves, veins, and proteoglycan ranging from macroscale to nanoscale levels (Figure 1). The design and architecture of branching in macromolecules with controlled properties are considered to be an important phenomenon inducing suitable properties. The latter is due to the presence of controlled structural-properties relationship in hyperbranched polymers (HPs) at macromolecular scale. In view of this, a considerable interest has been developed among the researchers around the world to focus on the processing techniques in the modification of these polymers. Berzelius1 for the first time has reported the synthesis of hyperbranched polyester (HPEs) resin using glycerol and tartaric acid in 1900 followed by Watson Smith (1901) and Baekland (1909).2 Watson and smith have separately reported the synthesis of HPEs using the concept of A2 monomer (Phthalic acid) and B3 monomer (glycerol), while the synthesis of HB-phenol formaldehyde was given by Watson using Smith principle. Kienle in 19293 investigated the preparation of HPEs involving Phthalic acid and glycerol of lower viscosity than that of HP polystyrene. The HPs have attained a significant position in both academia and industry long back following the introduction of Flory’s theory of macromolecules in 1952.4



Figure 1. Natural examples of branching (Human blood system, rivers, and trees). The present terminology of HPs was invented by Kim and Webster in 1988 to define the dendritic polymers that possess a randomly branched topology and can be prepared by single step polycondensation of AB2 type monomers.5 Prior to them, Kricheldorf and co-workers have reported the preparation of branched copolymers via AB and AB2 type monomers using a single step copolymerization in 1982.6 Afterwards, HPs were intensively explored, particularly during the last 30 years. These polymers possess a tree like structure of monomeric units called dendritic polymers, which are densely branched with reasonably large number of end groups. In HPs the repeating units of branches are originating from the central core. The core is characterized by its functionality that is the number of chemical bonds by which it can be attached to the external parts of molecules.7 The functionality of the core is normally considered to be three (e.g. amine) or four (e.g. ethylene di amine).8 The linear units of polymeric materials are attached to the core through their bonds in the form of arms, generally terminated with the multifunctional branched units. The larger units are introduced by the addition of linear units to the end groups beneath the polymer layers.7 The perfect attachment of number of these units within the molecule leading to the formation of dendrimer, while the imperfect attachments led to the processing of HPs. Generally, the HPs are classified as compact hyperbranched polymers (CHPs) and segmented hyperbranched polymers (SHPs).9 CHPs are having single or repeating units between two branching points. Their characteristic features are compact and branched structure along with end functional groups. SHPs and CHPs are analogues to one another, having long linear segments dispersed between two branching points, while the SHPs possess a low degree of branching (DB) in comparison to CHPs. The reactive groups in CHPs are located on the peripheral regions of polymers, on the other hand in SHPs they are present in the core. The rheological, chemical, thermal, flame retardancy, etc. properties have enhanced the research interest in these 4 ACS Paragon Plus Environment

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polymers.10 The poor mechanical properties of HPs are due to their non-entangled and less coiled structures. However, the incorporation of relatively long linear and hard moieties in HPs led to the entanglement of the polymer structure, there by generating the required acceptable mechanical properties. For example, the incorporation of long chain polycaprolactone (PCL) into hyperbranched polyurethane (HPUs) resulted in enhanced mechanical properties.11 This study also revealed that the properties of these polymers depend mainly on the length of the chain present between branching units rather than molecular weight. Moreover, the spherical shape, presence of functional end groups, low solution viscosity and high solubility makes them accessible to other reactants. Their high reactivity is attributed to a large number of reactive end-groups, which can be functionalized for a specific application.12 The DB and nature of end-groups have direct impact on the thermal properties of HPs like glass transition, i.e. the increase in DB and polarities of endfunctional groups increase the glass transition. The DB of these polymers can be determined by NMR spectroscopy.13 Literature also reported that the HPs have been prepared via polyfunctional organic moieties. These properties and simple synthesis process facilitate the facile chemical and structural modifications of HPs resulting in tremendous increase on number of publications. Their potential scope in various applications was observed since 1990. Since 1981 a large amount of work has been carried out on different types of HPs like HPUs, poly (ester amide)s, polyesters, polyether polyols

and inorganically modified HPs like

polystyrenes, polyborates, polyphosphates and phenolic resins.14–19 Interestingly, it is observed that most of the published reviews on HPs have focussed only on synthesis or applications aspects.20–23 However, the present review emphasizes on recent structural modification of HPs that results in enhancement of existing or emerging new properties, in addition to these chemical modifications have broadened the HP’s in various advanced technologies such as biological applications, storage devices, energy convertors, 5 ACS Paragon Plus Environment


etc. The article also points out a brief on their general structure, development, the challenges that are associated with the development of HPs, and future scope of research. It is important to mention that most of these points related to their applications have not been covered by earlier reports. 2. STRUCTURE AND PROPERTIES OF HPs Generally, HPs contain the initial unit (A) that could react (polymerize) with intramolecular B group and form AB bond monomer either through cyclization or via extra-added introduction of multifunctional core molecules.4 The units with one unreacted B group, two reacted or unreacted B groups represent linear (L), dendritic (D), and terminal (T) units, respectively. The repeating units are present in the form of L, D, and T that are introduced in macromolecular structure with the help of AB2-type monomers. There are two types of L units that may exist in a HPs when prepared with an asymmetric AB2 (or ABB') monomer.24 Different structural parameters are used to characterize the topology of HPs with reference to degree of branching which can be defined as: DB= no of dendritic units + no of terminal units / Total no. of Units = D +

T /D + T + L

⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ (1)

D is the number of fully branched units along with the number of T and L units present in the form of partially reacted units. In case of HPs having large molecular weight, the number of T units are very close to that of D units. Accordingly the equation (1) can be simplified as:25 DB =

/ !

⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ (2)

The equation (2) is very useful since ratios of L/D or L/T can easily be calculated using NMR spectrum. The more specific expression for the DB was obtained as a function of conversion based on equal reactivity ratio of all B groups, obtained by Frey, Muller and Yan et al. given below. The expression is found to be more helpful either in the prediction of DB at a given molecular weight or in the determination of degree of polymerization.26 6 ACS Paragon Plus Environment

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DB=

" #$"

⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ ⋯ (3)

The x is the fractional conversion of the group A. The value of x approaches to 1 on the completion of the reaction, while DB becomes 0.5. The DBs of most of the HPs based on AB2 monomers are close to 0.5. Interestingly theoretical and experimental DB values for these polymers are found to be similar.27 The DB is considered as essential characteristic parameter for HPs due to its close relationship with the various properties of polymers like glass-transition temperature (Tg), free volume, chain entanglement, mean-square radius of gyration, degree of crystallization (DC), mechanical strength, melting, solution viscosity, self-assembly behaviors and biocompatibility.28 Thus, the HPs with controlled properties can be developed to some extent by adjusting the DB. For example poly [3-ethyl-3-(hydroxyl methyl) oxetane] (PEHMO) with an increased value of DB, the Tg was found to linearly decrease while DC decreased exponentially (Yan and coworkers).29 Further, it was observed that the hyperbranched polyglycerol (HPG) exhibited a much higher capacity of supramolecular encapsulation of guest dyes compared to that of its linear analog.30 3. SYNTHESIS METHODOLOGY OF HPs HPs have been synthesized using various methodology broadly divided into two classes based on number of monomers used, the single monomer and the double monomer (DM) methodologies (Figure 2). These methodologies include mainly following approaches (i) Step-growth polycondensation of AB2 type monomers, (ii) Self-condensing vinyl polymerization (SCVP), (iii) Self-condensing ring-opening polymerization (SCROP), (iv) proton-transfer polymerization, (v) A2 + B3 methodology, and (vi) Couple-monomer methodology.23



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Figure 2. Synthesis routes for HPs. 3.1 Step-growth polycondensation of AB2 monomers The method is widely used in the synthesis of HPs, if two B groups react with A then it will be D unit, if only one B reacts with A then L unit, and if no B reacts T unit will form. Flory was the first to synthesize HPs via this approach.21 The polycondensation reaction involving AB2 monomers doesn’t have control over the reaction as a result of which it is difficult to control molecular weight and the polydispersity. Wooley et al. for the first time reported the synthesis of branched polyester (BPE) by this methodology (Figure 3).31 Later on, Frechet et al. followed similar method to synthesize other type of BPE by using

3,5-

bis(trimethylsiloxy) benzoyl chloride (AB2 monomer) in the presence of N, Ndimethyloformamide (DMF) with DB ranging 0.55-0.60, however, the properties of these polymers were not reported. Hobson and Feast reported the synthesis of branched polyamide having remarkable viscosity via Michael addition reaction at high temperature using Nacryloyl-α, ω-diaminoalkane hydrochloride.32 Recently Shi et al reported a one-pot solution polymerization of AB2 (A= alkyne group, B= two azide groups) monomers to synthesis the 8 ACS Paragon Plus Environment

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HPs having high M.Wt (≈ 240 kg/mol), low poly disparity (Mw/Mn < 1.3), and high DB (= 0.83) taking copper as catalyser to facilitate azide–alkyne cycloaddition reactions (CuAAC) for the first time. CuAAC polymerization demonstrates chain-growth mechanism exhibiting a constant increase in molecular weight with the addition of repeated monomer units at the end of chain, hence called “Living Chain-growth mechanism” results in the formation of uniform HPs.33

Figure 3. Schematic representation of AB2 based HPs based on Flory principle (Reprinted with permission from ref 31. Copyright 1994, Springer Nature.) 3.2 Self-condensing vinyl polymerization (SCVP) SCVP was developed as a substitute to AB2 method by Frechet (1995) for the synthesis of AB type vinyl monomers by using AB* monomers where A was vinyl monomers containing an inactive functional group which can initialize vinyl polymerization process upon activation and B was used as the initiating moiety.34



After the activation, initiation starts with vinyl group present on other molecules and finally it results in the formation of HPs at final stage.35 The method found wide applications of HPs because of large variety of monomers and ability to attain by various techniques based on the nature of initiating groups. Some of the most practiced polymerization methods in SCVP are cationic, anionic, surface initiator, photo-facilitated radical, reversible addition–fragmentation chain transfer (RAFT) polymerization, etc.36 The ease in regulation of these techniques reduced the chance of gelation during the synthesis and also makes it possible to control the amount of branching point that leads to the development of variety of branching structures. Among these SCVP techniques, RAFT-SCVP offers higher compatibility in mild conditions with more diverse solvents and functional monomers. Wang et al reported the RAFT-SCVP based on radical polymerization for the first time in 2003. After that this technique has attracted substantial interest, resulted in the development of different complexed copolymers by the utilization of numerous monomers that enables functionalization of products even after polymerization. Figure 4 exhibits a better understanding of RAFT-SCVP method by demonstrating RAFT mechanism.37 The thio carbonyl thiol is used as a key component (chain transfer agent) in this method at the early stage of polymerization to facilitate the achieving of equilibrium state between active and inactive chains. In the course of RAFT polymerization, selection of a suitable chain transfer agent is an important fact, which is mainly based on the activity of the monomer. For instance, vinyl amide and vinyl ester are less-activated monomers, while styrene, acrylate, and acrylamide are known to be moreactivated monomers. However, in HPs synthesis, if a more-activated monomer is used as a first block, the introduction of a less-activated monomer into the second block using similar chain transfer agent will be a difficult task. This limitation was resolved for the first time by Sudo et al., who developed a switchable RAFT material i.e. (4-vinyl) benzyl-N-methyl-N-(4-


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pyridyl) dithiocarbamate (chain transfer agent) for both types of monomers used in the synthesis of HPs.38

Figure 4. Schematic representation of mechanism of SCVP. 3.3 Self-condensing ring-opening polymerization (SC-ROP) The method is analogous to SCVP technique reported by Frechet and Hedrick to fabricate the hyperbranched polyester (HPE) using cyclic esters where the polymerization reaction initiated by opening the ring of cyclic monomer.39 Literature reveals that this technique has been used for the polymerization of various cyclic compounds were used to prepare hyperbranched polyamines (HPA), HPE, and HB polyethers.40,41 For instance, Liu et al. reported synthesis of Hyperbranched Polyphosphates (HPPs) where they synthesised 2-(2 hydroxyethoxy)ethoxy-2- oxo-1,3,2-dioxaphospholane (cyclic AB* inimer), which further used in the fabrication of HPhs.42 They used chlorine containing initiator (2-chloro-2-oxo1,3,2- dioxaphospholane) and hydroxyl cointaining monomer (diethylene glycol) to synthesize the AB* inimer, where on bulk reaction HPhs were obtained without using catalyst (Figure 5). 11 ACS Paragon Plus Environment


Figure 5. Schematic representation of SCROP (Reprinted with permission from ref 42. Copyright 2009, American Chemical Society, Washington, DC.) 3.4 Proton-Transfer polymerization (PTP) The PTP methodology is exclusively anticipated in the synthesis of HPs for biomedical applications. This method was introduced by Chang and Frechet that continued via transferring of proton in every propagation step.43 The initial step involves the proton abstraction by the initiator (anion) from the H-AB2 monomer that results in formation of reactive nucleophile. This reactive species initiates the reaction by reacting with another AB2 monomer that forms a dimer where one of the B functionalities act as nucleophile that reacts with the next monomer. This active dimer reacts with another H-AB2 molecule via PT reaction. In last step, polymer is prepared through the successive addition of nucleophiles and PT reactions. The branching is generated as a result of multiplicity of active B moieties during the polymerization. The effectiveness of this methodology is based on the activation 12 ACS Paragon Plus Environment

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of AB2 monomers. Therefore, PTP is more favourable when ABx or A2+B3 monomers are used and ring opening reaction takes place. For instance, the HB poly(hydroxyether) was synthesised for the first time using AB2 monomers where A was phenolic hydroxyl, and B was epoxide groups.43 This polymerization was continued through step-growth pathway where the nucleophile attack on the epoxide ring resulted in the deprotonation of the phenol. As a result, the opening of the epoxide ring produced an active dimer that could undergo PT reaction with another AB2 monomer generating a neutral and a phenolate anions under controlled condition. Chen et al. reported the synthesis of temperature-responsive HPEt (HB polyether) based on this methodology. The PTP occurred between the epoxy moieties (1,2,7,8-diepoxyoctane [DEO]) and a multifunctional group (ethylene glycol, triethylene glycol, diethylene glycol, etc.44 Recently, Gadwal et al. fabricated HPEt via PTP using AB2 monomers (A=thiol and B= epoxide moieties) having DB of 0.65-0.69. These HPEt had –OH and epoxide groups as reactive sites distributed throughout the HP structure. The mechanism of PTP using ABx monomer represented in Figure 6.45 These epoxide moieties were able to attach the aryl, alkyl, and ethylene oxide group via thiol-epoxy route, on the other hand the – OH functionalities were connected to the cationic ammonium moieties.



Figure 6. PTP of epoxide and thiol groups. (Reprinted with permission from ref 45. Copyright 2014, American Chemical Society, Washington, DC.) 3.5 A2 + B3 methodology Since the early years, this approach was used for the preparation of HPs via polycondensation reaction between the two identical functionalities of A and three similar functionalities of B (Figure 7). For instance, Shi et al. prepared an aromatic HPP that via polycondensation reaction between bisphenol-A (A2) and phosphoryl trichloride (B3).46 Generally, A2+B3 methodology contains following postulates: i)

lack of cyclization during propagation, ii)

same reactivity of monomeric and polymeric A2 and B3 groups, and iii) selective reactivity of A2 functional groups with B3 active sites.47,48 One of the advantages of this technique, is the availability of numerous types of monomers that facilitates the diverse synthesis of HPs like 14 ACS Paragon Plus Environment

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HPPs, hyperbranched epoxy (HE), hyperbranched poly(ester amide) (HPEA), HPE,, hyperbranched polyurethane (HPU),etc.49–54

Figure 7. Schematic representation of A2+B3 methodology. 3.6 Couple-monomer methodology The Couple-monomer methodology (CMM) was first reported by Yan and Gao.55 This methodology is based upon the unequal reactivity of specific functional groups present in reacting monomers. Thus the reacting monomers generate preferentially a single type of intermediate (ABn) via in situ polymerization to produce HPs. The most important and the basic step in CMM is the choice of suitable monomers as precursors. This technique resolves the problem of gelation which was disadvantageous to the previously mentioned A2+B3 method. There are number of literatures available that report the synthesis of various HPs by this method. Yan et al reported the synthesis of hyperbranched poly(urea urethane)s, poly(ester amine)s, poly(sulfone amine)s, and poly(amide amine)s) following this technique (Figure 8).56 In this approach the type of functional groups present on monomers are A, A’/B, B’, while the other such methods have also been developed using different

starting

monomers such as AA’+BB’, AA’+CB2, A2+B2+B4, etc. Karakaya et al synthesised oil based HPEAs in which they used phthalic anhydride or maleic anhydride as A2 monomer and diethanol amine as BB᾽2 monomer.57 The other modified methodology followed the A2+B2+B4 approach. Das et al synthesized sunflower oil modified HPU Fe3O4 nanocomposites via in15 ACS Paragon Plus Environment


situ polymerization technique using this approach.58 The resulting HPU nanocomposites were found to have significantly improved thermo-mechanical properties, shape-memory behaviour, antibacterial activity, biocompatibility as well as biodegradability in comparison to their pristine system. CMM approach minimises the cross-linking or cyclization of monomers during the synthesis, which results in termination of reaction by presenting various methodology based on the difference in reactivity of functional groups of monomers. For example in dissimilar functional groups of AA′ monomers if A′ is slightly more reactive than A, then CMM will be AA′ + B3 approach.

Figure 8. Synthesis route of HPs via different AA' and CBB' monomers. (Reprinted with permission from ref 56. Copyright 2004, American Chemical Society, Washington, DC.) Literature survey reveals that in addition to the aforementioned synthesis methodologies, there are other approaches, which have been recently practiced by different researchers across the globe. These methodologies, like click chemistry, Michael addition, emulsion polymerization, etc. have found wide scope in development of various materials for biological and non-biological applications. For instance, Wang et al. have reported a highly elastic, pH-responsive and biodegradable HP via cryo-aza-Michael addition polymerization 16 ACS Paragon Plus Environment

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technique using hyperbranched amine-terminated polyamidoamine and polyethylene glycol.59 Recently, Zhao et al. developed hybrid nano-structured hyperbranched materials through double mini-emulsion (water-in-oil-in-water) polymerization technique in the absence of surfactant. The new approach was attained by the addition of hyperbranched polyethoxysiloxane, and polymeric silica precursor to the oil phase, which was reformed to strong water@SiO2@polymer@SiO2 nano-capsules upon transformation of the silica precursor, followed by polymerization of the oil-phase.60

4. EFFECT OF STRUCTURAL MODIFICATION ON HPs PROPERTIES As it is discussed before, the presence of large number of functional end-groups purposed the introducing new or enhancing existing properties through chemical/or and physical modifications. These chemical and physical modifications mainly effect the miscellaneous, fluorescent/phosphorescent, amphiphilic, biodegradability and biocompatibility properties of HPs, which are discussed as follow: 4.1 Miscellaneous Properties of HPs 4.1.1 Magnetic The modification of polymers through doping was investigated for the first time in 1976 that showed the increase in electrical conductivity of polyacetylene by one million times.61 Thus, this interesting behaviour of modified polymers has led the researchers to develop new techniques for the preparation of efficient magnetic responsive polymeric materials (MRPM).62 For instance one of the most practiced technique was pyrolytic cerimization, by which the preparation of magnetic ceramics (MC) was achieved using organometallic polymers as precursors.63 However, the drawbacks associated with this technique was the effect of precursors on the activity of final product. Thus, the use of these precursors substituted by organometallic HPs. Organometallic HPs precursors enhanced the retention of pyrolyzed 17 ACS Paragon Plus Environment


entities and steadier growth of crystallites. In year 2000 Tang et al synthesised a hyperbranched precursor (hyperbranched poly [1,10-ferrocenylene-(methyl) silyne]) which exhibited high saturation magnetization (Ms= 51 emu.g-1) value compared to its linear (Ms= 3.5 emu.g-1) counterpart. However, similar results were obtained by Sharif et al.,62 via doping of linear polyaniline with magnetic nanoparticles (Fe3O4), therefore the need of alternative materials was on demand.

Soon, Tang utilized another co-monomer to synthesis HP using ferrocene-

containing precursor, which showed drastic increase in Ms value (= 51 emu.g-1).64 In 2004, Tang et all used Glaser-Hay oxidation coupling polymerization (OCP) to develop HB polytriphenylamine which showed higher Ms value (= 118 emu.g-1) and they observed that on decreasing the ligand concentration the Ms value decreased to 26 emu.g-1 which resulted in the reduction of electrons mobility, which was due to the entanglement of end functional groups. The other conventional method to improve the magnetic properties of HPs is the modification magnetic nanoparticles (MNPs) using active molecules (Figure 9).65 This approach was achieved through surface-initiated ring-opening polymerization technique reported by Wang et al 2009.66 They synthesised biocompatible nano-composite hydrophilic solution of stable HB polyglycerol-grafted Fe3O4 NPs with Ms= 30 emu.g-1, which did not have any effect on macrophages that made it a suitable for bio-medical application in form of magnetic resonance imaging (MRI). Recently, Zhao and co-workers developed water dispersible surface functionalized hyperbranched magnetic nano-particles.67 Initially, the magnetic centre (Fe3O4) was functionalized using ethanolamine and methyl acrylate that generated more number of functional end groups (amino groups) that resulted in the development of magnetic, watercompatible molecules. The obtained imprinted amino-functionalized nanoparticles showed high sensitivity towards chlorogenic acid and could be used for selective absorber of these materials.


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Figure 9. The synthesis process of hyperbranched polyyne via homo-polycouplings process using tri-ynes (1) followed by copolycouplings with Monoyne (2). (Reprinted with permission from ref 65. Copyright 2004, American Chemical Society, Washington, DC.) 4.1.2 Electrical Properties Previous section has highlighted the preliminary investigations on the magnetic, electrical and optical properties of these polymers through dopping of polyacetylene, which boosted its conductivity. This result encouraged researchers to examine other conductive polymers like polyanilines, polythiophenes, polypyrorolenes, etc. For instance, Xu et al. reported the synthesis of p-type conjugated hyperbranched polythiophenes comprising asymmetric substituted thiophene units. These conjugated HPs exhibited strong fluorescence behavior, effective intramolecular energy transfer, and a broadband absorption, which made them potentially useful materials for the development of efficient light emitting and photovoltaic devices.68 Albertson et al. have reported a number of linear and hyperbranched copolymers (via ‘‘A2 +Bn” (n≥2) approach) via bi coupling reactions between aniline-based pentamer and branched biocompatible polymer (poly(e-caprolactone)).69 These investigations showed that the HPs exhibited higher conductivity in comparison to their linear. The unique 3D structure of HPs and doping of 19 ACS Paragon Plus Environment


conducting polymers were mainly responsible for improved conductivity. The 3D structure resulted to the ordered distribution of emeraldine state of the aniline pentamer (EMAP) segments at peripheral position that enhance the conductivity of polymers, which makes HPs as a promising candidate for energy storage applications. The application of various HPs based on conducting polymers in lithium batteries and solar cells exhibits remarkable results.63 For example, the application of conducting HPs like hyperbranched polythiophenes, hyperbranched poly (p-phenylene vinylene) (PPV) as donors during the preparation of polymeric solar cells (PSCs) improves the performance with a reasonably high value of Power Conversion Efficiency (PCE) i.e., 3.91%.70 However, their efficiency has still place to be improved. HPs performance mainly depend on the nature of end capped groups. For instance, pyridyl end capped groups of HPs has a wide range of absorption with highest absorption coefficient, which is found to be suitable for the absorption of sunlight (PCE= 9.3%).71 The other example of energy storage materials based on conducting HPs is hyperbranched polyphenylene (HPP) prepared by pyrolysis technique.72 In this method Diels-Alder reaction was followed using AB2 monomers (A is the diene and B2 is the dienophiles) where functionalized cyclopentadienone (CP) was permeated into the nanochannels of an anodic aluminum oxide (AAO) membrane using in situ thermal process. The Diels-Alder reactions between alkyne moieties and cyclopentadienone at 250 °C resulted in the formation of Alkyl-substituted HPP, which on successive heating (at 800°C) formed a carbon-containing nanomaterials (CCNM) with comparative high surface area of 1140 m2 /g with 1D pores of 10-20 nm size. These HPPs showed a relatively high capacitance (304 F g-1) and cycle performance in an acidic electrolyte. Zeigler et al reported a new modified HPs with different pore size that enhanced their electrochemical stability (Figure 10).73 They used a non-basic aromatic tertiary amine (triphenylamine) as core and electrochemical stable (naphthalene diimide) terminal units while thiophene was used as spacer. This combination resulted in a stable system with more than 500 20 ACS Paragon Plus Environment

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cycles. They showed that by increasing the spacer amount (thiophene) the pore size increases, which decreases the reduction sites. By this modifications the prepared HPs achieved higher resistance to oxidation confirming through values of charge transfer resistances (Rct = 420.3 ohm) and equivalent series resistances (ESR= 7.7 ohm). On the other hand, Koga et al. reported the synthesis of an n-type π-conjugated hyperbranched polypyridine for the first time. The conjugated

HP

was

achieved

using

2,4,6-tribromopyridine

and

2,5-dibromopyridine

(copolymerization) via chain-growth condensation polymerization in the presence of a catalyst. They found that the presence of branching units enhanced the polymer response during the course of electrochemical doping, without affecting the highest occupied-lowest unoccupied molecular orbitals (HOMO–LUMO) levels.74 Thus, the investigations proved that HPs in the fabrication of energy storage devices have played a significant role due to their 3D structure, which makes the dispersion of nanoparticles more easy and efficient.

Figure 10. Synthesis of modified HPs with different pore size electrochemical stabile TPA(n)Th-NDI Polymers. (Reprinted with permission from ref 73. Copyright 2015, American Chemical Society, Washington, DC.) 4.1.3 Optical Properties 21 ACS Paragon Plus Environment


Utilization of linear polymers in the preparation of photopolymers with high refractive index (RI) changes (∆n), dimensional stability and great recording sensitivity for various applications such as data storage, information displays, holographic data, etc. have been investigated.75 For instance, Tomita et al, have reported a photopolymer nanocomposite system using inorganic nanoparticles (silica or titania) which have higher RI than that of polymerized monomer and organic monomers (methacrylate).76 The use of inorganic moiety in the preparation of photopolymers induces higher RI as compared to conventional photopolymers that increases the fabrication speed of photopolymers and reduces polymerization shrinkage. These features of inorganic nanoparticles improved the dimensional stability with the increase in recording speed that resulted in the low lightscattering photopolymer having greater contrast hologram on their perfect dispersion. It is experimentally proved that the increase in dispersion of nanoparticles in polymer matrix enhances the light intensity interface pattern, representing holographic control in nanoparticle distribution, which develops new application for photopolymers as photonic sensors. To achieve this, application of highly branched polymers such as dendrimers and HPs have been used. Among these, HPs are more preferred due to their ease of synthesis, high number of functional groups that increases the dispersion of nanoparticles without any aggregation or chain entanglement within the structure of photopolymers. The presence of large number of peripheral functional groups in HPs facilitates the control of optical properties like photosensitivity through photonic functionalization of end groups. For example, Tomita et al., have reported the synthesis of highly effective well-dispersed HP nanocomposite of methacrylate photopolymer in which HPs [hyperbranched poly (ethyl methacrylate) and hyperbranched polystyrene] were used as organic nanoparticles.77 They have confirmed the preparation of highly transparent photopolymer having 100% diffraction efficiency using HPs-dispersed (organic) nanocomposites and reported grafting mechanism is identical to that 22 ACS Paragon Plus Environment

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of inorganic dispersed photopolymers. The other significant use of HPs is the preparation of organic light emitting devices (OLEDs). The OLEDs due to their versatile properties have gained a high demand in the society since the early times.78 The interest in application of polymers having 3D structure (dendrimers) in the fabrication of OLEDs has begun in late 1980s due to poor stability and high operating voltage of crystalline materials. Thus, Adachi et al., proved that the application of amorphous moiety is promising method used to overcome these drawbacks.79 Hence, the application of low molecular weight and high molecular weight dendrimers was a successful method to be established.80 Thus, large number of research papers on application of amorphous polymeric moiety (dendrimers and HPs) with low molecular oxadiazoles like 2-biphenyl-5-(4-tert-butylphenyl)-1,3,4-oxiazole and high molecular like 7-bromo-9,9-dioctylfluorene-2-boronic acid through various methods like vacuum evaporations and tetrazole intermediate has been reported (Figure 11).81 For example, the employment of HPs in the preparation of white OLEDs have attracted enormous attention to overcome the drawbacks associated with the OLEDs based linear polymers such as low luminous and roll-off efficiency due to accordance of side chain interaction in polymer chains. A lot of work has been reported on light emitting HPs, while in this review we focus on a limited number of LE materials based on HPs.

Figure 11. Representation of the possible administrations of nonlinear optical chromophores 23 ACS Paragon Plus Environment


(arrows) in a branched structure: (a) main, (b) side, and (c) peripheral chains. (Reprinted with permission from ref 81. Copyright 2009, American Chemical Society, Washington, DC.) 4.2 Fluorescent and phosphorescence properties The fabrication of 8-hydroxyquinoline aluminum based two layer emitting OLED was initially reported by Tang et al. in 1987,82 While on the other hand Friend and co-workers have reported the synthesis of conjugated polymer [poly (p-phenylene vinylene) (PPV)] based LED in 1990.83 Simultaneously a number of HP based light emitting systems were designed for their application in various OLEDs. The full color display is generally required in the emission spectra of basic colors red, green and blue. Due to the inherently wide band gap of blue emitting materials the performance of emitting devices based on blue color are often found to be much lower than the green and red ones.78 It has been reported that the derivatives of Polyfluorene (PF) have revealed exceptionally high efficiency in comparison to devices based on blue emitting polymers.84 However, PFs and other polymeric light emitters are forming their aggregates and excimers in the solid state, resulting in fluorescence quenching.85 The HPs with a 3D topology may also prevent the aggregation and interlaced with in the polymer chains. Thus, the use of polyfluorene as a co block agent for the processing of conjugated HPs is considered to be a good approach.86 Recently Zhang et al reported the synthesis of hyperbranched copolymers using three chlorophors like fluorenone having green emission (green units), tris[1 phenylisoquinolinato-C2,N] iridium(III) having effective red-emission (red cores) and poly(9,9- dioctylfluorene) as blue emission (branches). They exhibited that the white-light emission can be attained by the variation in ratios of these three chromophores. The appropriate ratios of these chromophores (1:0.5 mol% of red core: green units) ensures the well-adjusted emission of three lights (red, green and blue) with great fluorescence quantum efficiency (FQE) of 28.1-47.2% in plane films featuring superior color rendering index of 87, high current efficacy (3.85 cd A-1) and brightness (3354 cd m-2 at 10.9 24 ACS Paragon Plus Environment

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mA cm-2).87 On the other hand, Feng et al (Figure 12) reported the synthesis of conjugated hyperbranched based phosphorescent polymers by the application of oxygen sensitive core [red phosphorescent iridium(III) complex] and conjugated fluorescent 9,9-dioctylfluorene (oxygen insensitive blue chromophore) in the backbone of HPs. They proved that the hyperbranched structure of this phosphorescence increased the O2 sensitivity of these materials due to increase in the number of functionalities.88

Figure 12. Design, synthesis, and chemical structures of Ir-HPC/PSMA (phosphorescent polymer) dots for hypoxia imaging and photodynamic therapy with its mechanisms. (Reprinted with permission from ref 88. Copyright 2017, American Chemical Society, Washington, DC.) 4.3 Amphiphilic properties The amphiphilic behavior of compounds (hydrophilic and hydrophobic) in biomedical applications is a crucial property. They are capable of forming self-assemble nano-sized structures (nanoparticles, nano-spheres, vesicles, micelles, and microcapsules) in an appropriate solvent (Figure 13).89–91 Therefore, many researchers fabricated self-assembly micelles by combining the hydrophilic and hydrophobic segments using amphiphilic linear copolymers. However, these systems failed to provide satisfactory performance due to 25 ACS Paragon Plus Environment


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disassembly in their structure on in vivo application because of the dilution of copolymers in bloodstream that led to the loss of their main properties. These drawbacks made scientists more concern for the development of new amphiphilic systems based on HPs. In early 1990 the structural advantages of highly branched polymers availed them through functionalization and modification as an alternate to surfactants or block copolymers.92 Later, in 2006 Kreutzer et al. developed a water soluble drug vector star-copolymer by successive atom transfer radical polymerization of two acrylate based monomers [n-butyl methacrylate and poly(ethylene glycol) methyl ether methacrylate] and macro-initiators (functionalized fourthgeneration hyperbranched polyester) following multistep process.93 They showed that the increase in loading of hydrophobic drugs in the carrier increased the branching and endgroups up to 27% by weight. Later the interest in fabrication of these drug carrier by multistep process changed towards the simple one step synthesis due to complexity in their preparation. For instance, Chen et al. synthesized functionalized self-assembly amphiphilic HP through one step process using commercially available aliphatic core (HB aliphatic polyester Boltorn H40), hydrophobic inner segments [poly(ɛ-caprolactone)], hydrophilic outer shell (poly(ethylene glycol)), and a targeting group (folic acid).94 They studied the drug release behavior of these nano-carriers and the effect of molecular weight on their size in which their size increases with increasing their molecular weight. Recently, Zhang et al. fabricated biodegradable amphiphilic HP through one-pot process by polyesterification of AB2 monomers (one –OH and two –COOH groups) and polyethylene glycol methyl ether (mPEG). They showed that the drug release of this biodegradable HP was enhanced compared to its linear one.95 Furthermore, Sudo et al. have reported the fabrication of a thermoresponsive

amphiphilic

HP

(hyperbranched

polystyrene-g-poly(N-

isopropylacrylamide)) for smart cell culture plates and cell sheet engineering applications.96,97 The HP was synthesizedby via grafting of poly(N-isopropylacrylamide) from the termini of 26 ACS Paragon Plus Environment

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hyperbranched polystyrene using RAFT polymerization, which was further solution casted to prepare thermoresponsive films. They found that the presence of hyperbranched structure resulted in uniform distribution and stablity of poly(N-isopropylacrylamide). In addition, they introduced polar groups (dimethylamino and carboxylic groups) in the structure of thermoresponsive HPs to investigated their effect of on cell adhesion/detachment properties. This study exhibited that the presence of polar groups tuned cell compatiblity properties of the HPs.98

Figure 13. Synthesis of upper critical solution temperature type poly (amino acid)-based [polyvinylpyrrolidone-b-polyureido(ornithine-co-lysine)] PVP-b-PUOL block copolymer via layer-by-layer assembly system. (Reprinted with permission from ref 90. Copyright 2017, American Chemical Society, Washington, DC.) 4.4 Biodegradability and Biocompatibility The biodegradability and biocompatibility of HPs are the most important properties which determine the suitability of these polymers in various applications (biosensors, tissue engineering, drug delivery, biomaterials, etc.).99,100 Many types of biocompatible HPs have been synthesized for biomedical applications, but these polymers could not be explored because of the lack of specificity on their action, low physicochemical properties, and lack of 27 ACS Paragon Plus Environment


selectivity. Therefore, researchers started to develop new HPs to overcome these disadvantages. The biocompatible polymers (polyglycerols) with increased hydrophilicity and improved chemical stability high biocompatibility was developed by Frey et al for the first time via ring-opening mechanism by AB2 monomers under a controlled polymerization condition. Nevertheless, these materials were not biodegradable, which prevented their invivo applications, as the accumulation of such polymers was harmful for living cells.100 Thus, the preparation of biocompatible HPs with high degradation in physiological condition became an important requirement. Consequently, HPEs gained much attention due to ease of their structural hydrolysis like commercially available Boltron Hx (where X= 20-40), Kizhakkedathu et al. prepared a biodegradable HPE (hyperbranched polyether) through the incorporation of ketal moiety into its backbone via ring-opening mechanism (KHPE) (Figure 14).101 The synthesized KHPEs showed high biocompatibility and biodegradability when subjected to cell viability assay (CVA) and blood compatibility assay (BCA) which showed their great potential in drug delivery applications. Huang et al synthesized biocompatible and biodegradable HPPs using 2-(2 hydroxyethoxy) ethoxy-2-oxo-1,3,2-dioxaphospholane inimer, which is hydroxyl functionalized cyclic-phosphate via self-condensing ring-opening (SCRO) polymerization that could be used in biomedical applications.42


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Figure 14. Scheme for the fabrication of poly (ketal hydroxy ethers) via anionic multibranching ROP. (Reprinted with permission from ref 101. Copyright 2012, American Chemical Society, Washington, DC.) Additionally, the use of HPs in target drug delivery and drug release attracted the researcher’s attention to develop smart HPs by modification in their structures that enables them to respond against the physiological changes (example changes in redox state, pH, temperature, etc.), which specifies their action.30 This property of HPs via structural modification is explained in details in section 5.3.1. 4.5 Shape-memory HPs (SM-HPs) Generally, the materials possessing the ability to remember a permanent shape and retain it upon temporary deformation when placed under different conditions (pH, temperature, pressure, electric, etc.) and applied stress are called SM materials. These materials have gained great attention from engineers and scientists because of their capacity to memorize two shapes at different environments, which gives them a great potential for various applications in actuators, sensors, media recorders, smart devices, tissue engineering, etc.102 29 ACS Paragon Plus Environment


The special peculiarities of HPs further fitted them for the development of SM materials, which was found to provide enhanced performance compared to their linear counterparts.103,104 For instance, Deka et al. fabricated a vegetable oil-based SM-HPU nanocomposites using multi-walled carbon nanotubes.105 They observed that the nano-tubes enhanced (98%) SM behavior of the material by effecting the soft segment (reversible phase) of the HPUs through hydrogen bonding. However, the aggregation of nano-tubes within the matrices was the limiting factor to their wide application. Sivakumar et al., reported SMHPUs using AB2 monomers.106 They utilized poly(e-caprolactone) as diol and cross-linked it with hydroxyl-terminated HPUs (AB2), which revealed that the cross-linked materials showed greater SM property than the pure polymer. On the other hand, Thakur et al., could minimize the aggregation of nanoparticles by developing a well dispersed graphene oxide (GO) SM-HPU nanocomposites. They observed that the chemical interactions between HPU and GO improved their dispersion within the matrix, which enhanced the toughness and shape recovery (99.5%) of the resultant polymers.107 Further, Li et al., fabricated lignin modified thermally stimulated SM materials using HP-based cross-linkers.108 The material was prepared through melt polycondensation reaction with alkyl chain of hyperbranched poly(ester-amine-amide), which was used as B3-A2-CB3ʹ segment as cross-linking moiety. Recently, Jeong et al., have synthesized hyper-OH, hyperbranched poly(amine-ester), using pentaerythritol tetraacrylate and diethanolamine through thiol-ene click chemistry, following Michael addition reactions. The resultant materials exhibited satisfactory SM behavior due to the presence of cross-linking multifunctional HP moiety present in the structure.109 In addition, Yang et al. designed a SM as CO2 capture membrane using hyperbranched polyimide. The branching center was 2,4,6-triaminopyrimidine, which prominently effected the gas absorption and SM properties of the developed membrane. They also observed that SM behavior and gas absorbing property of these HPs are prominently dependent on DB.110 30 ACS Paragon Plus Environment

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5. APPLICATIONS Initially the HPs were considered to have limited applications because of their brittle nature due to the chain entanglement in their structure. These limitations certainly had an impact on their applications in various fields as bulk materials. However their modifications with large number of functional groups and low viscosity have made them versatile in terms of applications. These modifications could be made mainly by two ways, which are structural (end-capping, terminal grafting, hyper grafting, etc.) and synthesis (waterborne, inorganic branching unite, nanocomposites, etc.).111–114 These methods facilitated the improvement of their existing properties as well as inducing new special peculiarities. The results obtained based on various studies pertaining to their properties after modification have revealed that these drawbacks could be resolved successfully. These distinct features made HPs to be suitable in numerous fields and applications, which are described in this section. 5.1 Coatings The structural modification of HPs in the form of primers, additives, and blend components attracted high attention. The introduction of several functional moieties that act as cross linking groups (vinyl ether, acrylate, epoxy ring, -OH groups, etc.) enhanced their solubility and induced low viscosity resulted in their excellent performance in their applications in form of solid and powder coatings.115,116 The use of –OH terminated HPEs has helped the development of coatings having low volatile organic compounds (VOC) (e.g. PU coatings).117 The key factor for the application of HPs as coating materials is their ability to maintain the ratio of functional groups of reactants, as in case if the ratio of functional groups is not maintained, the presence of large number of reactive functional groups results in the fast curing of coatings. Kim et al. exhibited that blending of HPP with linear polystyrene (PS) results in the decrease of viscosity, the shear rate at high temperatures, and the toughness along with an increase in thermal stability of 31 ACS Paragon Plus Environment


such HPs compared to their pure counterpart. They confirmed that the HPEs on blending with linear polymers like polyesters, polyamides and polycarbonates show full miscibility due to strong hydrogen bonding.118 The modified HPEs,119 polyesteramides (HPEAs)120 or polyethylene (HPEts)121 imines have found their use in polyolefin blends as dye carriers. These materials showed a reduction in melt viscosity and a homogenous distribution of dye in the matrix. Hong et al. found that the alkyl-modified aliphatic hyperbranched polyesters had a strong effect on rheological behaviour of Low Density Polyethylene.122 Nunez et al., reported the reduction in melt viscosity as a function of hyperbranched polymer concentration. 123 The introduction of various reactive functional groups (acrylate, methacrylate, epoxy, etc.) into the hyperbranched polymeric backbone allows their use in coatings and other polymeric resins. These facts enables HPs to be used as toughening,124,125 additives,126 Stabilizers,127 curing, crosslinking and adhesive agents128 in coating industry, which opened new field of applications for modified HPs, such as high solid , flame retardant,128 barrier for flexible packaging, etc.129 Often HPs in combination with long alkyl chains, provide desired flow and viscosity, for example the use of bis (hydroxymethyl) propionic acid (BHMPA) in the preparation of aliphatic HPEs led to their promising applications in the field of coatings.130 Literature survey currently reveals that the number of thermal and UV cured, epoxy, acrylate and alkyl ether functionalities based hyperbranched polymers have promisingly been used in high performance coatings.131 Recently, the vegetable oil and petrochemical based hyperbranched poly(esteramide)s under the trade name HybraneTM and polyethyleneimines under the trade name of LupasolTM. It is important to mention that the application of HPs in protection of metal surfaces (aluminium and iron) as anticorrosive coatings also gained a lot of attention due to their promising high stability under extremely corrosive environment. For instance, Sharif et al. recently reported the development of HP soya alkyd NC using Fe3O4 as magnetite nano-fillers, butylated melamine formaldehyde (BMF) as curing agent, and soya 32 ACS Paragon Plus Environment

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monoglyceride as precursors. They proved that the uniform dispersion of NPs within the HP matrix prevented the penetration of corrosive ions through the HP matrix, which improved the anticorrosive performance of resultant HP-NCs.132 Recently Armelinet al., developed highly effective anticorrosion coatings using strong adhesive and cross-linking poly(1,4disubstituted 1,2,3-triazole)s via y copper catalysed azide−alkyne cycloaddition. They observed that using this technique results in the formation of a strong interfacial interaction between organic coating and metal (aluminium) surface. They further found that the best films with superior anticorrosion properties, were attained with monomers having A2B3 type functionalities such as A2B3 polytriazoles (Figure 15).133

Figure 15. Synthesis of Linear and HB poly(1,4-disubstituted 1,2,3-triazole)s using Cu catalysed click reaction of A2B3 Azides for anticorrosion application. (Reprinted with permission from ref 133. Copyright 2017, American Chemical Society, Washington, DC.) 5.2 Sensors (chemical and biosensors) The imperfect, highly branched, surface-grafted functional HPs, such as branched poly (acrylic acid-co-acrylamide) has been successfully employed in the form of thin layer used in different sensors used for the detection of VOCs (Figure 16).134–136 For instance; Daniel et al. reported a chemical sensor by polymer grafting method to enhance the chemical selectivity of 33 ACS Paragon Plus Environment


sensors. They prepared an electrochemical sensor by crafting HB poly(acrylic acid) (HPAA) films functionalized ß -cyclodextrin-coated Au electrodes with ultrathin polyamine film. The HPAA film was used to act as the chemical receptor for ß -cyclodextrin molecules and the ultrathin polyamine film acts as the pH-sensitive filter that selectively allows charged particle to pass through and reach to ß-cyclodextrin receptor.137 So far, most of the work reported in the field of sensors deals with the linear biopolymers and highly branched polyamidoamine (PAMAM) or polypropylene imine (PPI) dendrimers.138–140 The self-assembled monolayers of these materials are fixed on the surface of the substrate and used as sensors. Variety of other dendrimers with different functional end groups have also been employed.141,142 Often, the preparations of assemblies or multilayer systems of HPs are effectively prepared for the better control of thickness, which induces improved sensor behaviour. Tsukruk et al. have intensively reviewed and reported that the modification of surface of sensors by branched macromolecules have superior surface properties than those of linear polymer or surface assembled monolayers.143 Strumia and Reemers et al. have further reported that the surface modification by dendritic molecules like poly(esteramide) and polyurethane have been potentially used to prepare the surface, which HPs retain the active and fine-tuned surface properties.144 Surface properties of hyperbranched polyesters were extensively studied by Voit et al.145 They investigated the effect of hydrogen bonding, swelling in aqueous medium, buffer solution, and protein adsorption on thin polymer films. They reported that the thin films of HPs under the dynamic states differ significantly from that of linear polymers. The direct application of hyperbranched thin film polymers in sensor devices is not completely explored. Voit et al. reported the use of aromatic hyperbranched polyesters with hydroxyl, carboxylic acid, or acetate end groups, are employed as sensitive layers in humidity sensors.146 These films respond rapidly and repeatedly to water vapour, with a linear 34 ACS Paragon Plus Environment

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dependency between the amount of humidity and the swelling of the films. The response significantly depended upon the functionality of the polyester. The strongest response for water was found in case of hyperbranched polyesters with hydroxyl end groups. However, the sensitivity changed significantly when different alcohol grades were used as analytics. Hyperbranched poly(carbosilanes) with different end groups have been studied in surface acoustic wave sensors, giving a good sensing response for molecules used as a models for nerve agents.147

Figure 16. Detection of SO2 molecules present in water using self-assembly, hyperbranched poly(3-ethyl-3-oxetanemethanol)-star-poly(ethylene oxide)-tertiary amine alcohol (HSPTAA) vesicles, which is functionalized through proton exchange with cresol red (CR). (Reprinted with permission from ref 136. Copyright 2017, American Chemical Society, Washington, DC.) 5.3 Therapeutic applications 5.3.1 HPs for drug delivery The previous section (section 4) has described the characteristic features of an ideal drug 35 ACS Paragon Plus Environment


carrier, which are the outstanding biocompatibility, biodegradability along with least cytotoxicity. The other important characteristics are the ability to form a stable drug-polymer complex, which can be transferred to the target site, followed by their efficient release, while retaining their pharmaceutical effects (Figure 17).100 These approaches has been partially attained by the development of hydrogel-based carriers and has been reviewed previously.148,149 However, these limitations could not be resolved completely and still the need for more advanced materials is needed. Therefore, HPs have emerged as the ideal delivery systems due to their ease of chemical modifications and their large number end groups. Administration of HPs in establishment of controlled, sustained, and smart drug release at target site has been extensively explored. Burt et al., described the fabrication of carboxylic acid modified HPs based on HPG and H40 precursors.150 These modified HPs were examined at physiological condition to determine their drug carrier property for controlled drug release. They observed that the modified HPs (HPG) formed a stable complex with drug, which showed a sustained release of cisplatin at the site of application for period of seven days that makes this polymer to be used in drug delivery application. The synthesis of biocompatible, biodegradable, and non-cytotoxic KHPE by Kizhakkedathu et al. showed the stimulus responsive behavior towards the change in environmental pH.149 They showed that the incorporation of both ketal groups (cyclic and acyclic) in KHPE synthesis could affect the rate of degradation from few minutes to more than hundred days in different pH level. On the other hand hyperbranched polypeptides (HPPt) gained the high attention in field of drug delivery due to their proteolytic stability, high solubility, and low cytotoxicity compared to their linear counterparts.151 However, their potential in the field of biomaterials drug delivery has not been investigated thoroughly. The stimulus responsive HPs have gained intense attention due to their controlled and sustained drug release behaviour. These polymers are specially designed to respond towards a 36 ACS Paragon Plus Environment

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minor change in their environmental pH, light, redox condition, and temperature that enhances their target drug release. Gong et al fabricated a HB H40-star-(PLA-b-PEG) copolymer, as a cancer-targeting drug carrier.152 In this biocompatible and biodegradable HP, folic acid was used as targeting group and the DOX (Doxorubicin, anticancer drug) was introduced to hydrophobic PLA segment by hydrazine (pH-sensitive) linkage. On exposer of drug-polymer complex to the acidic environment, the cleavage of pH-sensitive bonds occurs, which subsequently results in release of anticancer drug (DOX). On the other hand, Ji et al developed a photo-responsive HP through the modification of phospholipid hyperbranched polymers (HPHEEP) using photo-responsive segment, they introduced 2-diazo-1,2naphthoquinone-5-sulfonyl chloride (DNQ) a hydrophobic group, at the terminal chains of HPHEEP which could form micelle in water.153 This biodegradable HP micelle when exposed to UV irradiation gets destabilized due to the disintegration of DNQ moieties that initiates the drug release. The thermos-sensitive HPs also attracted researcher’s attention because of their quick respond to the temperature change in their surroundings. These HPs when exposed to different temperature, absorb the thermal energy that causes the spatial rearrangement in their structures that suitable them to be used where temperature changes occur. Recently, Wang et at., synthesized thermo-responsive HPs using two spatially ionized isomers.154The two thermo-responsive HPs were fabricated using two spatial isomers containing two different amide entities, i.e., the isobutyramide and acetamide groups, located in different layers of HPs. They observed that by the incorporation of these two amide groups in different positions (layers) the extraordinarily dissimilar transition behaviours was achieved. They confirmed that mechanism of these thermal changes was due to variations in packaging arrangement which was induced by different spatial position of amide moieties using NMR study. The HP which had dense-packed structure showed a characteristics microscopic phase separation in 37 ACS Paragon Plus Environment


the molecule due to less hydration compared to loose-packed structures that explained the change in the HPs transition temperatures. It is also important to mention that the loading of drugs into the HPs matrix is a crucial step towards the development of stable drug-polymer complex. Therefore, here a brief discussion is given on the improved functioning of HPs after the encapsulation and covalent attachment of drug in HPs matrix.

Figure 17. Schematic development of biocompatible and dual-pH responsive anticancer drug delivery micelles based on amphiphilic and multi-functional linear-hyperbranched copolymer. (Reprinted with permission from ref 100. Copyright 2017, American Chemical 38 ACS Paragon Plus Environment

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Society, Washington, DC.) The encapsulation of hydrophobic and hydrophilic guest molecules assisted by utilization of various HPs like HPEs, polyglycerols, polyester amides, and polyethylenimines, which are widely investigated during the past few years (Figure 18).155–157 One of the simplest method of drug encapsulation is the direct entrapment of small drug molecules within the nanocavities present in the HPs structures.158 Furthermore, the fabrication of self-assembled HPs that can form stable micelles containing drug molecules (encapsulation), which enhances the drug dissolution in the cavities present within the HPs. These drug-HPs complexes mostly exhibit higher stability compared to that of linear-based complexes under similar conditions.159 HPs have superior capability in encapsulation of drugs than their linear counterparts. For instance, Haag and coworkers studied the effect of chemical functionalization of HPEI on their drug loading capability.160 The fatty acid-functionalized HPs showed superior encapsulation property towards drug molecules comprising anionic moieties such as carboxylate, phosphate, sulfonate, acidic OH groups, etc. They further described that this modification improved the encapsulation capacity of HPEI up to 150 guest molecules. However, it is important to mention that HPs containing uni-molecular micelles contain less volume of inner cavities that limits them to encapsulate large amount of drug molecules. Whereas, multi-molecular micelles contain more hydrophobic centers, resulting in their improved encapsulating property and smart delivery. The drug molecules form noncovalent bonds with the multi-molecular micelles that enables them to encapsulate more than one type of drugs.156 Above-mentioned remarks reveal that the 3D structure of HPs play a critical role in their encapsulation ability.



Figure 18. Schematic representation of simple drug encapsulation using HPs as carrier. Albeit the effective performance of HPs in drug delivery, there are some limitations associated with their efficient ability in drug encapsulation and release. One of the main drawbacks is the comparatively rapid and uncontrolled drug release through the HP vectors. However, the HP-drug conjugates having covalent bound form stable complexes in buffer and water. The formation of covalent bond between the drug molecules and HPs depend on the types of functional groups present at the peripheral site of HPs, which shows the maximum impact on the activity of the drug conjugated HPs, which allows the control drug release.161 5.3.2 Protein delivery There has been a rapid advancement in the administration of active proteins and peptides in curing of a number of disorders, such as hormone deficiency, cancer, anemia, etc.162 One of the major challenges in protein drug administration is their poor bioavailability and weak hydrolytic stability.163 They deactivate during the course of fabrication, packing or transfer by chemical, physical and enzymatic means. Various systems have been developed from time to time to overcome these limitations in protein administration (Figure 19).164 Zhang et al. developed self-assembly poly lactic acid (PLA) copolymer nanoparticles using functionalized HPG for protein delivery (bovine serum albumin).165 The HPG-based systems showed high controlled protein release. They further showed that the released protein retained its original structure. Recently, they also reported that other functionalized PEG %-CD functionalized HPG) could be administrated in the insulin.166 They encapsulated the insulin by positively 40 ACS Paragon Plus Environment

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charged HPG-g-CD nanoparticles with 188-340 nm range. They demonstrated that the invivo administration of insulin-HPG-g-CD complex could permeate through the nasal epithelia membrane, lowering the blood glucose level, which proved their promising protein delivery.

Figure 19. Schematic structures of HPs along with their reaction and crosslinking with protein to form the nano-assembly, and its release behavior in reducing condition. (Reprinted with permission from ref 164. Copyright 2017, American Chemical Society, Washington, DC.) 5.4 Antimicrobial applications Microbial infection is an emerging serious complication in several areas like health care, medical devices, hygienic applications and drug delivery.167A great achievement has been attained during the last few years in antimicrobial treatment using a variety of antimicrobial agents. However, these antimicrobial agents suffer from several disadvantages like 41 ACS Paragon Plus Environment


antibacterial resistance, short-term antimicrobial ability and environmental toxicity because of their low molecular weight.168 Recently, polymer-based antimicrobial agents derived from linear polymers have gained considerable interest as a result of their low toxicity, enhanced antimicrobial activity and decreased potential for resistance.169,170 However, utilizing the single step approach for the preparation of HPs and ease of availability, the antimicrobial agents based on these polymers have gained much attention.171 The inorganic metallic nanoparticles (silver or gold nanoparticles) are widely used in biomedical science as antimicrobial agents. However, the main drawback associated with the metallic nanoparticles is that they often agglomerate, limiting their antimicrobial effect.172 Thus, the utilization of HPs proved to be the most effective method to overcome these limitations. Zhou et al developed a stable colloid of nanoparticle (AgNPs and AuNPs) using amine-functionalized HPs as stabilizer and reductant both through a green approach.173


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Figure 20. Synthesis rout of extremely active peptide-grafted antibacterial HP. (Reprinted with permission from ref 174. Copyright 2016, American Chemical Society, Washington, DC.) Gao et al. (Figure 20) reported self-assemble antibacterial peptide-grafted HP nano-sheets, which penetrated through bacterial cell-membrane by wrapping them resulting in bacterial death.174 The HPs were fabricated by Michael-addition reaction of free radical and thiol-ene following the ring-opening polymerization of N-carboxyanhydrides. These HPs could show effective biocidal activity against Gram-positive and Gram-negative bacteria due to their slightly positive charge which arisen due to the H-bonding in amine moieties. 43 ACS Paragon Plus Environment


Lia et al. developed surface active antimicrobial hyperbranched polyurea using covalently bonded quaternary ammonium compound to surface of HPs as antimicrobial agent using polycondensation reaction of AB2 monomers (A is secondary amine and B is blocked isocyanates).175 The HB polyurea was developed by the initial covalently grafting of siloxane containing isocyanate group with silanol groups present on the glass slides which facilitates the reaction between the central amino group of AB2 and blocked isocyanate functionality of factionalized glass slides. These HPs coatings exhibited high contact-killing abilities against adhering bacteria without any leaching of biocidal agents. The presence of QAC-molecules in HPs structure improves the adhesion of bacteria to the surface of polymer, limiting their growth by inducing localized distortion of lipid bilayer that ultimately resulted in death of bacteria. 5.5 HPs as Antifouling materials Biofouling has emerged as a serious concern for those polymeric materials used as artificial devices, immunoassays, drug delivery systems, biosensors and others.176 The process of protein absorption occurring on biomaterial implantation initiates a cascade of host responses, like bacterial infection, thrombus formation, blood coagulation, platelet activation and other undesirable responses.177 To solve these problems, there is an imminent need to develop novel antifouling materials. Over the last few years, linear polymers have proven to be promising candidates for the fabrication of antifouling materials mainly, PEG which is a hydrophilic, water soluble and a flexible polyether.178,179 Although, it is thermally unstable and undergoes rapid autoxidation to yield acids and aldehydes. Taking this into account, Haag and coworkers developed SAMs on the gold surface based on HPG that possessed similar protein resistance like that of PEG SAMs but a superior oxidative and thermal stability in comparison to PEG SAMs.179 In the recent years, a good quanta of antifouling materials based on HPG were reported.180 A novel HPG grafted microporous membranes 44 ACS Paragon Plus Environment

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were reported by Kang and coworkers via simple alkyne-azide click reaction. The phase inversion in an aqueous medium was utilized for the fabrication of these membranes from amphiphilic graft copolymers of poly (vinylidene fluoride) (PVDF-g-PDMAEMA). The prepared membranes exhibited better antifouling and a good resistance to protein adsorption. (Figure 21).181

Figure 21. Steps involved in the synthesis of HPG-grafted microporous membranes via 45 ACS Paragon Plus Environment


simple alkyne-azide click reaction. (Reprinted with permission from ref 181. Copyright 2012, American Chemical Society, Washington, DC.) Apart from the antifouling materials synthesized from HPG, some other HPs have also been utilized for antifouling properties.182 Voit and coworkers, synthesized polyester films using hyperbranched polymers with varying backbone structure, flexibility, DB and polarity.183 The protein active to protein repelling (film properties) i.e. were controlled by altering the backbone with aromatic, aromatic-aliphatic and aliphatic moieties. Wooley and coworkers, centered their efforts on the novel synthesis of cross-linked amphiphilic polymer networks (HFP-PEG) comprising of linear PEG and hyperbranched fluoropolymers as antifouling coating materials.184 The same researchers also reported a dual-mode surface for marine applications the beauty of which was that it combined both active and the passive antifouling modes. In this method, a new generation of HFP-star-PEG was obtained by crosslinking HFP containing ethylene glycol units with PEG. This was followed by deposition and followed by curing, where the passive antifouling surface was provided by the complex chemical heterogeneity and surface topography. An antifouling agent noradrenaline (NA) was used to decorate the HFP-star-PEG further for the active mode of fouling deterrence. These studies revealed a new approach for the combination of active antifouling moieties with passive antifouling networks, leading to the formation of antifouling coatings with superior properties.164 5.6 Bio-imaging Bio-imaging is a technique used for the better treatment of living organisms by visual imaging of internal organelles and tissues of living specimen along with visualization of their functions. This technique is more frequently used in comparison to that of positron emission tomography (PET) and magnetic resonance imaging (MRI). This is attributed to the use of the high light-sensitive materials and low cost. However, certain disadvantages are also 46 ACS Paragon Plus Environment

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associated with this technique like the absorption of radiations by tissues, auto-fluorescence, and light scattering prevented this technique to be the safest method of diagnosis.118 Therefore, researchers have developed new bio-imaging probes based on the combination of HPs and polymer-based fluorescent that showed a great potential in medical applications (Figure 22).185 These probes are divided in to two types: (i) HPs-fluorescent complexes like fluorescentproteins, small fluorophores, inorganic-fluorescent, and (ii) HPs with fluorescence properties.186 Although organic based fluorescent dyes are mostly used as fluorescent components as bioimaging probes, there are some disadvantages, associated with these materials such as poor stability (short half-life ) in blood, low photo-bleaching thresholds, low permeability through live cells membrane, and the absence of specificity in choosing target organs or cells.187 Therefore there is a scope for researchers to overcome these drawbacks associated with these systems through the synthesis of new fluorescent materials by encapsulation or conjugation of fluorescent agents by HPs. For instance, HPs-fluorescent complex was fabricated using fluorescein isothiocyanate (FITC) that could be easily linked to HPs covalently for bio imaging. To do so HB poly (sulfone-amine) (HPSA) with less cytotoxicity and high serumcompatibility was synthesized and covalently linked to FITC through the amino functionality of HPSA and isothiocyanate group present in FITC, which resulted in a more endosomalstable complex.188 Recently, Chen et al. fabricated cationic red‐emitting HP involving tetraphenylethene and pyridinium groups through A2+B3 methodology (where A2 is 1,2-diphenyl-1,2-di(ppyridylethenyl)phenylethene and B3 is 1,3,5-tris(bromomethyl) benzene) for cell imaging applications.189 The presence of tetraphenylethene induced aggregation‐enhance emission (AEE) character in to the HP with a high fluorescence quantum yield (FQY) (13.5%) and on 47 ACS Paragon Plus Environment


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conjugation with pyridinium group enhances the Stokes shift (greater than 200 nm), strengthening it towards external stimuli like solvent properties (solution phase), mechanical grinding ( solid form), and different pH (in aqueous). The cationic nature of the red‐emitting HP enables it to permeate through cell membrane that results in staining of cells and used as a stable fluorescent probe for bio-imaging application. On the other hand, Due et al. reported fluorescent hyperbranched conjugated polymers (FHCPs)

based

on

boron-dipyrromethene

(BODIPY)

and

poly[2-

methoxy-5-(2-

ethylhexyloxy)-1,4-(1-cyanovinylene-1,4-phenylene)] (CN-PPV), poly[9,9 dioctylfluorenyl2,7-diyl-co-1,4-benzo-{2,10-3}- thiadiazole] (PFBT) through Suzuki polymerization strategy having higher FQY (40%) than that of previously reported fluorescent materials. They have confirmed the cytotoxicity, biocompatibility, and cell specificity of these FHCPs through in vivo study by injecting these materials to zebrafish larvae for the successful bio-imaging applications.190


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Figure 22. Hyperbranched polylactide nanoparticles functionalized with N-Carboxyhexyl-4dimethylaminoethyloxy -1,8-naphthalimide (pH-sensitive N2 [green]) exhibiting aggregationinduced emission performance that shows high potential in intracellular pH sensing and bio imaging using N-Carboxyhexyl-4-methylpiperazin-1,8- naphthalimide (N1[blue]) as the reference. (Reprinted with permission from ref 185. Copyright 2015, American Chemical Society, Washington, DC.) 5.7 Energy Storage Applications 5.7.1 Thermal Energy Storage (TES) 49 ACS Paragon Plus Environment


Phase change materials (PCMs) are those substances that are able to store and release high amount of heat energy upon changing phase (melted state to solid state). PCMs can release sensible, thermochemical, and latent heat in TES devices (Figure 23).191 Among them, the latent heat have gained the importance in terms of energy management during 1973–1974.190 Different class of PCMs are produced based on the type of phase-change occurs, like: solid– liquid (S-L), solid–solid (S-S), and liquid–gas (L-G). Furthermore, the S-SPCMs are mostly used due to their good latent heat release during heating/cooling cycle, produced via chemical grafting, cross-linking, and blocking reactions between hard (polymers) and soft (PCM) segments.192,193 Later, linear polymers became an important candidate in development of organic PCMs because of their required physical and chemical properties that can be tailored.194 However, the presence of certain drawbacks such as the chain entanglement, low molecular weight, and presence of less functional groups in linear polymers emerged the need of HPs to overcome these disadvantages.195 Du et al. reported the synthesis of S-SHPU-PCMs using hyperbranched polyol (HPP), where poly(ethylene glycol) (PEG) and 2,2bis(hydroxymethyl) propionic acid as the core molecule and chain extender respectively.196 They have fabricated S-SHPU-PCMs by varying extend of crosslinking using various amounts of isophorone diisocyanate (IPDI) and HPP as polymer backbone while PEG-6000 was used as phase-change moiety. They showed that the proper DB in HP’s structure improved the phase-change capacity, good fusion enthalpy (123.5 J/g), the thermal cycling (TC 100- cycles)) and the TG analysis (≥250 °C) revealed its good thermal stability and performance. At the same time, Sundararajan et al. reported fabrication of HPU-based SSPCMs through A2 + B3 methodology where A2 was the PEG-IPDI and B3 was trimethylolpropane (TMP) containing IPDI at terminal position.196 Here, they have used high amount of phase-change moieties (PEG-8000). They proved that by increasing the PEG moiety the enthalpy of fusion associated with polymer increased to 145 J.g-1 along with 50 ACS Paragon Plus Environment

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phase-change temperature (55 °C). The enthalpy of synthesised S-SPCMs showed an increase on 20 TC without any chemical loss, which indicated its high thermal stability (300 °C).

Figure 23. Schematic representation of thermal energy storage material based on PEG

incorporated with HPU as S-S phase change material. (Reprinted with permission from ref 191. Copyright 2017, American Chemical Society, Washington, DC.) 5.7.2 Rechargeable batteries The interest in utilization of polymer-based electrolytes in development of lithium-ion (Liion) (rechargeable) batteries has begun in late 90’s. These electrolytes were used as thin layers or plastic-like batteries because of their performance and exclusive properties.197 The application of dry (solvent-free) polymer electrolytes in fabrication of Li-ion batteries enhanced the ion mobility drastically due to the presence of amorphous region in which mostly ionic-conduction occurred.198 However, these polymer electrolytes showed low conductivity at normal temperature, which was due to the presence of crystalline phase (nonconductive).199 The focus on the employment of branched polymers in the construction of Li-ion batteries attained by the dendritic polymers due to the availability of large number of functionalizing end groups (Figure 24),198,200 Later on the actual application of HPs in preparation of Li-ion batteries started using HPEO derivatives comprising 3,551 ACS Paragon Plus Environment


dioxybenzonate branching as branching moiety along with diethylene glycols (DEG), and triethylene glycols (TEG). These HPs were effective in the fabrication of Li-ion batteries due to their low Tg (below room temperature) due to their amorphous nature. However, one of the drawbacks associated with these materials is low mechanical properties, which inhibited their use at the site of application. This drawback could be solved to some extent by application of HPs-composite materials. For example, Wen et al investigated the role of inert ceramicbased fillers ( α- or λ-LiAlO) in fabrication of HPs-composite electrolytes using TEG benzoate having acetyl group at terminal position, and lithium salt (LiN(CF3SO2)) using solvent casting technique. They observed that on addition of these crystalline fillers (10 wt. %) to the amorphous phase (HPoly bis TEG benzoate) the Li-ion transference and ionic conductivity improved, but these materials showed poor thermal properties and low conductivity at low temperature.198 These limitations were assigned to the presence of high crystalline moiety in polymer electrolytes structures, which was attributed to the presence semi-crystalline poly(ethylene oxide) with high degree of crystallization, inhibiting the Li ions mobility through the amorphous phase. Therefore, researchers have developed an alternative method to overcome these problems i.e. the gel-based HP electrolytes due to existence of large number of copolymers, cross-linker, monomers such as PMMA, poly(acrylonitrile), poly(vinylidene fluoride), etc.201 Wang et al. have fabricated an electrolyte based on HB-gel copolymer via free radical polymerization using a HB oligomer (modified bismaleimide), homo-polymerization initiator (barbituric), and cross linker (poly(ethylene glycol) diacrylate).202 They have investigated the role of modified hyperbranched oligomer as additive, and the effect of salt concentration on high power application. They found that the additive has increased the free space along with the ion mobility (conductivity), which was 7.72 × 10−3 S/cm at 23 °C, the activation energy of which was found to be 5.41 kJ/mol. They also observed that on application of the prepared 52 ACS Paragon Plus Environment

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electrolyte in Li-ion battery (LiCoO2/GPE/MCMB) at room temperature resulted in the satisfactory current discharge maintaining the capacity of 84.2% at 8 cycles having acceptable electrochemical stability up to 4.2 V. This strategy partially helped in resolving the problem but their poor physical properties, less thermal stability, and safety issues (due to the leakage of additives) forced researchers to investigate to overcome these limitations. Till date number of HPs based electrolytes have been fabricated but they still failed to achieve the Li-ion conductivity threshold i.e. 1×10-3 S/cm. Wilms et al. have reported hyperbranched PEG Li-ion conductors through one-step random copolymerization of ethylene oxide and glycerol.203 They showed that even a slight change in the branching moiety (glycerol) induce a drastic effect on the electrolytes properties compared to their linear PEG. These properties were further studied by Lee and co-workers.204 They have reported a study on the effect of branching centre in Li-ion conductivity (using LiTFSI or LiN(CF3SO2)2 as Li-salts) of hyperbranched PEG (HPEG) along with the end-chain modification of hydroxyl terminated copolymers with permethylated precursor. They controlled the DB and maintained Li-ion solubility by varying the glycerol moiety, which resulted in the reduction of crystallinity without increasing nonpolar characteristic and found that upon addition of 8 mol % of branching moiety the crystallinity reached to its minimum level. They observed that at this concentration, the Li-ion conductivity increases by 100-fold in comparison with that of linear PEO at temperature less than 50 °C. Literature reveals that the silicone (Si) and its alloys are also the most promising candidates that act as anodic material in Li-ion batteries. These materials are used to improve the properties of Li-ion batteries by enhancement of their storage ability that can restore power for long distance vehicles on each charge cycle along with improvement in movable electronic devices. But upon lithiation of Si during the operation of battery the Li4.4Si complex is formed in which Si volume expands up to 300% compared to its initial form.205 53 ACS Paragon Plus Environment


This drastic change in the volume of Si causes the slow released of active compounds by pulverization, loss of contact between Si and carbon (conductive mediators), delamination of films from current receiver, and finally damage of solid electrolyte interface that are fatal to Li-ion batteries. It has been realized that these limitations can be overcome through the application of HPs. The application of HPs in preparation of Li-ion batteries is not limited only to the charge transfer enhancement, but also the high number of functionalities present at the end of the chain enabled them to be used as binder. Jeong et al. introduced HP-based binder for Si anodes using β-cyclodextrin as monomer which showed better performance compared to its linear structure. These HB β-cyclodextrin anode binders contained number of functional groups that could provide sites for Si particles to form hydrogen- bonding, thus increasing the contact points between the Si and conductive materials.206 These HPs could provide the self-healing property to the Li-ion batteries due to the multi-dimensional binding system. They further proved that the cycling property of the Li-ion batteries was enhanced reasonably in comparison with those of well-known hybrid binders. Moreover the recent trend has shown the development of new class of rechargeable Li-ion batteries, which is based on the replacement of Si moieties by sulphur compound that resulted in the formation of Lithium-sulphur (Li-S) batteries. These batteries exhibited higher capacity (fivefold) compared to that of Li-ion and Li-Si batteries. However, these batteries are associated with several limitations such as, the inherent low electrical conductivity associated with S elements, volume expansion of sulphur throughout cycling, and dissolution of polysulphides in electrolytes. These limitations can be fulfilled through mainly 3 techniques that improves sulphur stability in the Li-S batteries i) its encapsulation, ii) strengthening chemical bonds between the sulphur and additives, and iii) wrapping it by conductive polymers.207,208


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Figure 24. Preparation of the cationic β-CDp-N+ through the introduction of a quaternary ammonium cation into the backbone of β-cyclodextrin polymer. (Reprinted with

permission from ref 200. Copyright 2015, American Chemical Society, Washington, DC.) However, these methods found to be practically expensive and difficult to process.209 HPs have found a great scope in development of such batteries due to their special 3D structures. Wei et al. have developed an alternative method to enrich the Li-S batteries with Sulphur elements.206 They have fabricated soluble HPs by free radical ring-opening polymerization reaction using sulphur (S8) and 1,3-diisopropenylbenzene solution. The synthesised inversevulcanized HPs could be dissolved in polar organic solvents in large extent (400 mg/L) and upon functionalization (end-capping) with thiol-ene through click chemistry it became water soluble as well. These sulphur-rich soluble HPs used as cathode materials in Li-S batteries upon introducing into conductive ultralight graphene-based aerogels, which exhibited high initial specific capacities (1247.6 mA h/g) and cycling capacity ( 694.0 mA h/g) over 90 55 ACS Paragon Plus Environment


cycles. They evidenced that these enhancements in Li-S batteries are arisen as a result of chemical fixation of sulphur elements that prevents the loss of chemical contact with conductive moiety. Recently, Dhopte et al. synthesised a hyperbranched poly (amido graphene), which exhibited high electrical properties as a non-metallic electrodes for charge storage devices. The HP was developed via Michael addition of ethylene diamine, polyamines, triethylene tetra-amine, diethylene triamine, to acrylamide modified GO, followed by in-situ reduction. They observed that hyperbranching prevented the agglomeration of GO, which enhanced the surface area and d-spacing of GO stacks that improved the availability of ion storing sites. They further indicated that the longer amine chains (the flexible region) boosts the movement of ions within the material, exhibiting the time constant of 538 ms. The fabricated HP found to be highly stable (over 10,000 cycles) against volume change of electrodes (89% capacity retention) throughout the charging and discharging process at 10 A g-1.210 5.7.3 Solar Cells The application of linear polymeric materials (polyelectrolytes or conjugated polymers) in development of solar cells (PSCs) have attracted significant because of their flexibility, interfacial properties, ease of production, light weight, transparency, improved power conversion efficiency (PCE), inter/intra molecular charge transfer, etc.211,212 For instance the application of polymer-based electrolytes or conjugated polymer (example poly[(9,9-bis (3′(N,N-dimethyl-amino)propyl)-2,7-fluorene)-alt-2,7- (9,9-dioctylfluorene)]) as interfacial materials in the development of PSCs requires the surfactant-type side groups such as phosphate, amino, carboxyl, sulfonic, etc. But the linear polyelectrolytes has less number of these moieties that limits the presence of counter-ions around the main chains that results in less stability of the system when charge is passed. In a simple explanation, this means that the complete charge transfer to electrodes through interfacial dipolar channel does not take place 56 ACS Paragon Plus Environment

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and remaining charge will recombine on the interfaces. To overcome this problem the use of multi-channel dipoles on the interfaces can enhance the charge transfer and reduce the charge recombination on these interfaces. For example, Lv et al. fabricated alcohol soluble conjugated HPs as Cathode interlayer via A2+B3 polymerization using blend of high efficacy donor

(poly([4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]

dithiophene-2,6-

diyl]-alt-[2-(2′-thylhexanoyl)-thieno[3,4-b]thiophen-4,6-diyl])) and an acceptor (6,6-phenyl C71- butyric acid methyl ester ) for high-performance PSCs with PCE enhancement (7.55%).213 The significant increase in the PCE value was an extreme enhancement on comparison with reported values (0.3% to 7.55%). Later, Ai et al. reported the synthesis of single-junction PSCs with higher PCE (9.12%) by incorporation of two functionalized HPs (carboxylic acid-functionalized HB poly(ether ketone) and sulfonic acid-functionalized HB poly(ether sulfone)) to alter the cathode interface.214 Recently, Zhou et al. reported synthesis of one-pot HPs via A3+B2 methodology using low-cost precursors (where A3 was 3,6dibromo-9-(4-bromophenyl)-9H-carbazole (1), and B2 was 2,7- bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-9-(undecan-5-yl)- 9H-carbazole).215 They introduced a fully '-conjugated moiety using carbazole in the HPs structure. The presence of HB structure and carbazole moieties enhanced the hole-transporting properties of fabricated HPs that speed-up the charge regeneration, which was as a result of increase in number of UV absorber moieties and the drastic increase in PCE (14.07%) value because of branching. Figure 25 gives the schematic representation of hyperbranched conjugated polyelectrolytes synthesised via A2+B3 polycondensation approach.216



Figure 25. The systematic representation of hyperbranched conjugated polyelectrolytes (HBCPEs) having cationic (PNMe3+) and anionic (PSO3-) HPs, which are synthesised via A2+B3 Heck polycondensation reaction. (Reprinted with permission from ref 216. Copyright 2007, American Chemical Society, Washington, DC.) 5.8 Electroluminescence applications The phenomenon of light emission by materials on exposer to a strong electric field or on the passage of current is called electroluminescence. As it is discussed before (section 4), the utilization of HPs through various synthesis methodology in the development of HPs based LEDs has been reported (Figure 26).217 Among the various reported emissive-based conjugated polymers used for the development of PLEDs, conjugated HP-based emissive 58 ACS Paragon Plus Environment

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LEDs showed more advantage due to their large number of functional sites available for the stabilization of main chain wave-function as the increase in the number electron of deficient moieties results in the increase of electron affinity.218 The development of light emitting materials through AB2 methodology causes the defect in conjugated mass that can affect their luminous performance because of self-quenching.219 Therefore, Lu et al. synthesised partial-conjugated HPs using hard fluorescent conjugated moiety (2,5-dimethoxy-substituted distyrylbenzene), and non-conjugated soft spacers (trioxymethylpropane) via A2+B3 methodology that showed maximum luminous efficiency of 1.38 cd/A.220 Moreover, Guo et al. reported the development of light-emitting HPs via A2+A2+B3 where the first A2 was (2,7-bis(4,4,5,5-tetramethyl-1,3,2-dixaborolan-2-yl)29,9dioctylfluorene), the second A2 was 3,7-dibromo-dibenzothiophene-S,S-dioxide, and B3 was 1,3,5-tribromo-triphenylamine and poly(fluorene-co-dibenzothiophene-S,S-dioxide) act as the chain extender inducing branches.221 They also confirmed that on varying the molar ratio of branches, the emission properties of electroluminescence materials changed with maximum luminous efficiency of 4.5 cd/A. Sun et al. developed a novel conjugated HPs having white-light emitting segment (tris[1-phenylisoquinolinato-C2,N]iridium(III) red phosphorescent iridium complexes) (Ir(piq)3), which are covalently bonded to blue-light emitting segments (polyfluorene).220 They observed that the triplet–triplet annihilation could successfully be stopped due to the HB structure of conjugated polymers. Simultaneously, they found that the synthesized HPs exhibited comparatively better photo-physical, electrochemical, thermal properties, and enhanced fluorescence quantum yield (57–77%) and white-light emission was attained by the incorporation of 0.1 mol% Ir(piq)3 into the backbone of conjugated polymers. Later, Wu et al. reported synthesis of hybrid white-LEDs using HB hybrid copolymers (fluorescence/phosphorescence) using orange-light emitting branches 9,9dioctylfluoren and bis(1-phenyl isoquinoline)(acetylacetonato) iridium(III) (Ir(piq)2acac) as 59 ACS Paragon Plus Environment


3D-structured and the spiro[3,3]heptane-2,6-dispirofluorene as the core.222 The 3D-structured core induced powerful fluorescence and a morphological stability in electroluminescence by preventing the rotation of neighbouring acryl groups that decreases intermolecular interactions and compact packing of the chromophores in solid form. These white-LEDs was fabricated by altering the mol ratios of Ir(piq)2acac to achieve the maximum current (4.0 cd/A), maximum luminance efficiency (6777.3 cd/m2 at 18.3 V), and maximum white-light emission (on 0.04 mol % of Ir(piq)2acac). The HB morphology also enhanced the spectral, and the thermal stability along with amorphous nature of the films.223 Recently, Sun et al. have studied the effect of hyper-branching on the performance of their synthesized whiteemitting LEDs.224 They fabricated 3 types of linear and hyperbranched white-light conjugated polymers based on polyfluorene using orange-emission Ir(III) complexes. They found that the HB conjugated polymers (HCPs) could transfer the energy from the polymer backbone to the Ir(III) complexes more effective than that of linear ones which resulted in better electroluminescence performance having maximum current efficiency (1.73 cd/A) and maximum luminance (2469 cd/m2).


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Figure 26. Schematic route for the development of single polymer, star shaped, three-armed conjugated polymer system having

red emissive centre [TN, tris(4-(3-hexyl-5-(7-(4-

hexylthiophen-2-yl)-benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)phenyl)amine],

a

green

dopants [BT, benzothiadiazole], and blue emissive arms [PF, polyfluorene] with the process of energy transfer. (Reprinted with permission from ref 217. Copyright 2016, American Chemical Society, Washington, DC.) 5.9 Catalysis The special structural and properties of HPs further broadened the application area of the polymers as promising catalytic materials in chemical and biochemical reactions. The presence of large number of functional end-groups enables the introduction of catalytically 61 ACS Paragon Plus Environment


active moieties to their highly branched structure (Figure 27). Furthermore, other peculiarities of HPs like low degree of chain entanglement could enhance the catalytic activity of these peripheral groups by exposing these active sites to the reactants. In addition, the free volume of HPs would increase the transport rate of reactants and products. Recently, it was found that sulfonic acid (SA) functionalized HPs (SA-HPs) act as good solid acid-catalyst materials. For example, the selective conversion of cellulose to levulinic acid using SA functional catalysts is one of the main industrial contests,225,226 which could be solved partially by the application of functionalized HPs. In this regard, Vyver et al., prepared sulfonated hyperbranched poly(arylene oxindole)s via A2+B3 polycondensation reaction, where isatin and 1,3,5-tri-(4phenoxybenzoyl)-benzene were A2 and B3 monomers, respectively.227 These water-soluble acid catalysts HPs resulted in approximately 30 mol% levulinic acid (yields) from cellulose after 3 h reaction at 443 K. Rcently, Nabae et al. fabricated number of catalytic HPs via functionalization of various HP matrixes with different active molecules and investigated the effect of branching along with immobilization (grafting) on their activity.228 For instance, they developed a soluble SA-HPs (hyperbranched poly (ether sulfone)) as solid catalyst using AB2 monomer (4,4'-(m-phenylene-dioxy)-bis-(benzenesulfonyl chloride)), having SO3H groups as catalytic active sites at terminal positions. Their investigations revealed that the catalytic behaviour of aromatic HPs increased by increasing the SA content. However, the fabricated HPs suffered from low solution stability, which reduced their reusability and recyclability. Therefore, they grafted the fabricated HPs onto a carbon black (CB) to resolve the solubility problem.229 Later, they synthesised insoluble heterogeneous HB-based catalyst by functionalization of aromatic hyperbranched poly(ether ketone) using 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO, catalytic site), which was designed for selective (aerobic) oxidation of alcohols. They observed that the highly active terminals of HPs induced higher catalytic activity to the subsequent materials.230 On the other hand, they 62 ACS Paragon Plus Environment

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reported a carboxylic acid (CA) functionalized aromatic HP-based catalyst using symmetric AB2 monomer via one-step polycondensation reaction, which was soluble in polar solvents.231 The selective catalytic activity of the material was due to the conversion of peripheral CA groups to ethanol when treated with thionyl chloride in presence of catalyst (trimethylamine). The catalytic activity of fabricated material was determined by its ability to hydrolyse cellulose to glucose. However, these materials were mostly grafted on CB or polyimide nanoparticles. Therefore, they further investigated the effect of graphene (used for immobilization of HPs) on the catalytic behaviour of TEMPO functionalized HPs. They found that the synergetic effect of graphene and HPs improved the stability and recyclability of materials.232 Besides, they developed TEMPO functionalized hyperbranched polyimide as a heterogeneous catalyst via A2+B3 polymerization approach.233 The fabricated catalytic materials successfully oxidized benzyl alcohol to benzaldehyde aerobically in a selective manner.



Figure 27. Schematic representation of end-group functionalization of HPs with catalytically active groups and their grafting on (nano) substrates. 5.9 Flame retardant polymers In early times, the development of flame retardant materials (FRMs) was made through incorporation of halogenated additives in polymer matrix. However, the environmental concerns has changed the trend in the production of such materials to halogen-free FRMs (e.g. phosphorus–nitrogen (PN)). The facile structural modification of HPs has motivated researchers to utilize them in the development of such FRMs.234 For instance, Zhu et al. synthesized FRHPs by modification of HPUs using A2+B3 methodology.235 They have synthesized the phosphorus-based triols (B3) as branching center, 2,4-toluen diisocyanate (TDI) as A2 monomers, and hydroxyethyl acrylate (HEA) as UV-curable moieties. These UV-curable FR-HPUs showed high FR performance with high limiting oxygen index 64 ACS Paragon Plus Environment

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(LOI=27).

Later, Wang et al. have developed HB polyphosphate ester (HPPE) via A2

(bisphenol-A) + B3 (phosphoryl trichloride) polycondensation reaction.46 They observed that the thermal stability and FR properties of synthesized epoxy-based HPPE improved with an increase in LOI value from 23 to 31. Huang et al.236 reported similar UV-curable FR-HPs using A2 (piperzine) + B3 (tri(acryloyloxyethyl) phosphate) methodology. They observed that the physical, thermal, and mechanical properties of these HPs enhanced on addition of 20% B3 monomers that resulted in highest LOI of 47. Qian et al. fabricated HB-based triazine precursor via elimination reaction between cyanuric chloride, and ethylenediamine having satisfactory synergistic effect with ammonium polyphosphate in FR-polypropylene.237 Duan et al. reported PN-containing HPs via a transesterification reaction between 2-carboxyethyl (phenyl) phosphinic acid and tris (2-hydrooxyethyl) isocyanurate.238 They found that the presence of 10% of PN-HP could enhance the FR behavior by improving LOI value up to 30%. Recently, Indulekha et al. described the fabrication of HB silicone polymers (HSPs) with special molecular architecture that exhibited high thermal and inherent FR properties.239 They showed that the branching in structure of polysiloxanes results in FR polymers even in the absence of fillers or additives. They have prepared a set of terpolymers-based HPs (MeDPhDViT) having methyl, phenyl, and vinyl moieties by copolymerization of dimethyldiethoxysilane,

diphenyldimethoxysilane,

and

vinyltriethoxysilane

(DMDES,

DPDMS, and VTES), these HPs were cured in the presence of catalyst. They found that on increasing ViT content (from 13.4 to 53.3 mol %) in the prepared terpolymers, the branching increases that results in enhancement of LOI by 11% (26 to 37%). Literature shows that the incorporation of nano-fillers, such as graphene oxide (GO), in enhancement of FR properties of hyperbranched polymers has attracted considerable attention. Hu et al. reported the utilization of GO during the synthesis of FR-HP, where GO was functionalized through its carboxylic site with N-aminoethyl piperazine followed by the 65 ACS Paragon Plus Environment


reaction with di(acryloyloxyethyl)methylphosphonate forming functionalized GO (FGO).240 Consequently, the FGO was dispersed into the cross-linked hyper branched polyethylene that enhanced the FR and thermal stability of the resultant HP. They found that the dispersion of nano-fillers played an important role in enhancement of FR properties of materials, which increase the LOI of FR-HP (18.5% to 20.5%) (Figure 28).240 These investigations proved that the branching in polymers improves the performance of the HPs, which allows them to be utilized in various applications that require high performance materials.

Figure 28. Fabrication of FGO in HP architecture. (Reprinted with permission from ref 240. Copyright 2014, American Chemical Society, Washington, DC.) FUTURE PROSPECTS The current review article has illustrated that the imperfect 3D-structure of HPs have provoked the serious efforts in their applications in various biological and non-biological fields. These applications are mainly drug delivery, tissue engineering, bio-imaging, 66 ACS Paragon Plus Environment

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chemical/bio sensors, energy storage, light emitting devices, etc. Based on the current developments in HPs especially with reference to their bio applications, it seems a great deal of potential for HPs is still unexplored. The recent studies revealed that HPs are suitable materials in solving the simple and complex problems associated with biological fields, but their potential in tissue engineering, biosensors, and stimuli responsive material has not been explored to the maximum. The special structural properties of HPs like free space within their matrix and large number of end-groups can ease their application and functionalization in tissue engineering and organ repairing applications. Among the various challenges that researchers face during the processing and application of HPs, the in-vivo studies on live organisms (such as mammalians), is the most critical one. Therefore, extensive experimental work is still required to prepare suitable HPs, which show no negative impact on human cells on in-vivo application. It is also important to mention that the development of new hybrid materials using polymeric and non-polymeric moieties (nanoparticle, metals, inorganic moieties, etc.) can emerge new properties compared to that of conventional HPs. Such modifications can be made to improve the storage capacity of HP-based batteries, optical, thermal and luminescence properties of HP-LEDs. The other area that HPs are not utilized to their maximum potential is the development of inherent antimicrobial HPs. The presence of large number of end-groups in HPs, which can be functionalized with bactericidal materials, make them suitable candidates for the development of surface-active polymers. Further, these antimicrobial polymers can be fabricated as active coatings for medical devices, tiles, swimming pools, pipelines, marine industry, microbial inducing corrosion protection, etc. Moreover, these bactericidal HPs can find wide scope of application in other industrial areas, for instance, food and pharmaceutical industries as active packaging materials, because they possess almost all the



criteria/properties required for packaging materials. They can restrict the growth of bacteria on food and medicinal materials, which subsequently increases material’s shelf life. In addition, the utilization of HPs as catalysis, which is a new area, requires more investigations due to the limited number of published reports. The developed HP-based catalysts mainly suffer from less stability and recyclability. We believe that one of the effective method to overcome these limitations may be the development of new HPs containing multifunctional hybrid materials (inorganic-organic) as branching moieties due to their higher stability in solution form. However, the development of these materials will be a challenge as the selection of such branching agents depends on many factors like area of application, extend of solubility, the possible interactions with other constituent of HPs, and their stability in solution form. Furthermore, the fabrication of SM-based HPs are also associated to certain drawbacks, which minimize their applications in broad spectrum. According to our observations these limitations are the long relaxation time (time required to return to their original shape), limited synthesized materials, and the methodologies. However, we believe that these problems can be solved (to some extent) by development of new methodologies or modification of conventional techniques. These modifications could be achieved by designing of branching centers with nanoparticles to improve their mechanical strength and the development of highly cross-linked network possessing suitable soft and rigid segments. The rigid segments can be obtained by tailoring the nano-materials, which further increase their compatibility with the soft segments of the matrix. These materials further can find wide applications in both biological (ex: artificial organs, tissues, muscles, etc.) and nonbiological (ex: data storage devices, gas absorbing membranes) areas.


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7. SUMMARY The emergence of a new polymer architecture followed by the linear, branched and crosslinking in HPs has attracted a considerable attention of researchers in recent years. The current review article has discussed various aspects of these polymers, such as, their synthesis methodology, properties, and the structural modification that resulted in various applications. According to the well-known fact which says that any particle in its nano-size exhibits superior properties than that of its original size, here in same manner we can conclude that almost all polymers show better properties in their hyperbranched form. These properties are mostly rheological, crystallinity, melting behaviour, glass transition, thermal, electronic, encapsulation, self-assembly behaviour and biomedical applications that are highly related to branching in these polymers. However the problems associated with controlling the DB, and its relationship with their structural-based properties are still limited. In this review some miscellaneous properties like electric, optical and magnetic have also been discussed. Further, suggestions have been made to reduce the DB fraction of HPs. In the current research scenario, a great deal of research work yet to be explored on HPs, pertaining to the field of bio-applications such as gene and protein delivery as well as tissue engineering. However, applications such as dental resin, modification of surfaces to enhance biocompatibility, and biodegradability of such HPs needed further investigations. Among these potential applications, an imperfect structure of HPs in comparison with dendrimers, made their structural tailoring easier. The broad polydispersity of HPs is likely complicate their characterization as well as their evaluation for biomedical applications. However, for practical purposes, the broadening in DB led to various modifications in the properties of HPs. On the other hand, for several types of HPs such as Boltorn and polyglycerol, a welldefined structure can be obtained with reasonable polydispersity. Thus, it is expected that



these polymers could play even more important role in various industrial and biological fields especially in biomedical sciences than imagined before. ACKNOWLEDGMENTS One of the authors, Shahidul Islam Bhat, is thankful to UGC India for financial support.


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