Effect of Macromonomer Branching on Structural Features and

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Effect of Macromonomer Branching on Structural Features and Solution Properties of Long-Subchain Hyperbranched Polymers: The Case of Four-Arm Star Macromonomers Nairong Hao,†,‡,∥ Ahmad Umair,‡,∥ Mo Zhu,‡ Xiaozheng Duan,§ and Lianwei Li*,†,‡

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†

Food Science and Processing Research Center, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, China 518060 ‡ Department of Chemical Physics, University of Science and Technology of China, Hefei, China 230026 § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China 130022 S Supporting Information *

ABSTRACT: Following our recent work on AB3 macromonomer systems (Macromolecules, 2019, 52, 1065−1082), this work focuses on AB4 systems to further explore the effect of macromonomer branching on structural features and solution properties of longsubchain hyperbranched polymers (LHPs). Compared with ABn macromonomers (n < 3), our results reveal that AB4 macromonomers tend to undergo less interchain coupling but more intrachain self-cyclization during “click” polyaddition process. Via the triple-detection SEC characterization of resultant LHPs, both intrinsic viscosity ([η]) and radius of gyration (Rg) are found to be scaled to the macromonomer molar mass (Mmacro) and the total molar mass (Mhyper) as [η] = K η ,AB4Mhyper νM macro μ (ν ≈ 0.40, μ ≈ 0.25, and Kη, AB4 ≈ 1.72 × 10−2 mL/g) and R g = HR ,AB4Mhyper αM macro β (α ≈ 0.52, β ≈ 0.20, and HR, AB4 ≈ 1.54 × 10−3 nm). The obtained results clearly demonstrate that (i) the self-similar behavior of LHPs is almost independent of the detailed structures of the macromonomer (AB2, AB3, and AB4) and (ii) the AB4 system presents Mmacro-dependent solution properties (μ ≈ 0.25 and β ≈ 0.20), which is similar to the result for the AB2 system but totally different from the observed Mmacro independence for the AB3 system (μ ≈ 0 ≈ β). A combination of experimental observation and Langevin dynamics simulation of ABn dendrimers and LHPs unambiguously verifies our previously proposed conjecture; that is, the unique synergistic effect of segment back-folding and segment interpenetration exists only in moderately branched LHP systems but not in highly branched ABn LHP systems (n > 4).

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INTRODUCTION Hyperbranched polymers (HPs) have received much attention in the past two decades due to their unique properties such as high solubility, high number of terminal functional groups, and low intrinsic viscosity. At present, all reported HPs could be classified into two categories, that is, short-subchain hyperbranched polymers (SHPs) that contain single or several repeating units between two branching points, and longsubchain hyperbranched polymers (LHPs), which have long linear segments dispersed between branching points. Benefiting from long subchains, LHPs exhibit unique properties such as a weaker steric hindrance effect, diverse terminal group functionalization, better mechanical properties, and unique phase behavior. Therefore, LHPs have been receiving increasing attention over recent years. To date, the controlled synthesis of LHPs with predesigned branching parameters has remained significantly challenging. Among various synthetic approaches, polymerization using ABn (n > 2) macromonomers has been proven to be one of the most effective strategies to control the structural parameters, such as the average length of branching subchains (Ls, the © XXXX American Chemical Society

average segment length between two branching points), and the subchain length distribution (DL).1−7 Originally, the classic ABn monomer strategy (n > 2) could be traced back to Flory’s pioneering work on the discussion of the ABn monomer strategy for the preparation of SHPs.8 So far, a variety of SHPs has been synthesized via this strategy; for example, AB2,9−11 AB3,12−20 AB4,21−24 and even AB621 monomers have been previously reported for the exploration of the structure− property correlation for SHPs. In contrast, although there are numerous studies reporting the preparation of LHPs by the AB2 macromonomer strategy,1−7,25−41 including our contributions,42−49 little attention was paid to the study of LHPs prepared by ABn macromonomers (n > 3). Very recently, our group reported the study regarding the structure−property relationship of LHPs prepared by threearm AB3 macromonomers.50 The results revealed the unique synergistic effect of segment back-folding and segment Received: May 30, 2019 Revised: August 2, 2019

A

DOI: 10.1021/acs.macromol.9b01103 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

is exactly composed of one-fourth of a macromonomer (one arm, Ls = Larm), accordingly, leading to a narrower subchain length distribution and a more uniform segment density distribution. In this work, we extend the study to AB4 LHPs prepared using four-arm star macromonomers. Experimental characterization and computational simulation were employed to confer insight into the macromonomer branching effect in LHPs. Importantly, the obtained results unambiguously verified our previously proposed conjecture; that is, the unique synergistic effect of segment back-folding and segment interpenetration exists only in moderately branched LHP system.

interpenetration on the structural features and solution properties of AB3 LHP systems. In the original paper, we mentioned that although the increase in degree of branching of macromonomers could promote the segment back-folding, it might simultaneously lead to a significant decrease in the trend of segment interpenetration. Consequently, the segment interpenetration might be completely prohibited for highly branched ABn LHP systems with a much stronger macromonomer branching effect. Moreover, we previously hypothesized that such a kind of synergistic effect might only exist for ABn LHP systems with n ranging from 3−5. In this work, we aim to further test our conjecture by constructing and characterizing the AB4 LHP system. Similar to the AB3 system, there exist three possible topological structures for the AB4 macromonomer system, that is, linear, five-arm and four-arm star structures (Scheme 1a−c). For the linear structure (Scheme 1a), the four B groups

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EXPERIMENTAL SECTION

Materials. Unless stated otherwise, all chemicals were obtained from commercial suppliers and were used as received. Dimethylformamide (DMF, Sinopharm, AR) was dried with anhydrous magnesium sulfate and then distilled under reduced pressure prior to use. Chloroform, acetone, dichloromethane (DCM), and tetrahydrofuran (THF) from Sinopharm were distilled over CaH2 just prior to use. Styrene (Sinopharm, 97%) was passed through a basic alumina column to remove inhibitors, distilled under vacuum, and stored at −20 °C. Copper(I) bromide (CuBr, Alfa, 98%) was washed with glacial acetic acid to remove soluble oxidized species, filtered, washed with diethyl ether, and dried under vacuum. 18crown-6 (Aldrich, 98%), N,N,N′,N′,N″-pentamethyl-diethylenetriamine (PMDETA, Aldrich, 99%), lithium aluminum hydride (LiAlH4, Aldrich, 99%), phosphorus tribromide (PBr3, Aldrich, 99%), propargyl bromide (Aldrich, 98%), dimethyl 5-hydroxyisophthalate (DMHIP, Aldrich, 99%), 4,4′-dinonyl-2,2′-bipyridine (dNbpy, Aldrich, 98%), sodium azide (NaN3, Aldrich, 99%), and methanol (Sinopharm, 99.8%) were used as received. 1,3-dibromomethyl-5propargyloxy-benzene (DBMPB) was prepared according to our previously published procedure.42 Analytical Methods. NMR spectra were recorded at 300 K on a Bruker Avance III Ascend 500 (500 MHz) spectrometer with the delay time (d1) set to 8 s by using deuterated chloroform (CDCl3) as the solvent and tetramethylsilane (TMS) as the internal standard. FTIR spectra were recorded on a Bruker Tensor 27 Fourier transform spectrometer. Size Exclusion Chromatography. The molar mass distributions of macromonomer and hyperbranched polystyrene samples were characterized using a standard size exclusion chromatography (SEC) system in our own lab47 where the relative number- and weightaverage molar mass (Mn and Mw) were obtained based on a conventional polystyrene calibration method (six polystyrene standards ranging from 7.65 × 102 to 2.21 × 106 g/mol). The absolute Mn and Mw were obtained from the measurements of a triple-detection size exclusion chromatography (TD-SEC) system. The TD-SEC system is equipped with a refractive index detector (RI), a multi-angle light scattering detector (MALS), and a viscosity detector. The instrumentation consists of a Waters 1515 isocratic HPLC pump with 5 mm Waters Styragel columns (the efficient exclusion limit of the column system is 5 × 102 ≈ 1 × 107 g/mol), a Waters 717 PLUS autosampler, a Waters 2414 RI detector, a MALS detector (Wyatt mini-DAWN HELEOS-II) with a scattering volume of 0.07 μL, an 18angle light scattering detector at a wavelength of 690 nm and 220 W power, a Wyatt Visco Star viscometer detector, and a Waters Breeze data manager. The eluent was HPLC-grade THF with a flow rate of 1.0 mL/min. Prior to injection, the sample solutions were filtered through PTFE membranes (0.45 μm pore size). The TD-SEC system was carefully calibrated with two polystyrene standards: (1) a polystyrene standard with peak molar mass (Mp) ≈ 3.01 × 104 g/mol was used to calibrate the detector normalization coefficient; and (2) toluene solvent and a polystyrene standard with Mp ≈ 2.21 × 106 g/ mol were used to calibrate the voltage calibration constant. For quantitative study, 50 μL of polymer solution was injected, and the polymer concentration was fixed at 15 mg/mL for all samples.42−47

Scheme 1. Schematic Illustration of the Topological Structures of Long-Subchain Hyperbranhced Polymers (LHPs)a

a (a) Linear AB4 macromonomer, (b) five-arm star AB4 macromonomer, and (c) four-arm star AB4 macromonomer where the highlighted regions with different colors represent branching subchains with different lengths (Ls).

are located at the same chain end (correlated), that is, A∼∼∼∼B4; for the five-arm star structure (Scheme 1b), the reactive A group is located at one arm end, and the four B groups are located at the other four-arm ends (non-correlated), that is, (A∼∼)(∼∼B)4; while for the four-arm star structure (Scheme 1c), the A group is located in the star center, but the four B groups are located at different arm ends (noncorrelated), that is, A(∼∼B)4. In principle, linear AB4 and five-arm star macromonomers (Scheme 1a,b) can only realize partial control over parameters Ls and DL. Namely, for the linear AB4 macromonomer, the unreacted B groups can easily lead to the extension of the internal subchain by one macromonomer length (Lm) repeatedly, resulting in the variability of the subchain length (Ls = kLm, k > 1). Moreover, for the linear AB4 structure, the spatially strong correlation among four B groups inevitably results in unequal reactivity for the four B groups, further leading to the non-ideal branching patterns for AB4 LHPs. In contrast, the use of a four-arm star macromonomer can guarantee that each branching subchain of resultant AB4 LHPs B


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Macromolecules Considering the large size (>50 nm) and high molar mass (>106 g/ mol) of polymer samples in this study, the observation length “1/q” might be much smaller than Rg where q is the scattering vector and Rg is the radius of gyration. Thus, the Berry plot was used for the extraction of molar mass and size information for all samples during the analysis of light scattering data. To perform calculations with the Berry method, which is a fit to [KC/R(θ)]1/2 versus q2 according to the following equation KC = Rθ

1 (1 + ⟨R g ⟩2 q2 /6) Mw 2

C), 4.73 (2H, CHCCH2O), 5.15 (4H, CCH2O−), 3.94 (12H, COOCH3−), 7.06 (2H, CH2OCCH), 7.16 (1H, CH2OCCHCCH−), 7.84 (4H, CH2OCCH−), 8.34 (2H, CHCCOO−). To a stirred solution of LiAlH4 (1.38 g, 36.3 mmol) in dry THF (50 mL) at 0 °C was dropwise added the intermediate compound I (4 g, 7.8 mmol) in THF (50 mL) under nitrogen. The reaction mixture was refluxed overnight. After quenching with water, the reaction mixture was filtered, dried with anhydrous Na2SO4, and concentrated to dryness under vacuum. Intermediate compound II was obtained as a yellow powder (yield ≈ 3.1 g, 86%). The crude intermediate compound II (2.5 g, 5.3 mmol) was added in dry CHCl3 (80 mL), and then PBr3 (10.5 g, 38.7 mmol) in dry CHCl3 (20 mL) was added dropwise under nitrogen. After refluxing overnight, the mixture was extracted with CH2Cl2 (200 mL) and saturated aq. NaHCO3 (50 mL). The aqueous phase was again extracted with CH2Cl2 (100 mL), and the combined organic phase was dried with anhydrous Na2SO4. The solution was concentrated, and the crude product was purified by column chromatography over a silica gel eluting with hexane/chloroform (2/3, v/v) to give a white powder (yield ≈ 3.2 g, 83%). 1H-NMR (CDCl3, ppm): δ 2.54 (1H, CHC), 4.73 (2H, CHCCH2O), 5.15 (4H, CCH2O−), 4.43 (8H, COOCH3−), 6.94 (4H, CH2OCCH−), 7.02 (4H, CH2OCCH− & CHCCH2Br), 7.13 (1H, CH2OCCHCCH−). Preparation of Four-Arm Star AB4 Polystyrene Macromonomers. Into a 100 mL dry glass tube with a magnetic stirring bar, initiator (400 mg, 0.55 mmol), 4,4′-dinonyl-2,2′-bipyridyl (dNbpy, 1.8 g, 44 mmol), THF (11.36 g), and styrene (23.3 g, 0.22 mol) were added successively. After mixing thoroughly, the polymerization tube was degassed by two freeze−vacuum−thaw cycles, and CuBr (328 mg, 2.2 mmol) was added into the frozen solution. The polymerization tube was degassed by one more freeze− vacuum−thaw cycle before being sealed under vacuum. The sealed tube was immersed in an oil bath at T = 110 °C. After the polymerization was carried out for 0.5 h, the tube was rapidly stopped in liquid nitrogen. The polymer solution was then diluted with THF and passed through a short column of neutral alumina to remove metal salts. After precipitation two times by the addition of polymer solution into methanol, the star AB4 polystyrene macromonomer was obtained after being dried under vacuum at T = 45 °C overnight (yield ≈ 2.4 g, 70%, PSS, Mn = 6100 g/mol). 1H-NMR (CDCl3, ppm): δ 2.40 (1H, CHC), 4.63 (2H, CHCCH2O), 4.81 (4H, CCH2O−), 2.10 (8H, Ar−CH2−CH2−), 1.45 (122H, CH2− CH−Ar), 1.87 (61H, CH2−CH−Ar), 4.50 (4H, CH2−CH−Br), 6.80 (305H, Ar). Following the similar protocol, three star AB4 polystyrenes with different molar masses, that is, 6100 g/mol (PSS), 10,800 g/mol (PSM), and 22300 g/mol (PSL), were synthesized. Azidation Substitution Reaction. A 25 mL round-bottom flask was charged with PSS (2.0 g, 0.32 mmol), DMF (20.0 mL), and NaN3 (412 mg, 6.3 mmol). The mixture was allowed to stir under nitrogen at room temperature for 24 h. The mixture was diluted with CH2Cl2 (1.6 mL), and the insoluble inorganic salt was removed by filtration. The filtrate was precipitated into an excess of cold methanol. The sediments were redissolved in CH2Cl2, passed through a neutral alumina column to remove residual sodium salts, and then precipitated into an excess of methanol. After being dried in a vacuum oven overnight at 45 °C, the macromonomer PS S functionalized with one alkyne group and four azide groups was obtained (yield ≈ 1.5 g, 75%). 1H-NMR (CDCl3, ppm): δ 2.40 (1H, CHC), 4.63 (2H, CHCCH2O), 4.81 (4H, CCH2O−), 2.10 (8H, Ar−CH2−CH2−), 1.45 (122H, CH2−CH−Ar), 1.87 (61H, CH2−CH−Ar), 3.93 (4H, CH2−CH−N3), 6.80 (305H, Ar). Following the similar protocol, three star AB4 polystyrene macromonomers with different molar masses, that is, PSS, PSM, and PSL, were synthesized. Preparation of Hyperbranched Samples from AB4 Macromonomers Via Azide−Alkyne Cycloaddition Reaction. Compared with our previous protocol,42 a slightly modified procedure was employed. Into a 5 mL dry glass tube with a magnetic stirring bar, PSS (1.0 g, 0.16 mmol), PMDETA (265 μL, 1.27 mmol), and DMF (1.4

(1)

/(NAλ04)

2

and q = (4π/λ0)sin(θ/2) with C, where K = 4π (dn/dC) dn/dC, NA, and λ0 being the concentration of the polymer solution, the specific refractive index increment, Avogadro’s number, and the wavelength of light in vacuum, respectively. In this work, the dn/dC values were determined to be 0.177−0.183 mL/g for hyperbranched samples in THF. Preparative Size Exclusion Chromatography. The LC98II high performance liquid chromatograph (Beijing Wenfen Analytical Instrument) consisted of a P98II isocratic HPLC pump with three Waters Styragel columns at 35 °C (guard, HR6, HR5, and HR3; the efficient exclusion limits are 2 × 105−1 × 107, 5 × 104−4 × 106, and 5 × 102−3 × 104 g/mol, respectively), a Z98 autosampler, a Shodex RI201H detector, a UV98II variable wavelength UV−visible spectrophotometer (wavelength range: 190 to 700 nm), and a UC-3265 component collector. The eluent was HPLC-grade THF with a flow rate of 1.0 mL/min. Dynamic and Static Light Scattering. A commercial LLS spectrometer (ALV/DLS/SLS-5022F) equipped with a multi-τ digital time correlator (ALV5000) and a cylindrical 22 mW UNIPHASE He−Ne laser (λ0 = 632.8 nm) as the light source was used. In static light scattering, the angular dependence of the absolute excess timeaverage scattering intensity, known as the Rayleigh ratio R(θ), can lead to the weight-average molar mass (Mw), the root-mean-square gyration radius ⟨Rg2⟩1/2 (or simply written as ⟨Rg⟩), and the second virial coefficient A2 by using KC 1 = (1 + ⟨R g⟩2 q2 /3) + 2A 2 C R(θ) Mw

(2)

The extrapolation of R(θ) to q → 0 and C → 0 leads to Mw. The plots of [KC/R(θ)]C→0 versus q2 and [KC/R(θ)]q→0 versus C lead to ⟨Rg⟩ and A2, respectively. For large hyperbranched polymers, qRg ≫ 1, so the Berry plot is used. In static LLS, the scattering intensity was recorded at each angle three times, and each time was averaged over 10 s. In dynamic light scattering (DLS),51 the Laplace inversion of each measured intensity−intensity time correlation function G(2)(q,t) in the self-beating mode can lead to a line-width distribution G(Γ) where q is the scattering vector. For dilute solutions, Γ is related to the translational diffusion coefficient D by (Γ/q2)q→0,C→0 → D, so that G(Γ) can be converted into a transitional diffusion coefficient distribution G(D) or further to a hydrodynamic radius distribution f(Rh) via the Stokes−Einstein equation, Rh = (kBT/6πη0)/D, where kB, T, and η0 are the Boltzmann constant, the absolute temperature, and the solvent viscosity, respectively. The polydispersity index Mw/ Mn was estimated from Mw/Mn ≈ (1 + 4μ2/⟨D⟩2) where μ2 = 2 ∫∞ 0 G(D)(D − ⟨D⟩) dD. In DLS experiments, we used a fixed small angle (13°) to ensure that the effect of extrapolating to the zero angle is minimum. The time correlation functions were analyzed using both the cumulants and CONTIN analysis. Synthesis of Pent-Functional AB4 ATRP Initiator. To a stirred solution of DBMPB (3.2 g, 10 mmol) and DMHIP (4.7 g, 22.3 mmol) in acetone (35 mL) were added potassium carbonate (3.1 g, 22.4 mmol) and 18-crown-6 (64 mg, 0.24 mmol). The reaction mixture was refluxed under nitrogen for 48 h. After filtration, the mixture was concentrated, and the crude product was purified by column chromatography with silica gel eluting with DCM/MeOH (100/3, v/v) to give a white powder of intermediate compound I (yield ≈ 4.6 g, 89%). 1H-NMR (CDCl3, ppm): δ 2.54 (1H, CH C


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Macromolecules

Scheme 2. Schematic Synthetic Route of Four-Arm Star AB4 Polystyrene Macromonomer and Long-Subchain Hyperbranched Polystyrene (HPS)

ÄÅ É l ÅÅi y12 i y6ÑÑÑ o o ÅÅjj σ zz ÑÑ o j z σ o o o 4εLJÅÅÅÅjjj zzz − jjjj zzzz ÑÑÑÑ + εLJ , rij ≤ rc o o WCA j z j z Å Ñ r r Uij (rij) = m ÅÅk ij { o k ij { ÑÑÑÖ ÅÇ o o o o o o 0, r > rc o n ij

mL) were added successively. After mixing thoroughly, the polymerization tube was degassed by two freeze−vacuum−thaw cycles, and CuBr (90 mg, 0.61 mmol) was added into the frozen solution. The polymerization tube was degassed by one more freeze−vacuum−thaw cycle before being sealed under vacuum. The sealed tube was immersed in an oil bath at 35 °C for 24 h. The mixture was then diluted with THF and quickly passed through a neutral alumina column. CuBr and PMDETA were further removed from resultant hyperbranched polystyrene by the repeating precipitation with THF and methanol. Finally, the pure precipitate of hyperbranched polystyrene HPSS was collected by filtration and then dried under vacuum overnight at 45 °C (yield ≈ 0.8 g, 80%). Following the similar protocol, three HPS samples were synthesized. Hydrolysis of Four-Arm Star AB4 Polystyrenes. PSS (5.0 mg, 0.82 μmol) was first dissolved in DCM (0.6 mL), and then a solution of BBr3 (25% in DCM, 0.4 mL) was added. The solution was sealed in a glass vial under nitrogen and stirred vigorously at room temperature. After 0.5 h, aqueous Na2CO3 (saturated solution) was added followed by the dilution with THF. The combined organic phase was extracted, dried over Na2SO4, and used for SEC measurement. Results showed that the degree of hydrolysis reached 100% in a few minutes, unlike our previous observation for the case of AB3 macromonomers. Therefore, no significant kinetic data could be collected to present here. Fractionation of HPSM by Preparative SEC Technique. The HPSM sample was dissolved in THF at room temperature with a concentration of ∼10.0 g/L, and then the polymer solution was filtered into a glass vial through a PTFE filter (0.22 μm pore size). The injection into the SEC system was accomplished using an autosampler where 0.1 mL of solution was injected into the SEC system at a time interval of 40 min. The SEC fractionation was repeated 50−150 times for a given sample, and 10−20 fractions were typically collected. Finally, 2.0−10 mg of the sample was obtained for each polymer fraction after concentration. Langevin Dynamics Simulation. The simulation details are the same as the published procedures in our previous work. Briefly, the chemical heterogeneity was ignored for simplicity, and the beadspring model was employed to coarse-grain the hyperbranched polymers. We modeled each macromonomer as a connection of several spherical segments and used the Weeks−Chandler−Andersen (WCA) potential to account for the repulsive excluded volume interactions between segments i and j separated with a distance rij

(3)

where εLJ and σ denote the energy and length parameters, respectively. We set the cutoff rc to be 1.12246σ. In addition, we adopted the finite extensible nonlinear elastic (FENE) potential to account for the chain connection between consecutive segments É ÄÅ 2Ñ ÅÅ ij rb yz ÑÑÑÑ 1 2 Å Å j z UFENE(rb) = − kR 0 lnÅÅÅ1 − jj zz ÑÑÑ j R 0 z ÑÑ ÅÅ 2 k { ÖÑÑ (4) ÇÅÅ where rb denotes the distance between connected segments. The Langevin equation for the simulation can be cast into mi

d2rik dt 2

= − ζi

d2rik dt 2

− ∇k Ui + fik

(5)

where i denotes the ith segment in the simulation, k is the component of the position vector, t is the time, and mi and ζi represent the mass and the friction coefficient of the ith segments, respectively. Ui and f i are the potential and the random force acting on the ith segments where f i satisfies the fluctuation−dissipation theorem with its magnitude given by kBTζi/dt . The details of construction of simulation models for ABn LHPs (n = 2, 3, and 4) with random conformations could be found in our previous work.50 The simulation was also performed for symmetric dendrimer models composed of ABn macromonomers (n = 2, 3, and 4) to better understand the segment back-folding property. The molecular information including the interpenetration factor (Ψ) and radial density functions [ρ(r)] was analyzed. In the calculation of Ψ, the second virial coefficient A2, which describes the interactions between pairs of molecules, can be obtained by52,53 ∞ ÄÅ ÉÑ Å i U (R ) y ÑÑ 2πNA zz ÅÅ jj Ñ 2Å zz − 1ÑÑÑdR A2 = − 2 R ÅÅexpjj− j z Å ÑÑ M ÅÅÇ k kBT { ÑÖ (6) 0

∫

where U(R) represents the intermolecular interaction between chains whose centers of mass are separated by a distance R. For each pair of generated chain conformations, chain 2 was put on a certain position with a distance of R and at the random angles θ and φ from the center of mass of chain 1 in the polar coordinates. The intermolecular interactions between the two polymers can be obtained through D


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Macromolecules M

U (R , θ , ϕ) =

M

four-arm star AB4 macromonomers with different average molar masses (Mn) were obtained and denoted as PSS, PSM, and PSL according to the relative size of the chain length where the subscripts “S”, “M”, and “L” are the abbreviations for “short”, “middle”, and “long”, respectively. The above-obtained molecular parameters for four-arm star macromonomers are apparent values if we consider their branching feature. Thus, we attempted to “cut” the star chains into individual linear arms to analyze the real molecular information. The cleavage of ether linkage in star macromonomers was carried out in DCM by using BBr3 as the reagent, and the reaction was found to be completely achieved within a few minutes. After treatment with saturated Na2CO3 solution, the hydrolyzed product was obtained. Scheme S1 shows the hydrolytic process, and the SEC characterization results of fully hydrolyzed products are summarized in Table 1.

∑ ∑ UijWCA(rij) i , m1 j , m2

(7)

where the summation terms correspond to values of the WCA potential for intermolecular interactions between segments i and j in chain 1 and chain 2. We performed a large amount of calculations for U with different random angles to fully explore the different orientations for every particular pair of conformations. We defined the LJ parameters εLJ and σ and the monomer mass m as the LJ units and fixed the size of the simulation box to be L = 100 LJ units. To avoid the bond crossing, the parameters for the FENE potential were taken as k = 30 LJ units and R0 = 1.5 LJ units. We set the temperature to be 1 LJ unit. The damp factor (m/ζ) was chosen as 100 LJ units, and the time step of the simulation was set to be t = 0.005 LJ units. Our simulations were performed with the large-scale atomic/molecular massively parallel simulator (LAMMPS).54 The results were obtained by averaging 103 to 104 statistically independent samples after the system equilibration.

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Table 1. SEC Characterization of the Fully Hydrolyzed Products of Four-Arm Star Polystyrene Macromonomers

RESULTS AND DISCUSSION Preparation and Characterization of Four-Arm Star AB4 Macromonomers. Scheme 2 shows our strategy for the synthesis of star AB4 polystyrene macromonomers A(∼∼∼B)4 and the corresponding AB4 long-subchain hyperbranched polystyrenes (LHPs). The pent-functional ATRP initiator with one alkyne group and four benzyl bromide groups was first prepared. The 1H-NMR characterization confirms the chemical structures of the key intermediate compound (tetraester) and the target molecule (Figure 1 and Figure S1). The

sample

Mn (g/mol)a

Mp (g/mol)a

Mw (g/mol)a

Mw/Mna

PSS,2‑arm PSM,2‑arm PSL,2‑arm

3300 5600 12500

3400 6000 15100

3700 6200 15500

1.12 1.10 1.16

a

Data were determined based on the conventional linear polystyrene calibration method.

Further, we calculated the peak molar mass (Mp,cal) for star AB4 macromonomers based on the following equation: Mp,cal = 2Mp,2‑arm + Minitiator where 2Mp,2‑arm represents the determined peak molar mass for individual two-arm linear chain and Minitiator represents the molar mass for the initiator (Minitiator ≈ 700 g/mol). The branching feature of the four-arm star AB4 macromonomers can be reflected in the calculation results summarized in Table 2. Namely, the ratios Mn,cal/Mn,app, Mp,cal/ Mp,app, and Mw,cal/Mw,app are estimated to be 1.10−1.19, 1.13− 1.18, and 1.09−1.17, respectively, indicating that the previously determined molar masses based on the conventional calibration method are significantly underestimated. It is well known that the radius of gyration for n-arm star and linear chains in good solvents can be correspondingly expressed as Rg,star = b[(3n − 2)/n2]1/5Nstar3/5 and Rg,linear = bNlinear3/5,55 where Nstar and Nlinear represent the degree of polymerization for star and linear polymers, respectively, and b represents the Kuhn monomer size. Thus, for a given chain size (Rg,star = Rg,linear), the MABn/Mlinear ratio can be theoretically derived to be ÄÅ É ÅÅ n2 ÑÑÑ1/3 Å ÑÑ Å Mstar /Mlinear = Nstar /Nlinear = ÅÅ Ñ ÅÅÇ 3n − 2 ÑÑÑÖ (8) Clearly, Mstar/Mlinear ≈ 1.09 and 1.17 for n = 3 and 4, respectively.55 Figure 3 shows that the Mp,cal/Mp,app values (1.13−1.18) for the AB4 system are significantly higher than those (1.09−1.11) for the AB3 system reported in our previous study, which also satisfactorily agrees well with the calculated results from eq 8. Considering that Mstar/Mlinear is equal to Mcal/Mapp, the theoretical value for Mcal/Mapp should also be ∼1.17 for four-arm star polymers in good solvents, which is pretty close to the experimentally determined Mp,cal/Mp,app ratios for four-arm polystyrenes (1.13−1.18 in Table 2). The above comparison unambiguously confirms the branching feature of resultant four-arm star macromonomers. Here, we

Figure 1. 1H-NMR spectra of the tetra-ester-functionalized intermediate compound and pent-functional AB4 initiator.

well-known atom transfer radical polymerization (ATRP) was adopted for the polymerization of styrene, and the monomer conversion was controlled below 60% to maintain high chainend functionality. As shown in Figure 2a, all the prepared AB4 polystyrenes show narrow molar mass distributions with polydispersity indexes (Mw/Mn) between 1.11 and 1.19, indicating a good control over polymerization process. Three

Figure 2. SEC curves of (a) star AB4 polystyrene macromonomers (PSS, PSM, and PSL) and (b) corresponding arm chains (PSS,2‑arm, PSM,2‑arm, and PSL,2‑arm). E


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Macromolecules Table 2. Apparent and Calculated Molecular Parameters of Four-Arm Star Polystyrenes sample

Mn,appa (g/mol)

Mp,appa (g/mol)

Mw,appa (g/mol)

Mw/Mna

Mn,calb (g/mol)

Mp,calb (g/mol)

Mw,calb (g/mol)

Mn,cal/Mn,app

Mp,cal/Mp,app

Mw,cal/Mw,app

PSS PSM PSL

6100 10800 22300

6300 11200 26100

6900 12000 28000

1.13 1.11 1.19

7300 11900 25700

7500 12700 30900

8100 13100 31700

1.19 1.10 1.15

1.15 1.13 1.18

1.17 1.09 1.13

a

Data were determined based on the conventional linear polystyrene calibration method. bData were calculated based on the following equations Mn,cal = 2Mn,2‑arm + Minitiator and Mp,cal = 2Mp,2‑arm + Minitiator where Mn,2‑arm and Mp,2‑arm represent the determined number-average molar mass and peak molar mass of individual two-arm linear chain and Minitiator represents the molar mass of the initiator (Minitiator ≈ 700 g/mol).

work, we believe that the discussion based on the Rg−M relation could give at least semiquantitative comparison to reveal the branching nature of macromonomers. In addition, the predicted values are consistent with the experimental values, which also indicates that the calculation method is reliable. Finally, the azide-functionalized macromonomers were obtained via an efficient bromine/azide substitution reaction, which is confirmed by the 1H-NMR and FTIR characterization (Figures S2 and S3). It can be seen that the signal for protons located at the arm ends of bromine-functionalized star polymers at 4.50 ppm (CHBr, Hg in Figure S2) vanishes, and a new signal at 3.93 ppm (CHN3, Hg in Figure S2) appears for azide-functionalized star polymers, which indicates that the azidation reaction was complete. In order to determine the chain-end functionality, PSs was chosen as a model sample as the peak intensity for this sample was high enough in NMR measurement due to lower molar mass, and the calculation for area was precise. The average azide chain-end functionality was found to be 95%. The chain-end functionality was determined by the ratio for the area corresponding to the peaks Hb, Hc, and Hg in Figure S2 by using the equation f = 100[2.5Hg/(Hb + Hc + Hg)], where “f ” is the functionality (%). The successful installation of azide groups is also reflected in the appearance of a NNN antisymmetric stretching absorption band near ∼2090 cm−1 in FT-IR spectra (Figure S3). In the following discussion, we prefer to use the peak molar mass (Mp,cal) as the feature parameter to represent the molar

Figure 3. Macromonomer molar mass (Mmacro) dependence of Mp,cal/ Mp,app for AB3 and AB4 macromonomers where Mmacro = Mp,cal is used as the x variable.

use Rg to discuss results obtained from SEC separation (not Rh) for the following reasons: (i) As far as we know, the Rg−M scaling relation for star-like chains with different arm numbers has been extensively studied experimentally and theoretically, and related results have been reported and could be easily found, while for the Rh−M scaling relation, it has not been seriously studied theoretically! (ii) Rh might be a more appropriate parameter. In the polymer community, Rh is widely accepted as one of the most reliable chain parameters for the universal calibration of SEC results. However, according to the work of Sun et al.,56 Rg is also shown to satisfactorily correlate with the partitioning of linear polymers or polymers with low branching densities. Considering the nature of low branching density for the macromonomers (four-arm stars) used in this

Figure 4. SEC curves for macromonomers and corresponding hyperbranched polystyrenes prepared at different macromonomer concentrations: (a) HPSS, (b) HPSM, and (c) HPSL where the reaction was conducted at T = 35 °C. (d) Macromonomer concentration dependence of the ratios (Ac/At) for the AB4 system. The data was recorded using an RI detector. F


Article

Macromolecules mass information of macromonomers (Mmacro), that is, Mmacro = Mp,cal, as the peak information is generally more accurate and less affected by the side products in the low or high elution time in SEC characterization where some polymer impurities such as the dead chains and coupled dimers might exist. Overall, the result demonstrates the necessity of careful characterization of star macromonomers before further investigation on the structure−property relation of LHPs. Effect of Macromonomer Branching on Interchain Coupling and Intrachain Cyclization. Similar to AB2 and AB3 LHP systems in our previous work,41,50 the azide/alkyne click chemistry was utilized to prepare hyperbranched polystyrenes. The addition of Sn(EH)2 was to suppress the potential Glaser coupling between alkyne groups.57 A combination of 1H NMR and FTIR characterization confirms that hyperbranched samples are successfully synthesized (Figures S2 and S3). Figure 4a−c shows the SEC curves of resultant hyperbranched polystyrenes (HPSS,M,L) prepared with different macromonomer concentrations at T = 35 °C. As shown, the elution peaks for these AB4 LHPs significantly shift to a higher molar mass region after the click coupling reaction, signifying the formation of huge hyperbranched chains. In order to quantify the chain extension precisely, we show the ratio of peak areas (Ac/At) for the cyclized macromonomer (Ac) and entire hyperbranched chain (At). It should be noted that as the concentration of the macromonomer for the click reaction increases, the chain extension also increases, but after a threshold, the chain extension starts to decrease (Figure 4d). Indeed, some insoluble material was observed during polymerization at C = 1.0 g/mL, and we believe that the insoluble ultrahigh molar mass fractions were removed during membrane filtration while preparing the samples for SEC measurements, which resulted in the decrease of average molar mass of soluble fractions. It is worth noting that a similar observation was previously reported by Hutchings et al.,32 while optimizing the Williamson coupling reaction of AB2 macromonomers for the synthesis of hyperbranched polymers, but no reasonable explanation was given. In theory, ABn macromonomers do not form crosslinked polymers during click reactions. Therefore, it is not clear why such insoluble polymers are produced, which we will explore in the following work. Another well-known fact is that the length of the macromonomer decides the fate of the click reaction; the higher the molar mass of the macromonomer, the lower the concentration needed to get higher chain extension. Keeping these factors in mind, we first checked a range of macromonomer concentrations for PSS to perform an effective click reaction, and it is unambiguous that the chain extension increased from 0.3 to 0.7 g/mL before it started decreasing. This finding gave us a clear idea about what concentrations could be used for PSM and PSL to prepare the hyperbranched samples with a molar mass high enough to get a good signal-tonoise ratio in triple detection and stand-alone light scattering. The hyperbranched samples prepared at the highest concentrations were used for further studies. As revealed in our previous work,7,41,50 the macromonomer branching has a significant impact on the intrachain cyclization during the click coupling process. Thus, we need to further discuss two specific parameters: (i) the elution peak ratio (Mp,cyclized/Mp,precursor) of molar mass for the cyclized product (Mp,cyclized) and star macromonomer precursor (Mp,precursor, Mp,precursor = Mp,macro) in SEC curves and (ii) the ratio of peak areas (Ac/At) for the cyclized macromonomer (Ac) and entire

hyperbranched chain (At). Enlarged SEC curves of cyclized products of hyperbranched polystyrenes (HPS1−3) and their corresponding macromonomer precursors are shown in Figure S4. Simple Gaussian mode was used to fit the peak for cyclized macromonomers in SEC curves, and the fitting parameters are summarized in Table S1. In our previous work, the result already proved that more than ∼95% of the residual AB2/AB3 macromonomer chains are cyclized, indicating the complete consumption of initial macromonomer chains. Nevertheless, to quantify the mass fraction of the cyclized product in resultant AB4 HPSs, 1H-NMR and SEC were combined to characterize the fractionated samples. For this purpose, by taking HPSM as a model sample, a preparative SEC system was used to fractionate the polydispersed HPSM to get the “macromonomer” fraction (Figure S5). Figure 5 shows the 1H-NMR characterization result for the macromonomer fraction. Compared with the PSM macro-

Figure 5. 1H NMR spectra of azide-functionalized PSM (black), HPSM (red), and macromonomer fraction (blue).

monomer, the chemical shift of characteristic protons exhibits completely different dependence on the macromonomer fraction. Namely, the signal for proton Ha shifts from 4.6 to 5.1 ppm, indicating the quantitative consumption (85−95%) of alkyne groups for the macromonomer fraction in the click reaction. Overall, the characterization result unambiguously confirms the high degree of cyclization for four-arm star macromonomers during the click reaction. The analysis result of the molecular information for cyclized macromonomers shows that (i) the Mp,cyclized/Mp,precursor ratios for AB4 macromonomers (0.94−0.98) are significantly larger than the values for AB2 (0.79−0.82) and AB3 (0.87−0.91) systems in a similar molar mass range (Figure 6a),50 (ii) the values of Ac/At for AB4 macromonomers are also much larger than those for AB2 and AB3 macromonomers for a given Mmacro (Figure 6b), and (iii) both Mp,cyclized/Mp,precursor and Ac/At show a strong dependence on macromonomer molar mass. Mathematically, half of the chain segments will undergo intrachain cyclization for an AB2 macromonomer, one-third for an AB3 macromonomer, while only one-fourth of chain segments will undergo intrachain cyclization in the case of G


Article

Macromolecules

Figure 7. (a) Total molar mass (Mhyper) dependence of the intrinsic viscosity ([η]) of HPSs, HPSM, and HPSL measured using the TDSEC system in THF at T = 35 °C where the data for AB3 and AB2 hyperbranched and linear polystyrene are also plotted for comparison. (b) Total molar mass (Mhyper) dependence of the radius of gyration (Rg) of HPSS, HPSM, and HPSL measured using the TD-SEC system.

Figure 6. (a) Macromonomer molar mass (Mmacro) dependence of the elution peak ratio (Mp,cyclized/Mp,precursor) where Mp,cyclized and Mp,precursor represent the peak molar mass for the cyclized product and star macromonomer (Mp,precursor = Mmacro), respectively. (b) Macromonomer molar mass (Mmacro) dependence of the weight fraction (Ac/At) of the cyclized product for HPSS,M,L samples where the reaction was conducted at T = 35 °C. The data for hyperbranched samples prepared from V-type AB2 macromonomers41 and AB3 macromonomers50 are also presented for comparison. The polymerization concentrations for AB2 and AB3 systems are similar, while the polymerization concentrations for the AB4 system are significantly higher.

result. In the data processing, [η] and Rg are plotted as a function of the total molar mass (Mhyper) of hyperbranched chains. Based on the Mark−Houwink−Sakurada (MHS) plot of [η] versus Mhyper in Figure 7a, we have three main findings: (i) compared with the data for AB2 and AB3 LHP systems reported in our previous work (pink and green regions), the increase in the macromonomer branching effect further leads to a decrease in [η] for the AB4 LHP system at a given Mhyper, which is consistent with our anticipation; (ii) the slopes in MHS plots are ν = 0.40 + 0.01 for AB4 LHPs, which are very close to ν ≈ 0.40 for the AB2 system and ν ≈ 0.39 for the AB3 system. Such an observation further supports that the macromonomer branching has little effect on the self-similar behavior of LHPs; and (iii) [η] is found to be Mmacrodependent for AB4 LHPs, and this observation is different from the result for the AB3 systemnamely, AB3 LHPs with different subchain lengths almost overlap in a narrow region (blue region). Based on the analysis of [η] data, the Mmacro dependency is found to only exist in AB2 and AB4 LHP systems. Specifically, [η] could be expressed as a function of variables Mhyper and Mmacro as

the AB4 macromonomer, which reasonably explains why the determined Mp,cyclized/Mp,precursor ratios are much larger for the four-arm star macromonomer at a given molar mass (Figure 6a). Second, the larger Ac/At ratios for the AB4 system in Figure 6b indicate that the star chains are more likely to undergo the intrachain cyclization, which is even more than the AB3 system, instead of the interchain coupling reaction. Apparently, the alkyne group located at the center of the AB4 star macromonomer is expected to suffer from even stronger steric hindrance (inset in Figure 6b) due to its much higher segment density in the star center, which is not favorable to the interchain coupling reaction. Clearly, the enhanced branching effect for the macromonomer leads to easier intrachain cyclization but more difficult interchain coupling. Effect of Macromonomer Branching on Structural Features and Solution Properties of LHPs: TD-SEC Study of Unfractionated Samples. As discussed above, the main purpose of this study is to clarify whether the synergistic effect of segment back-folding and segment interpenetration only exists in moderately branched LHP systems. Thus, these AB4 LHP samples were first characterized using a well-calibrated TD-SEC system to investigate the fundamental solution properties such as the intrinsic viscosity ([η]) and the radius of gyration (Rg). The calibration process was detailed in our published work.41 The cumulative/ differential molar mass distribution curves are summarized in Figure S6. It is well known that [η]−Mν and Rg−Mα are two fundamental scaling equations describing the self-similar behavior of topological macromolecules, where ν and α are scaling exponents. Figure 7a,b summarizes the characterization

[η] = K ηMhyper νM macro μ

(9)

On the basis of experimental data, the fitting results for different systems are summarized as follows Kη, AB2 ≈ 3.57 × 10−2 mL/g, ν ≈ 0.40, and μ ≈ 0.25 for the AB2 LHP system; Kη, AB3 ≈ 2.9 × 10−1 mL/g, ν ≈ 0.39, and μ ≈ 0 for the AB3 LHP system; and Kη, AB4 ≈ 1.72 × 10−2 mL/g, ν ≈ 0.40, and μ ≈ 0.25 for the AB4 LHP system. On the other hand, on the basis of theoretical prediction58−60 for randomly hyperbranched polymers (AB2type) with uniform subchains, Rg should be scaled to Mhyper and Mmacro as R g = HRMhyper αM macro β H

(10) DOI: 10.1021/acs.macromol.9b01103 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules where α = 1/2 and β = 1/10 for ideal hyperbranched chains in a good solvent. Experimentally, we previously confirmed that for the AB2 LHP system, HR, AB2 ≈ 1.75 × 10−2 nm, α ≈ 0.47, and β ≈ 0.083; and for the AB3 LHP system, HR, AB3 ≈ 3.60 × 10−2 nm, α ≈ 0.47, and β ≈ 0 (Figure 7b, blue region). For the AB4 system, we have Rg ≈ Mhyperα with HR, AB4 ≈ 1.54 × 10−3 nm, α = 0.52 + 0.01, and β ≈ 0.20 (Figure 7b). The analysis of Rg data unambiguously further confirms the dependency between solution properties and Mmacro for the AB4 system. Compared with AB2 and AB3 systems (α ≈ 0.47), we notice that the AB4 system shows slightly larger α value (α ≈ 0.52), which seems unreasonable because AB4 LHPs actually possess the highest branching densities. We believe that the origin for this phenomenon could be mainly attributed to the limitation of applicability of the Berry method for highly branched systems. In principle, in the analysis of TD-SEC data, the range of scattering angles (q: scattering vector) should be appropriately selected according to molecular information of the probed sample (molar mass and degree of branching); namely, for a sample with a higher degree of branching, the data in the lower q regime should be used in the fitting. Practically, the range of scattering angles was not adjusted on purpose in this study to avoid the influence of data selection on the fitting result, which eventually leads to an overestimation of Rg for polymer fractions with higher molar masses. Overall, the simultaneous measurements of [η] and Rg crossvalidate the reliability of obtained data in TD-SEC measurement. A comparison of experimental results for ABn (n = 2, 3, and 4) LHPs clearly demonstrates that (i) the macromonomer branching leads to an ignorable influence on the self-similar behavior and (ii) only the AB3 system shows the unique Mmacro-independent solution properties. In other words, the synergistic effect of segment−segment interpenetration and segment back-folding phenomena does not exist in highly branched ABn LHP systems (n > 4). Three HPSM polymer fractions were obtained using a preparative SEC system. Figure S7 shows the hydrodynamic radius distributions [f(Rh)] of HPSM fractions measured by DLS in THF. TD-SEC measurement shows that the fractionated samples present slightly different characteristics in Rg−Mhyper plots compared with the unfractionated sample (Figure 8). Each fractionated sample implies a linear relationship between Rg and Mhyper over the whole molar mass range (4.0 × 105−1.5 × 106 g/mol), while for the unfractionated sample, a slightly curved feature is observed in the molar mass range lower than 1.0 × 106 g/mol (Figure 8). The comparative study of unfractionated and fractionated samples unambiguously clarifies that it is the large polydisper-

sity that accounts for the partially curved feature observed in Rg−Mhyper plots for highly polydispersed AB4 LHPs. A similar phenomenon was previously observed for the AB3 system. Thus, precautionary measures should be taken while dealing with highly branched systems with high polydispersities. Further Discussion Based on Experimental Result and Langevin Dynamics Simulation. In our recent work of the AB3 system,50 the solution properties ([η] and Rg) were found to be almost independent of Mmacro, and the results implied that the local chain segment density of AB3 LHPs is almost constant and irrelevant to Mmacro (internal subchain length). Meanwhile, we attributed such an abnormal independency between Mmacro and solution properties to the unique synergistic effect of segment interpenetration and segment back-folding. Our statement was rationalized by quantitatively analyzing the segment−segment interpenetration factor (Ψ) and the segment back-folding probability (Pb). Physically, Ψ is mainly affected by the free space inside the internal cavity of one hyperbranched chain,61,62 and Pb is mainly affected by the local branching density around the branching point. Specifically speaking, the coupling reaction should occur mainly between AB3 macromonomer chains in the initial stage of the interchain coupling process and then between hyperbranched oligomers in the later stage. Physically, hyperbranched oligomers with longer subchains and lower branching densities are more interpermeable due to the larger free space inside their cavities (smaller Ψ). Thus, one ABn hyperbranched oligomer chain composed of longer internal subchains finds it easier to undergo interpenetration with other small macromonomers/oligomers, facilitating the occurrence of interchain coupling reaction in its interior space. In the case of hyperbranched oligomers composed of the shortest subchains, the interpenetration might be completely inhibited. Consequently, for a given Mhyper, the overall molecular sizes and local draining properties of hyperbranched samples with different subchain lengths could eventually become similar at the end of click polymerization (the case of the AB3 system). In addition to the occurrence of interchain interpenetration, the local concentration of the azide group in the internal space of one ABn hyperbranched oligomer chain needs to be high enough to facilitate the interchain coupling. This is strongly affected by the trend of segment back-folding. The diffusion process of chain segments from the outer layers into the inner space is a well-known “back-folding” characteristic in branched polymers.63,64 Obviously, for AB2 LHPs, a much lower Pb is expected due to their much lower local branching densities, that is, the synergistic effect of segment interpenetration and segment back-folding does not exist in the AB2 system. In other words, the lacking of segment back-folding could suppress the occurrence of coupling reactions inside the internal cavities of one oligomer chain. Compared with ABn systems (n < 3), the local segment density is much higher for the AB4 system, which further leads to a decrease in the trend for segment interpenetration and an increase in the trend for segment back-folding. Physically, the segment interpenetration might be completely prohibited in the AB4 system. To quantify the trends of segment interpenetration and segment back-folding, we further performed Langevin dynamics simulation for the AB4 system to elucidate the discrepancy in structural properties of ABn LHPs and dendrimers (n = 2, 3, and 4). The details for the construction of simulation models for ABn LHPs and dendrimers can be

Figure 8. Total molar mass (Mhyper) dependence of the radius of gyration (Rg) of the unfractionated and fractionated HPSM measured by TD-SEC in THF. I


Macromolecules

Article

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CONCLUSIONS Using three well-characterized four-arm star polystyrenes with different molar masses (Mmacro) as macromonomers, we have elucidated the effect of highly branched macromonomers on the structural features and solution properties of long-subchain hyperbranched polymers (LHPs). The results reveal that the higher the branching degree of macromonomers, the higher the suppression of chain extension and the enhancement of macromonomer self-cyclization during the click interchain polymerization process, which is attributed to the stronger steric hindrance effect. Furthermore, via a combination of experimental study of the total molar mass (Mt)-dependent intrinsic viscosity ([η]) and radius of gyration (Rg) for AB2, AB3, and AB4 LHP systems and the computational study of the segment interpenetration and segment back-folding property for dendrimers and LHP model systems, we reveal that (i) the self-similar behavior for LHPs is not sensitive to the local segment density or branching pattern and (ii) both [η] and Rg are dependent on Mmacro for AB2 and AB4 LHP systems but independent of Mmacro for the AB3 LHP system, so the unique synergistic effect of segment back-folding and segment interpenetration only exists in moderately branched ABn LHP systems. For highly branched ABn LHP systems (n > 4), the increase of subchain length leads to the enhancement of the trend for segment interpenetration but the reduction of the trend for interchain coupling, which eventually could lead to a different segment density and draining property with different subchain lengths. In general, the AB4 LHP system verified our previously proposed conjecture. Moreover, its existence complements the influence of macromonomer branching on the solution properties of hyperbranched polymers, which is useful to simplify the quality control of hyperbranched polymer products in industrial production.

referred to the Experimental Section and our published work. The radial density function [ρ(r)] was analyzed only for longsubchain dendrimers, but the interpenetration factor Ψ was analyzed for both LHPs and dendrimers. Figure 9 quantitatively shows how the radial density ρ(r) of G2−G5 AB4 dendrimers changes with the generation. Similar

Figure 9. Simulation results of the radial density [ρ(r)] of segments belonging to G2−G5 generations for dendrimers comprising AB4 macromonomers (Ls = 10).

to our previous results for AB2 and AB3 systems, the present simulation results illustrate that the ρ(r) curve for the AB4 system changes from a unimodal distribution to a multimodal distribution as the generation increases, indicating that the broadening of the distribution curve is more significant for larger branched chains. A similar trend is also expected for large hyperbranched chains (analogues of G5 AB4 dendrimers). Table 3 further shows the simulation result of Ψ for different types of LHPs and long-subchain dendrimers. The results illustrate that for a given macromonomer type and subchain length, the Ψ values for dendrimers are significantly larger than ABn LHPs, which is reasonable and can be attributed to the more condensed molecular structure of dendrimers and stronger steric hindrance effect. As expected, AB2 systems present a stronger trend for segment interpenetration than AB3 systems (1.15−1.64), reflected in the smaller Ψ values (0.76− 1.10). Quantitatively, Ψ = 2.20 for the AB4 dendrimer (Ls = 10) and Ψ = 2.00 for AB4 LHPs (Ls = 10), signifying hard sphere-like structures. It is worth noting that Ψ is predicted to be ∼1.60 for an ideal hard sphere, which is even smaller than the values for the AB4 system. Such a discrepancy could be due to the ignored multibody interaction in the simulation. In Lederer and Burchard’s work,62 Ψ for dendrimers is stated to be 1.81−2.31, a larger value than 1.60, expected for hard spheres, but they did not provide a rationale for this increment. Anyway, the much larger Ψ value for the AB4 system clearly demonstrates the prohibition of the segment interpenetration process in AB4 LHPs, which explains why the synergistic effect is only observed in the AB3 system. Overall, the lack of segment back-folding in the AB2 system and the lack of segment interpenetration in the AB4 system reasonably account for the origin of differences in dependence between solution properties and Mmacro for different ABn systems.63,64

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01103. Table of the fitting parameters of cyclized macromonomers; schematic for hydrolysis of the ether linkage at branching points; 1H-NMR spectra of the initiator and intermediate compound; 1H-NMR spectra and FTIR spectra of bromine- and azide-functionalized macromonomers and hyperbranched polymers; enlarged SEC curves of cyclized products of hyperbranched polystyrenes and their corresponding macromonomer precursors; SEC curves of PSM, HPSM, and “macromonomer” fraction; molar mass distribution functions of hyperbranched samples; and hydrodynamic radius distributions of HPSM fractions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected].

Table 3. Simulation Results of Interpenetration Factor (Ψ) for Different Branched Polymers interpenetration factor

AB2 dendrimer (Ls = 10)

AB2 LHPs (Ls = 5)

AB2 LHPs (Ls = 10)


AB3 LHPs (Ls = 5)

AB3 LHPs (Ls = 10)


AB4 LHPs (Ls = 10)

Ψ

1.10

0.87

0.76

1.64

1.21

1.15

2.20

2.00

J


Article

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(16) Fei, Z.; Li, B.; Bo, Z.; Lu, R. Synthesis and properties of hyperbranched conjugated porphyrins. Org. Lett. 2004, 6, 4703−4706. (17) Li, X.; Su, Y.; Chen, Q.; Lin, Y.; Tong, Y.; Li, Y. Synthesis and characterization of biodegradable hyperbranched poly(ester-amide)s based on natural material. Biomacromolecules 2005, 6, 3181−3188. (18) Adeli, M.; Rasoulian, B.; Saadatmehr, F.; Zabihi, F. Hyperbranched poly(citric acid) and its application as anticancer drug delivery system. J. Appl. Polym. Sci. 2013, 129, 3665−3671. (19) Zou, L.; Shi, Y.; Cao, X.; Gan, W.; Wang, X.; Graff, R. W.; Hu, D.; Gao, H. Synthesis of acid-degradable hyperbranched polymers by chain-growth CuAAC polymerization of an AB3 monomer. Polym. Chem. 2016, 7, 5512−5517. (20) Testud, B.; Pintori, D.; Grau, E.; Taton, D.; Cramail, H. Hyperbranched polyesters by polycondensation of fatty acid-based ABn-type monomers. Green Chem. 2017, 19, 259−269. (21) Miravet, J. F.; Fréchet, J. M. J. New hyperbranched poly(siloxysilanes): variation of the branching pattern and endfunctionalization. Macromolecules 1998, 31, 3461−3468. (22) Liu, Z.-D.; Chang, Y.-Z.; Ou, C.-J.; Lin, J.-Y.; Xie, L.-H.; Yin, C.-R.; Yi, M.-D.; Qian, Y.; Shi, N.-E.; Huang, W. BF3·Et2O-mediated Friedel−Crafts C−H bond polymerization to synthesize π-conjugation-interrupted polymer semiconductors. Polym. Chem. 2011, 2, 2179−2182. (23) Wu, W.; Ye, C.; Yu, G.; Liu, Y.; Qin, J.; Li, Z. New hyperbranched polytriazoles containing isolation chromophore moieties derived from AB4 monomers through click chemistry under copper (I) catalysis: improved optical transparency and enhanced NLO effects. Chem. − Eur. J. 2012, 18, 4426−4434. (24) Wu, W.; Li, Z. Further improvement of the macroscopic NLO coefficient and optical transparency of hyperbranched polymers by enhancing the degree of branching. Polym. Chem. 2014, 5, 5100− 5108. (25) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Hyperbranched poly(ethylene glycol)s: a new class of ion-conducting materials. Macromolecules 1996, 29, 3831−3838. (26) Trollsås, M.; Hedrick, J. L. Hyperbranched poly(ε-caprolactone) derived from intrinsically branched AB2 macromonomers. Macromolecules 1998, 31, 4390−4395. (27) Trollsås, M.; Kelly, M. A.; Claesson, H.; Siemens, R.; Hedrick, J. L. Highly branched block copolymers: design, synthesis, and morphology. Macromolecules 1999, 32, 4917−4924. (28) Choi, J.; Kwak, S.-Y. Synthesis and characterization of hyperbranched poly (ε-caprolactone)s having different lengths of homologous backbone segments. Macromolecules 2003, 36, 8630− 8637. (29) Hutchings, L. R.; Dodds, J. M.; Roberts-Bleming, S. J. HyperMacs. Long chain branched analogues of hyperbranched polymers prepared by the polycondensation of AB2 macromonomers. In Macromolecular symposia; Wiley Online Library: 2006; pp 56−67. (30) Hutchings, L. R.; Roberts-Bleming, S. J. DendriMacs. Welldefined dendritically branched polymers synthesized by an iterative convergent strategy involving the coupling reaction of AB2 macromonomers. Macromolecules 2006, 39, 2144−2152. (31) Dodds, J. M.; De Luca, E.; Hutchings, L. R.; Clarke, N. Rheological properties of HyperMacslong-chain branched analogues of hyperbranched polymers. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 2762−2769. (32) Clarke, N.; De Luca, E.; Dodds, J. M.; Kimani, S. M.; Hutchings, L. R. HyperMacs−long chain hyperbranched polymers: A dramatically improved synthesis and qualitative rheological analysis. Eur. Polym. J. 2008, 44, 665−676. (33) Hutchings, L. R. DendriMacs and HyperMacs−emerging as more than just model branched polymers. Soft Matter 2008, 4, 2150− 2159. (34) Hutchings, L. R.; Dodds, J. M.; Rees, D.; Kimani, S. M.; Wu, J. J.; Smith, E. HyperMacs to hyperblocks: a novel class of branched thermoplastic elastomer. Macromolecules 2009, 42, 8675−8687.

Lianwei Li: 0000-0002-1996-6046 Author Contributions ∥

N.H. and A.U. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Scientific Foundation of China Projects (21774116 and 21404103) and Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm2019015) are gratefully acknowledged. We are also grateful to the Network and Computing Center, CIAC, CAS, and Computing Center of Jilin Province for the essential support.

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REFERENCES

(1) Gao, C.; Yan, D. Hyperbranched polymers: from synthesis to applications. Prog. Polym. Sci. 2004, 29, 183−275. (2) Zheng, Y.; Li, S.; Weng, Z.; Gao, C. Hyperbranched polymers: advances from synthesis to applications. Chem. Soc. Rev. 2015, 44, 4091−4130. (3) Voit, B. I.; Lederer, A. Hyperbranched and Highly Branched Polymer Architectures- Synthetic Strategies and Major Characterization Aspects. Chem. Rev. 2009, 109, 5924−5973. (4) Trollsås, M.; Atthoff, B.; Claesson, H.; Hedrick, J. L. Hyperbranched poly (ε-caprolactone)s. Macromolecules 1998, 31, 3439−3445. (5) Hutchings, L. R.; Dodds, J. M.; Roberts-Bleming, S. J. HyperMacs: highly branched polymers prepared by the polycondensation of AB2 macromonomers, synthesis and characterization. Macromolecules 2005, 38, 5970−5980. (6) Kong, L. Z.; Sun, M.; Qiao, H. M.; Pan, C. Y. Synthesis and characterization of hyperbranched polystyrene via click reaction of AB2 macromonomer. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 454−462. (7) Yan, J.-J.; Hong, C.-Y.; You, Y.-Z. An easy method to convert the topologies of macromolecules after polymerization. Macromolecules 2011, 44, 1247−1251. (8) Flory, P. J. Molecular size distribution in three dimensional polymers. VI. Branched polymers containing ARBf‑1Type units. J. Am. Chem. Soc. 1952, 74, 2718−2723. (9) Hutchings, L. R.; Agostini, S.; Hamley, I. W.; Hermida-Merino, D. Chain Architecture as an Orthogonal Parameter To Influence Block Copolymer Morphology. Synthesis and Characterization of Hyperbranched Block Copolymers: HyperBlocks. Macromolecules 2015, 48, 8806−8822. (10) Li, P.-Y.; He, W.-D.; Chen, S.-Q.; Lu, X.-X.; Li, J.-M.; Li, H.-J. Formation of long sub-chain hyperbranched poly(methyl methacrylate) based on inhibited self-cyclization of seesaw macromonomers. Polym. Chem. 2016, 7, 4842−4851. (11) Jikei, M.; Suzuki, M.; Itoh, K.; Matsumoto, K.; Saito, Y.; Kawaguchi, S. Synthesis of hyperbranched poly(L-lactide)s by selfpolycondensation of AB2 macromonomers and their structural characterization by light scattering measurements. Macromolecules 2012, 45, 8237−8244. (12) Hawker, C. J.; Chu, F. Hyperbranched poly(ether ketones): manipulation of structure and physical properties. Macromolecules 1996, 29, 4370−4380. (13) Lach, C.; Müller, P.; Frey, H.; Mülhaupt, R. Hyperbranched polycarbosilane macromonomers bearing oxazoline functionalities. Macromol. Rapid Commun. 1997, 18, 253−260. (14) Yoon, K.; Son, D. Y. Syntheses of hyperbranched poly(carbosilarylenes). Macromolecules 1999, 32, 5210−5216. (15) Jaumann, M.; Rebrov, E. A.; Kazakova, V. V.; Muzafarov, A. M.; Goedel, W. A.; Möller, M. Hyperbranched Polyalkoxysiloxanes via AB3-Type Monomers. Macromol. Chem. Phys. 2003, 204, 1014−1026. K


Article

Macromolecules (35) Konkolewicz, D.; Gray-Weale, A.; Perrier, S. Hyperbranched polymers by thiol−yne chemistry: from small molecules to functional polymers. J. Am. Chem. Soc. 2009, 131, 18075−18077. (36) Xu, J.; Tao, L.; Boyer, C.; Lowe, A. B.; Davis, T. P. Combining Thio−Bromo “click” chemistry and RAFT polymerization: a powerful tool for preparing functionalized multiblock and hyperbranched polymers. Macromolecules 2010, 43, 20−24. (37) Konkolewicz, D.; Poon, C. K.; Gray-Weale, A.; Perrier, S. Hyperbranched alternating block copolymers using thiol−yne chemistry: materials with tuneable properties. Chem. Commun. 2011, 47, 239−241. (38) Li, Z. A.; Wu, W.; Qiu, G.; Yu, G.; Liu, Y.; Ye, C.; Qin, J.; Li, Z. New series of AB2-type hyperbranched polytriazoles derived from the same polymeric intermediate: Different endcapping spacers with adjustable bulk and convenient syntheses via click chemistry under copper (I) catalysis. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1977−1987. (39) Barbey, R.; Perrier, S. Synthesis of polystyrene-based hyperbranched polymers by thiol−yne chemistry: a detailed investigation. Macromolecules 2014, 47, 6697−6705. (40) Jikei, M.; Uchida, D.; Matsumoto, K.; Komuro, R.; Sugimoto, M. Synthesis and properties of long-chain branched poly(ether sulfone)s by self-polycondensation of AB2 type macromonomers. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1825−1831. (41) Zhu, M.; Hao, N.; Yang, J.; Li, L. A comparative study of intrachain cyclization and solution properties of long-subchain hyperbranched polymers prepared via Y-type and V-type macromonomer approaches. Polym. Chem. 2018, 9, 2830−2842. (42) He, C.; Li, L.-W.; He, W.-D.; Jiang, W.-X.; Wu, C. “Click” Long Seesaw-Type A∼∼ B∼∼ A Chains Together into Huge Defect-Free Hyperbranched Polymer Chains with Uniform Subchains. Macromolecules 2011, 44, 6233−6236. (43) Yang, J.; Li, L.; Jing, Z.; Ye, X.; Wu, C. Construction and Properties of Hyperbranched Block Copolymer with Independently Adjustable Heterosubchains. Macromolecules 2014, 47, 8437−8445. (44) He, C.; He, W. D.; Li, L. W.; Jiang, W. X.; Tao, J.; Yang, J.; Chen, L.; Ge, X. S.; Chen, S. Q. Controlling the formation of longsubchain hyperbranched polystyrene from seesaw-type AB2 macromonomers: Solvent polarity and solubility. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3214−3224. (45) Li, L.; Wang, X.; Yang, J.; Ye, X.; Wu, C. Degradation kinetics of model hyperbranched chains with uniform subchains and controlled locations of cleavable disulfide linkages. Macromolecules 2014, 47, 650−658. (46) Li, L.; Lu, Y.; An, L.; Wu, C. Experimental and theoretical studies of scaling of sizes and intrinsic viscosity of hyperbranched chains in good solvents. J. Chem. Phys. 2013, 138, 114908. (47) Li, L.; He, C.; He, W.; Wu, C. Formation kinetics and scaling of “defect-free” hyperbranched polystyrene chains with uniform subchains prepared from seesaw-type macromonomers. Macromolecules 2011, 44, 8195−8206. (48) Li, L.; He, C.; He, W.; Wu, C. How does a hyperbranched chain pass through a nanopore? Macromolecules 2012, 45, 7583− 7589. (49) Li, L.; Zhou, J.; Wu, C. Intrachain folding and interchain association of hyperbranched chains with long uniform subchains made of amphiphilic diblock copolymers. Macromolecules 2012, 45, 9391−9399. (50) Hao, N.; Duan, X.; Yang, H.; Umair, A.; Zhu, M.; Zaheer, M.; Yang, J.; Li, L. How Does the Branching Effect of Macromonomer Influence the Polymerization, Structural Features, and Solution Properties of Long-Subchain Hyperbranched Polymers? Macromolecules 2019, 52, 1065−1082. (51) Ioan, C. E.; Aberle, T.; Burchard, W. Structure Properties of Dextran. 2. Dilute Solution. Macromolecules 2000, 33, 5730−5739. (52) Rubio, A. M.; Freire, J. J. Monte Carlo Calculation of Second Virial Coefficients for Linear and Star Chains in a Good Solvent. Macromolecules 1996, 29, 6946−6951.

(53) Withers, I. M.; Dobrynin, A. V.; Berkowitz, M. L.; Rubinstein, M. Monte Carlo simulation of homopolymer chains. I. Second virial coefficient. J. Chem. Phys. 2003, 118, 4721−4732. (54) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1−19. (55) Teraoka, I. Polymer solutions: an introduction to physical properties; John Wiley & Sons: 2002. (56) Sun, T.; Chance, R. R.; Graessley, W. W.; Lohse, D. J. A Study of the Separation Principle in Size Exclusion Chromatography. Macromolecules 2004, 37, 4304−4312. (57) Leophairatana, P.; Samanta, S.; De Silva, C. C.; Koberstein, J. T. Preventing Alkyne−Alkyne (i.e., Glaser) Coupling Associated with the ATRP Synthesis of Alkyne-Functional Polymers/Macromonomers and for Alkynes under Click (i.e., CuAAC) Reaction Conditions. J. Am. Chem. Soc. 2017, 139, 3756−3766. (58) Zimm, B. H.; Stockmayer, W. H. The dimensions of chain molecules containing branches and rings. J. Chem. Phys. 1949, 17, 1301−1314. (59) Isaacson, J.; Lubensky, T. C. Flory exponents for generalized polymer problems. J. Physique. Lett. 1980, 41, 469−471. (60) Gay, C.; de Gennes, P. G.; Raphaël, E.; Brochard-Wyart, F. Injection Threshold for a Statistically Branched Polymer inside a Nanopore. Macromolecules 1996, 29, 8379−8382. (61) Burchard, W. Solution properties of branched macromolecules. In Branched polymers II; Springer: 1999; pp 113−194, DOI: 10.1007/ 3-540-49780-3_3. (62) Lederer, A.; Burchard, W. Hyperbranched Polymers: Macromolecules in between deterministic linear chains and dendrimer structures; Royal Society of Chemistry: 2015. (63) Kavyani, S.; Amjad-Iranagh, S.; Dadvar, M.; Modarress, H. Hybrid Dendrimers of PPI(core)-PAMAM(shell): A Molecular Dynamics Simulation Study. J. Phys. Chem. B 2016, 120, 9564−9575. (64) Zhang, H.; Zhu, J.; He, J.; Qiu, F.; Zhang, H.; Yang, Y.; Lee, H.; Chang, T. Easy synthesis of dendrimer-like polymers through a divergent iterative ″end-grafting″ method. Polym. Chem. 2013, 4, 830−839.

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