Complex Branched Polymers for StructureâProperty Correlation

Perspective pubs.acs.org/Macromolecules

Complex Branched Polymers for Structure−Property Correlation Studies: The Case for Temperature Gradient Interaction Chromatography Analysis Lian R. Hutchings* Durham Centre for Soft Matter, Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom ABSTRACT: The idiom “can't see the wood for the trees” would appear uniquely apt in describing attempts to characterize structural dispersity in complex branched polymers, especially dendritically branched polymers, by size exclusion chromatography (SEC) alone. SEC is incapable of resolving polymers with nearly identical hydrodynamic volumes which may differ in molecular weighta particular problem in the characterization of branched polymers. This paper discusses in detail the variety of synthetic methodologies used to synthesize complex branched polymers including H-shaped polymers, comb-branched polymers, and with particular emphasis on dendritically branched polymers, also known as Cayley trees and DendriMacs. The advantages and limitations of each method are discussed, supported by a review of recent reports of temperature gradient interaction chromatography (TGIC) analysis of such polymers. With one eye to the future, recent results should provoke the polymer chemists to devise improved synthetic strategies, but it is clear that TGIC is now an indispensable technique for revealing structural dispersity in complex branched polymers and allowing teams involved in structure−property correlation studies to see “the wood and the trees”. dendritic11,46,47 polymers, and dendrimer-like star polymers.48−57 Regardless of the terminology, this branched polymer architecture is the simplest hierarchically branched (branch-on-branch) architecture with symmetry about a central branch point, and for this reason they are an optimal structure for studying the rheological properties of branched polymers. As is the case with traditional dendrimers, increasing layers of branching can be added, and the resulting structures are similarly described as having increasing numbers of generations of branching (see Figure 1). Such polymers with up to seven generations of branching have been prepared (by different methods) by both Hirao et al.50 and Gnanou,52 and variants have been prepared in which the linear segments making up different generations vary in both molecular weight42,46 and polymer type.57−59 For example, Hirao et al. describe the synthesis of dendrimer-like star branched polymers with alternating generations of poly(methyl methacrylate) and polystyrene.54 Moreover, numerous synthetic strategies have been described in the literature for the synthesis of well-defined dendritically branched architectures. The linear segments are inevitably prepared using a living or controlled chain-growth polymerization mechanism including living anionic polymerization,42,43,46,50,51 anionic ring-opening polymerization,49 ringopening polymerization of cyclic esters,48 and atom transfer radical polymerization.60−63 There is a similarly wide range of

1. INTRODUCTION It has long been understood that the physical properties of polymeric materials correlate strongly to the molecular architecture of the constituent polymer chains. Parameters such as molecular weight, dispersity, and chain branching have huge implications not only for the final solid-state properties of a polymer but also for properties in the melt such as rheology. Significant progress has been made in recent decades in understanding and predicting the relationship between chain branching and the rheology of polymers;1−14 however, although branching is very common in industrial polymers, attempting to develop this understanding using only industrially produced materials, which often have a very broad distribution of structures, is not a practical proposition. It is this long held realization15 that has driven polymer chemists to design (and synthesize) model branched materials with the overarching aim of synthesizing polymers which are structurally homogeneous. Thus, for more than two decades, the synthesis and characterization of well-defined, narrow dispersity, model branched polymers has been fundamental in the development of our understanding of the dynamics of entangled polymer melts and the contribution of the polymer chemist in this field is beyond question. During this period star-branched polymers,16−23 H-shaped polymers3,24−29 and combs14,30−41 have provided a stern testing ground for theoretical developments in the tube model of entangled polymers. More recently, strategies have been devised (by us and others) to synthesize yet more complex, hierarchically, branched architectures variously described as DendriMacs,42−44 Cayley trees,4,13,45 © 2012 American Chemical Society

Received: March 16, 2012 Revised: April 23, 2012 Published: May 17, 2012 5621

dx.doi.org/10.1021/ma3005422 | Macromolecules 2012, 45, 5621−5639

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be accepted and/or overcome. While on the one hand we must continue to strive to improve the synthetic methodologies, we must also strive to rigorously characterize our polymers to detect and attempt to quantify the presence of imperfect byproduct structures and provide accurate structural data to help validate existing models and evolve improved models of branched polymer rheology. Where structural perfection is not possible, the next best thing is a perfectly characterized, imperfect structure. It has long been assumed (until recently) that purification by fractionation of the inherently structurally disperse products of such syntheses is sufficient to remove imperfectionsfalse assumptions often reinforced by a monomodal, narrow molecular weight distribution obtained by size exclusion chromatography (SEC). However, it has been known for half a century that SEC columns separate polymers by molecular size in solution (hydrodynamic volume) rather than molecular weight, and moreover it has also been long realized that the significant dispersity in the molecular weight of even simple branched polymers does not translate into measurable differences in molecular size (by SEC)for example, the difference between a five- and six-arm star is practically impossible to detect by SEC.69 Given this unavoidable limitation of SEC, it is clear that monomodal SEC chromatograms with dispersity values well below 1.1 could be masking significant structural heterogeneity, and such heterogeneity is almost certain to introduce uncertainty into comparisons of experimental rheology data and predictions of that rheology based upon theoretical models. Validation (or otherwise) of such models is not trivial at the best of times, and differences between theoretical predictions and experimental data lead to the uncomfortable question, is there a problem with the model or is there a problem with the materials (or both)? However, in recent years, temperature gradient interaction chromatography (TGIC), a technique developed more than 15 years ago by Chang70 at Pohang University of Science and Technology (Republic of Korea), has emerged as a powerful tool to complement SEC and to provide a opportunity for more detailed characterization of polymers and, in particular, complex branched architectures. Briefly, for this is not a paper about TGIC per se but its application to the characterization of complex branched polymers, TGIC separation is driven by enthalpic interactions between the solute molecules and the stationary phase, and these interactions, which can be controlled by temperature variation during the elution, are to a first approximation proportional to the molecular weight, NOT the hydrodynamic volume,71,72 and careful control of solvent and temperature can lead to substantially improved resolution of branched polymers over SEC. Superior resolution in TGIC also arises as a result of far less band broadening than with SEC. This Perspective will focus in detail on the existing strategies adopted to synthesize complex branched polymers, specifically for structure−property correlation studies, with particular emphasis on hierarchically branched dendritic polymers and with some mention of H-shaped and comb polymers. The advantages and potential limitations of these strategies will be discussed in terms of their ability to produce structurally homogeneous branched molecules and the origin of possible defect byproduct structures. This will be followed by a review of recent reports where TGIC has been used in parallel with SEC to characterize some of the described branched architectures (and their defect byproducts) and the implications of structural dispersity on furthering our understanding of the

Figure 1. Schematic representation of G1 and G2 DendriMacs.

strategies that have been exploited for the introduction of branch points and, just as dendrimers can be made via either a divergent64,65 or convergent66 approach, so can their long-chain branched analogues. However, the focus of this Perspective is not to give a detailed overview of the many and varied synthetic approaches to prepare dendritically branched polymers. That has been more than adequately achieved in a number of recent review articles44,56,67 and a recently published Perspective entitled “Dendritic and Hyperbranched Polymers f rom Macromolecular Units: Elegant Approaches to the Synthesis of Functional Polymers” by Perrier.68 Instead, the aim of this paper is to discuss in detail complex branched polymer architectures and in particular dendritically branched materials, which have been designed and synthesized specifically as model polymers for rheological studies, and to outline the relative merits and limitations of the approaches which have been exploited in these careful synthesesalways emphasizing the necessity for a very high degree of molecular homogeneity. Without exception, the synthetic methodologies which have been developed to produce such polymers are complex, multistep procedures and rely on the use of living anionic polymerization for the generation of the linear segments. Although complex branched polymers can be made using other controlled chain-growth polymerization mechanisms, the inherently higher dispersity in chain length introduced through the use of these polymerization mechanisms makes them less suitable for structure− property correlation investigations. All of the following described strategies based on anionic polymerization therefore exploit, to a lesser or greater extent, reactions that are extraordinarily sensitive to traces of environmental impurities, and a direct result of such complex synthetic strategies is the almost unavoidable formation of imperfections and structural heterogeneity as a result of incomplete linking reactions and premature termination of living anionic polymerization. It is worth stating at this point that the presence of small amounts of defect structures can have a limited or negligible impact on many of the physical properties of branched polymers, and in many cases model polymers contaminated with low levels of imperfections have been tremendously useful in advancing our understanding of polymer physics. However, rheological measurements are very often extraordinarily sensitive to defect structures, and a high degree of structural perfection is crucial in testing existing models of branched polymer rheology. While structural imperfections may be a disappointment to the synthetic chemistand the author of this paper can admit to that from bitter experiencelimitations in our chemistry must 5622


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rheology of branched polymers. Finally, there will be some thoughts on the future of this field.

Scheme 1. Synthesis of H-shaped Polybutadiene Using DCMSDPE as Linking Agent (Reproduced with Permission from Ref 26)

2. SYNTHESIS OF COMPLEX (MODEL) BRANCHED ARCHITECTURES FOR STRUCTURE−PROPERTY CORRELATION STUDIES Although there are many groups around the world involved in the synthesis of complex branched architectures and this number is growing, not all of these syntheses aim to produce branched polymers with a high degree of structural homogeneity specifically for structure−property correlation studies. However, there have been a significant number of developments in this field in recent years, and the community of scientists following this fieldchemists, physicists, mathematicians, and engineers from both academia and industryis vibrant. We will focus here on the key advances in this field in recent years. Recent developments in the synthesis of model dendritically branched polymers will be discussed, and in some cases the same strategies have been previously developed for the synthesis of H-shaped polymers and model combs. However, the synthesis of very well-defined combs with multiple (many) branches at precise, predefined positions on the backbone remains a challenge. The synthesis of branched architectures will be discussed in order of increasing complexity. 2.1. H-Shaped Polymers. For many years H-shaped polymers were prepared by a strategy in which the crossbar of the H-shaped polymer was prepared by anionic polymerization using a difunctional initiator. The living chain ends were then end-capped with excess methyltrichlorosilane, and the arms of the H-polymer, formed as independently produced living chains, were coupled to the remaining unreacted chlorosilane groups.28,73 However, this approach can result in the production of high molecular weight defects as a result of backbone coupling as reported previously.28 While this previously reported work was unremarkable in terms of the synthesis, it is notable as being one of the earliest examples of work in which TGIC was used to characterize structural heterogeneity in complex branched polymers, and this characterization will be discussed later. However, more recently Mays et al. described26 a novel approach to synthesize Hshaped polymers based on a route developed earlier by Hadjichristidis for the synthesis of dendritically branched polymers46 (discussed in detail below). This H-polymer synthesis was designed specifically to overcome the high molecular weight defect structures alluded to above. This synthesis uses 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE) (see Scheme 1) as a linking agent/terminating agent and relies upon the fact the carbanion of a living polymer reacts preferentially with a chlorosilane group in the presence of a vinyl group, and in the case of DCMSDPE, the steric hindrance of the two phenyl groups hinders reaction between the living polymer and the double bond, thereby rendering the reaction of the carbanion with the chlorosilyl bonds very selective. Briefly, the synthesis was achieved by titrating 2 equiv of living polybutadiene with DCMSDPE followed by the addition of sec-butyllithium which reacts with the double bond of the diphenylethylene moiety to generate a macroinitiator. Addition of butadiene monomer results in the growth of a new arm and the synthesis of a living three-arm star, which upon addition of dichlorodimethylsilane couple to form the Hpolymer. Fractionation of the final product resulted in a polymer with a narrow molecular weight distribution and an

apparently high degree of structural homogeneity. Although this route overcomes some of the limitations of the earlier syntheses, it is not without its problemsa point highlighted in a subsequent paper (to be discussed later in the section on TGIC characterization) in which H-shaped polymers produced by this route were characterized by TGIC, revealing hitherto unseen structural heterogeneity. A nearly identical approach exploiting the linking agent DCMSDPE was also used by Hadjichristidis and co-workers74 to produce H-shaped polybutadiene and H-shaped polybutadiene with a single branch attached to the backbone (see Figure 2). As will become apparent, the synthesis of branched polymers using this general approach is complex but efficient, relying on anionic polymerization and living coupling reactions. Following fractionation, monomodal SEC chromatograms initially suggest a high degree of structural homogeneity; however, subsequent analysis by TGIC would suggest

Figure 2. Schematic of H-shaped polybutadiene (a) and H-shaped polybutadiene with one arm attached to backbone (b) and (c) synthesized by Hadjichristidis. Reproduced with permission from ref 74. Copyright 2009 John Wiley and Sons. 5623


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diphenylethylene. These initially disperse polymers were fractionated to produce samples of combs with a dispersity index of 1.1 and an average of about 4 arms per molecule. These combs were characterized by linear rheology and in an attempt to better understand the distribution of structures were also characterized by TGIC (see later). However, although TGIC was able to generate excellent data for the distribution of arms per comb, TGIC cannot give any information about the (distribution in) molecular weight between branch points. The other general strategy used to synthesize combs attempts to control the molecular weight between arms but in doing so introduces other structural heterogeneities. In this strategy76 a step growth approach is adopted to couple backbone polymer, produced by anionic polymerization using a difunctional initiator, to arms, end-capped with a difunctional reactive group, usually a chlorosilane (see Scheme 2). In this case the molecular weight between branch points (“l”) is fixed, but the molecular weight of the backbone “L” will be inherently disperse, as will the number of arms per comb molecule. Following this approach, we would expect a dispersity index of 2.0 (according to Carothers' theory), and indeed the dispersity of the crude product was reported as being between 2.1 and 2.5,76 although the crude product was fractionated to narrow the distribution. As well as producing a distribution of combbranched polymers, this strategy also presents the possibility for cyclization and as such may not be particularly well suited for preparing polymers for structure property correlation studies. There are however a very few examples where a careful iterative approach has been taken, with the aim of controlling all of the molecular variables to generate a highly defined structure. In 2000, Hadjichristidis77 described the first successful synthesis of a comb (co)polymer with a precisely controlled structure, composed of a polyisoprene backbone and two polystyrene arms. A stepwise iterative methodology (see Scheme 3) was followed in which living polyisoprene was added to 1,4bis(phenylethenyl)benzene to introduce a 1,1-diphenylethylene (DPE) moiety at the chain end. The resulting DPE-functionalized polyisoprene was then reacted with a stoichiometric equivalence of living polystyrene to attach the first arm to the polyisoprene chain. The resulting living diblock copolymer with a DPE-derived anion was then used as a macroinitiator for the anionic polymerization of isoprene to extend the backbone and in doing so preparing a living 3-arm star-branched polymer. Repeating this process, addition of an arm and then extending the backbone results in the formation a comb branched copolymer having two polystyrene arms, in which all of the molecular variables can be controlled. However, the need to

otherwise. The source of possible heterogeneity and the potential problems associated with a strategy based on the use of DCMSDPE are discussed in detail below in the context of its use in the synthesis of the more complex dendritically branched polymers. 2.2. Comb/Graft Polymers. One might define a combbranched polymer as one having multiple branches pendant to the backbone in contrast to those H-shaped polymers described above which are characterized by the presence of branches at both ends of the polymer backbone. H-shaped polymers might therefore be considered to be a subclass of combs. The chemist wishing to make well-defined comb/graft polymers is faced with the problem of trying to control the exact position of the branch points and therefore the molecular weight between branches. It is easy to control the molecular weight of the backbone and arms independently, but it is not trivial to create a comb with many arms in which the chain length of the backbone “L” (Scheme 2) has a narrow dispersity, the number Scheme 2. Synthesis of Comb-Branched Polymer via Step Growth Coupling of Backbone Polymer Polymerized by Anionic Polymerization Initiated by Difunctional Initiator and Arms End-Capped with Difunctional Chlorosilane

of arms is constant for all molecules, and the molecular weight between arms “l” (Scheme 2) is also constant with a low dispersity. Two common approaches have been adopted, neither of which produces a structurally homogeneous comb. The first general approach produces a well-defined polymer backbone decorated with a number of reactive functionalities, to which can be grafted prepolymerized arms. There are a variety of ways of doing this, but to a greater or lesser extent, the number of reactive functionalities and their distribution along the backbone is uncontrolled. The net result of which is a distribution of the number of arms per comb molecule and a variation in the molecular weight between branch points (“l”, Scheme 2). These materials can be fractionated to narrow the distribution, but it is not possible to eliminate structural heterogeneity. A recent example75 of such a strategy involved the chloromethylation of a well-defined polystyrene backbone to introduce a number of benzyl chloride moieties, onto which were grafted living chains of polystyrene, end-capped with

Scheme 3. Synthesis of Comb Branched Copolymer with Polyisoprene Backbone and Two Polystyrene Arms77 (Reproduced with Permission from Ref 78)

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Scheme 4. Synthesis of Polystyrene Comb Branched Polymers with Precise Control over Backbone Chain Length, Arm Length, and Number and Position of Arms (Reproduced with Permission from Ref 78)

tion studies is the Cayley tree, DendriMac, or dendritically branched polymer. Dendritically branched polymers of the structure shown in Figure 1 might be considered to be long chain branched analogues of classical dendrimers, differing only in that they possess a polymer chain, sometimes of high molecular weight, between branch points. Furthermore, just as there are two general strategies for the synthesis of dendrimers, there are similarly two general approaches for preparing dendritically branched polymers: a divergent and convergent approach. A divergent approach involves a method in which the molecule is built from the core out toward the periphery. As the number of generations increases, there is an increase in the number reactive sites at the chain ends and with each new generation potential problems increase. First, any incomplete reaction of these terminal groups, either functional group modification or chain coupling, leads to imperfections in the subsequent generation, and moreover, the probability of imperfections resulting increases as the growing molecule increases in generations. Furthermore, as the generation number increases, with a concomitant increase in the number of reactive groups, it can become progressively more difficult to detect the result of incomplete reaction at the terminal groups. Characterizing chain-end modification reactions by NMR and detecting any possible imperfections can be more challenging in the case of the macromolecular dendritically branched polymers

control the stoichiometry in these reactions, the challenging nature of living anionic polymerization, and in particular its sensitivity to the presence of traces of impurities meant that this reaction scheme did not proceed without the production of defect byproducts, but the final fractionated comb had a PDI of 1.08. This general concept was subsequently modified and extended greatly by Hirao et al.78,79 In the most recent paper,78 Hirao describes a methodology in which the backbone is first prepared via an iterative approach, such that the backbone is decorated with DPE groups at fixed positions along its length and therefore with a precisely controlled molecular weight between functional groups (see Scheme 4). Addition of living polystyrene chains to the DPE functionalized backbone yields the comb branched polymer. This extremely well-designed strategy is iterative and time-consuming but has the particular advantage of coupling all of the arms to the backbone in a single reaction step. Fractionation was required to purify the resulting comb, but the contamination with byproducts appeared to be less of an issue than with the original methodology described by Hadjichristidis. However, to the best of this author’s knowledge, these combs have not been subjected to TGIC analysis nor have they been the subjected to rheological characterization. 2.3. Dendritically Branched Polymers. One of the optimal branched architectures for structure−property correla5625


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Scheme 5. Methodology Developed by Hadjichristidis for the Synthesis of Dendritic Polymers (Reproduced with Permission from Ref 81. Copyright 2002 John Wiley and Sons)

Figure 3. SEC analysis of two-level dendritically branched polymer synthesized by Hadjichristidis. Reproduced with permission from ref 81. Copyright 2002 John Wiley and Sons.

of the type described here, where the concentration of chain ends is very low with respect to the molecular mass. Moreover, although the increase in molecular weight of increasing generations maybe significant and possible to detect by SEC, detecting the presence of imperfections arising as a result of incomplete linking reactions in a divergent approach may be impossible by SEC due to the small differences in hydrodynamic volume, and it is possible that such imperfections may not be resolvable from the desired structure even by TGIC. The alternative convergent approach offers distinct advantages in that the molecule is built from what will ultimately become

the periphery, and at each step, growth is designed to occur via a very limited number of reaction steps. Furthermore, in the case of long-chain branched analogues of dendrimers, each coupling reaction can be analyzed by SEC. Incomplete reaction results in partially coupled material which is often easily detected, and in some cases, imperfectly branched material can be removed by fractionation. However, where the molecular weight difference between the desired and defect structure is small, the resolution limitations of SEC may fail to detect defect structures. Although careful analysis by techniques such as NMR, light scattering, and osmometry have also often been 5626


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while this approach does allow for the isolation of samples of the intermediate structures for characterization such a strategy does not offer the option to isolate intermediates for purification. Hadjichristidis, however, substantially improved this methodology in subsequent work by using 4(dichloromethylsilyl)diphenylethylene (DCMSDPE) (see Scheme 6) as the linking agent.46 As mentioned above, in the

used in tandem with SEC, analysis results using these conventional polymer characterization tools are insufficient to establish the purity of polymers with complex architectures,80 and the identification of defect structures for many years remained elusive. We will demonstrate later that TGIC offers a solution to this problem as a result of far greater resolution than SEC in these cases, and TGIC has emerged as a “must-have” technique for any group working in this field. One of the pioneering approaches to make well-defined dendritically branched polymers for structure−property correlation studies was published by Hadjichristidis,81 who adopted a living convergent strategy. This group used living anionic polymerization and a terminating/linking agent, 4(chlorodimethylsilyl)styrene (CDMSS; see Scheme 5), to which a living polymer chain (produced by anionic polymerization) is added to produce a macromonomer. Underpinning the entire synthetic strategy is the fact that the carbanion of a living polymer chain reacts preferentially with CDMSS through the chlorosilane group rather than the vinyl group, and this linking agent was the forerunner to DCMSDPE which was described above for its use in the synthesis of H-shaped polymers. This strategy was broadly successful in producing dendritically branched polymers with a high degree of a structural homogeneity, suggested by SEC which showed a monodisperse peak and low dispersity index (1.05−1.07) for the products (see Figure 3). However, there are a number of reasons to suspect the resulting products contain some dispersity both in terms of the resulting architecture and in the chain length of some of the linear segments, especially the inner polyisoprene segment. Several unavoidable problems affect this synthesis. First, while the reaction of the living polymer chains with CDMSS clearly occurs preferentially with the chlorosilane moiety, it does not do so exclusively. The authors report that about 5% of the living polymer chains react with the vinyl group rather than the chlorosilane, resulting in the production of a distribution of high molecular weight byproducts showing up as a broad peak, described as “in-chain living homopolymer”, to lower elution volumes in SEC (see Figure 3). In the following step, a sample of living polymer (see Scheme 5) is added to react with the vinyl group of the macromonomer to create an “in-chain living polymer”. This reaction was accompanied by the termination of a portion of the living chains. In the final two steps, isoprene monomer was added to the “in-chain living polymer” which acts as a macroinitiator, to produce a three-arm living star which was finally coupled to trichloromethylsilane to give the designed two-level dendritic polymers. It is very likely that the addition of isoprene to the macroinitiator could result in further termination (due to the accidental introduction of environmental impurities), which in turn could produce another defect and, more importantly, affect the molecular weight of the polyisoprene chain forming the inner segment of the final producta particular problem since the molecular weight and dispersity of this inner segment cannot be measured directly, only indirectly by subtraction of the molecular weight of the macroinitiator from the “off center living polymer”. Although imperfect selectivity of the reaction between the living carbanion and CDMSS is a problem, possibly a more challenging issue for such a strategy is the reliance throughout on maintaining a rigorously, impurity-free environment whereby traces of atmospheric impurities (H2O, CO2, and O2) result in unwanted side reactions such as premature termination of living moieties and incomplete coupling reactions. Moreover,

Scheme 6. General Reaction Scheme for the Synthesis of Second-Generation Dendritic Polymers (G-2) (Reproduced with Permission from Ref 46)

case of DCMSDPE, the steric hindrance of the two phenyl groups of the DPE derivative hinders reaction between the living polymer and the double bond, thereby rendering the reaction of the carbanion with the chlorosilyl bond far more selective than with CDMSS. In this later work, 2 equiv of living (polybutadiene) polymer was added to DCMSDPE by titration (monitored by SEC) to give a coupled polybutadiene chain with a molecule of DCMSDPE in the middle of the chain. Addition of sec-butyllithium to this “in-chain double bond PBd” (see Scheme 6) results in reaction with the double bond, which in turn creates a macroinitiator for the polymerization of butadiene. The resulting living three-arm star was added to trichloromethylsilane to yield a two-level dendritic polymer. It is clear when comparing the SEC profiles of the original procedure using CDMSS and the improved methodology using DCMSDPE (Figures 3 and 4) that the latter offers much better control over the resulting structure. However, it is also possible to see from the SEC of the final (crude) product (Figure 4d) where probable defect structures might arise. A shoulder at 22− 23 mL (retention volume) in Figure 4d indicates the presence of some residual “in-chain double bond” or deactivated “inchain living polymer” due either to incomplete reaction with sBuLi or termination of the macroinitiator upon addition of subsequent monomeralthough the latter possibility would seem more likely. However, either possibility will have implications for the actual molecular weight of the subsequently polymerized 5627


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Figure 4. SEC analysis of the various steps in the synthesis of G-2 dendritic polymer: (a) living PBdLi, (b) macromonomer, (c) dendron, and G-2 dendritic polymer before (d) and after (e) fractionation. Reproduced with permission from ref 46.

Scheme 7. Iterative Divergent Approach Developed by Hirao To Produce Dendrimer-like Branched PMMA (Reproduced with Permission from Ref 83)

“inner” polybutadiene segment. This is of some concern since the molecular weight of this segment can only be obtained indirectly from the characterization of the dendron since the

linear segment itself cannot be isolated.46 However, following fractionation, a monodisperse chromatogram was obtained with a quoted dispersity of 1.05. Moreover, this strategy was 5628


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all cases, there is a slight increase in Đ with increasing generations where theory would suggest that the Đ should decrease; this may suggest the presence of some structural heterogeneity. This approach has been extended to produce analogous branched polymers with up to seven generations of branching,50 branched polymers comprising of linear segments derived from more than one polymer type,54 and discussed in detail in a recent review article.67 Although to the best of our knowledge these PMMA branched polymers have not been subjected to TGIC to probe for the presence of structural heterogeneity, examples of these branched polymers with 1−4 generations of branching, each with an identical molecular weight have been the subject of rheological studies.45 Our contribution to this very active area of polymer science has been the development of DendriMacs, which are analogous to classical dendrimers in terms of both structure and the mode of synthesis. Where dendrimers may be synthesized by the condensation of low molecular weight AB2 monomers, DendriMacs are dendritically branched and prepared from α,ω,ω′ trifunctional AB2 condensation macromonomers hence the name DendriMacs. In an initial proof of concept study we prepared DendriMacs with 2 and 3 levels of branching (see Figure 1) from polystyrene macromonomers.43 DendriMacs were conceived and designed solely for structure− property (rheological) studies, and as such, when considering not only the design but also the synthesis of the macromonomer building blocks, the primary aims were to retain control over molecular weight and dispersity but also to quantitatively introduce the relevant chain-end functionalities that would facilitate subsequent polymer−polymer coupling reactions, and in common with the work of others described above, we adopted living anionic polymerization as the optimal technique for macromonomer synthesis. The appropriate end groups were introduced using 3-tert-butyldimethylsiloxy-1propyllithium, a lithium initiator containing a protected primary alcohol functionality and a readily synthesized diphenylethylene derivative containing two protected phenol groups in a controlled termination reaction, to yield polystyrene AB2 macromonomers with narrow molecular weight distributions.43 Deprotection of the alcohol groups by a mild acid hydrolysis and conversion of the primary alcohol to an alkyl chloride resulted in macromonomers which could be used to build up branched polymeric architectures using a Williamson coupling reaction (see Scheme 8). In contrast to the work of Hirao and in common with that of Hadjichristidis, we adopted a convergent coupling strategy and in this way were able to synthesize both G1 and G2 polystyrene DendriMacs. The convergent coupling strategy involves an iterative series of Williamson coupling reactions and end-group (primary OH to Cl) modification reactions. In our earliest attempts the Williamson coupling reactions were carried out selectively between an alkyl chloride functionality present on one macromonomer and the two phenol functionalities present on the other macromonomerthe phenol groups being selectively deprotonated under mild basic conditions (potassium carbonate). Following such a series of reactions, G1 and G2 dendrons were prepared, which in a final coupling reaction with 1,1,1-tris(4-hydroxyphenyl)ethanea trifunctional core yielded G1 and G2 DendriMacs (see Scheme 8). The described “condensation macromonomer” approach is not perfect by any stretch of the imagination but offers some distinct advantages over those discussed above. Although the polymerization of the macromonomers by anionic polymerization is plagued by the

successfully extended to synthesize dendritic polybutadiene with three levels of branchingand again, following fractionation, narrow dispersity SEC chromatograms were obtained. This strategy is effective and elegant but not perfect (none is), and three principal issues can be identified, namely (i) the reliance throughout on reactions which are sensitive to traces of environmental impurities, (ii) the inability to isolate intermediates for purification, and (iii) the inability to directly characterize the molecular weight of any linear segments apart from the outer arms. The implications of these issues and the generation of possible structural heterogeneity were recognized by this group themselves who in a subsequent paper47 reporting the linear and nonlinear rheology of these very dendritic polymers reported that “It is dif f icult to pinpoint the ef fect of dendritic topology on the relaxation dynamics without accurate molecular characterizations for arm molecular weights, polydispersity at each generation and structural heterogeneity due to possible imperfect linking reactions in the complicated multiple-step synthesis”. However, one particular advantage of this strategy is the relative speed (weeks I suspect) with which samples may be produced. Ironically, NOT isolating any of the intermediate samples for purification but proceeding with an approach exploiting “living coupling reactions” is very efficient in terms of time, and indeed, a wide range of dendritically branched polymers, with both 2 and 3 levels of branching, and a wide variety of linear segment lengths have been produced, many of which have been the subject of structure−property studies.11,47 A further notable contribution to this field is the work of Hirao et al., who developed an iterative divergent coupling strategy to synthesize dendritically branched polymers based on methacrylate monomers. Poly(methyl methacrylate) (PMMA) requires the use of a sterically bulky initiator to prevent side reactions between the initiator and monomer, and initiators derived from reaction between diphenylethylene (DPE) and butyllithium are often used.82 In this case anionic polymerization was used to prepare living chains of poly(methyl methacrylate) which were functionalized at the α (initiating) chain end through the use of a diphenylethylene derived initiator carrying two silyl-protected benzyl alcohol groups83 (see Scheme 7). The branched polymers were prepared by coupling reactions between the living functionalized PMMA chains and benzyl bromide moieties attached to the chain ends at the periphery of the growing “dendrimer-like branched polymer”an excess of living chains (1.5 mol equiv) with respect to benzyl bromide groups was used to ensure efficient coupling. Following coupling, the silyl-protected benzyl alcohol groups were converted to benzyl bromide groups to allow coupling of the next generation of living PMMA chains. This iterative process was repeated to yield a branched polymer with four levels of branching. Each functional group transformation reaction (silyl-protected benzyl alcohol to benzyl bromide) was followed by NMR, and each chain coupling reaction was followed by SEC. The crude products were fractionated to remove the excess unreacted PMMA chains to yield a polymer with a monomodal SEC trace and analysis which indicated a dispersity index of 1.03. Although the reactions appear to proceed exactly as predicted, it is doubtful that SEC would offer sufficient resolution to detect low concentrations of lower molecular weight defect structuresfor example, in branched polymer G3 (Scheme 7) it is inconceivable that SEC could resolve the desired structure from a defect structure in which one (or more) outer arms were missing. It is also notable that although the dispersity index (Đ) values (from SEC) are low in 5629


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of course in some way a direct consequence of one of the major advantagesthat is the ability to isolate, characterize, and purify each intermediate; purification by fractionation can be a time-consuming process and especially so if the coupling reactions are not clean and quantitativewhich they were not!43 The macromonomer coupling reactions must be carried out in a “good” solvent for the polymer and Williamson coupling reactions (SN2) are promoted by the use of aprotic solvents with a high dielectric constant such as THF and DMF, both of which were investigated. Reactions in THF were impractically slow; however, the coupling reactions proceeded rapidly in refluxing DMF, reaching completion in a matter of a few hours. Unfortunately at reflux, DMF undergoes partial degradation, and the extent of coupling reactions was somewhat limited by side reactions between the macromonomer and impurities generated by the degradation of DMF. As a result, coupling reactions were far from quantitative, and SEC analysis showed increasingly complex and multimodal distributions including peaks corresponding to uncoupled and partially coupled starting material. Of course, fractionation resulted in purification of the polymers; the desired intermediates and final products were isolated with narrow molecular weight distributions, but the purification process was laborious and resulted in the loss of substantial amounts of material. However, the efficiency and extent of the coupling reaction was dramatically improved during investigations in a parallel project into the synthesis of HyperMacs84,85 which exploited similar coupling reactions to make less well-defined, hyperbranched polymers from analogous macromonomers. In this project the extent and rate of coupling were enhanced by an order of magnitude by replacing the chlorine with bromine (better leaving group) and using cesium carbonatea particularly efficient base for Williamson coupling reactions of this type in DMF due to the greater solubility of both cesium carbonate and the resulting phenolate.86 These two modifications appeared to work in a synergistic fashion, increasing the rate of the coupling reaction, allowing the reactions to be carried out at temperatures as low as 30−40 °C.85 Although these dramatic improvements are welcome it should be pointed out that polystyrene is not the ideal polymer from which to make complex branched polymers for structure− property correlation studies. In order to obtain meaningful rheological data from such a model polymer, it is desirable and

Scheme 8. Synthesis of G1 and G2 Polystyrene DendriMac via Williamson Coupling of AB2 Macromonomers43

usual sensitivity to impurities, following end-capping by the DPE derivativea reaction which is close to quantitativeand termination, environmental impurities are no longer an issue. All of the subsequent coupling reactions and end-group modifications can be carried out under far less rigorous conditions. Thus, the molecular weight of the linear segments can be controlled independently, and each linear segment can be analyzed directly to obtain the molecular weight. This is simply not possible in the approach adopted by Hadjichristidis which leads to one source of uncertainty. Moreover, a second distinct advantage of the DendriMac synthesis is that the (intermediate) product of each coupling reaction can be isolated, not only for analysis but also for purification by fractionation before proceeding to the next step. This offers a far higher level certainty when it comes to structural integrity. There is no doubt that these are major advantages but our approach, like all the described strategies for the synthesis of complex branched architectures, has its limitations and disadvantages. Although in this initial study the desired branched architectures were realized, yields were modest and the full reaction scheme took many months to completefar longer I suspect that it would take to prepare an analogous polymer but some of the other methods described here. This disadvantage is

Scheme 9. Synthesis of G1 Polybutadiene DendriMac by Modified/Improved Synthetic Methodology42

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Figure 5. SEC analysis (top left), TGIC analysis (right), and schematic representation of (possible) branched architectures formed during the synthesis of H-shaped polymer by chlorosilane route. Reproduced with permission from ref 28.

probably necessary that the molecular weight between branch points be several times the entanglement molecular weight, Me. Polystyrene has one of the higher known values of Me (ca. 16 kg mol−1),23 and constructing a DendriMac from macromonomers with molecular weights approaching 100 000 g mol−1 would be extremely challenging. An obvious solution to this issue is to use a polymer with a lower value of Me, and thus the methodology was readily adapted to produce DendriMacs from polybutadiene which has a value of Me a little under 2000 g mol−1.19 The synthesis of polybutadiene macromonomers follows essentially the same reaction scheme as that described earlier for polystyrene macromonomers; however, polybutadiene is insoluble in the “textbook” aprotic polar solvents for Williamson coupling reactions (dimethylformamide, dimethylacetamide, acetone, etc.). Overcoming this required a certain amount of trial and error; however, a mixed solvent of tetrahydrofuran and dimethylacetamide (50/50 v/v) proved successful, the former being a good solvent for polybutadiene but with a modest dielectric constant whereas the latter is a poor solvent for polybutadiene but has a high dielectric solvent. The improved Williamson coupling conditions were combined with a further modification in which the intermediate three-arm asymmetric star was produced using chlorosilane coupling42 (see Scheme 9). Following this modified/improved approach, model polybutadiene DendriMacs have been produced and, following fractionation, subjected to rheological studies. In a very recent paper87 we have described a highly collaborative effort to design in silico a two-level (G1) polybutadiene DendriMac with the optimal molecular structure for rheological studies, its subsequent synthesis, molecular weight characterization (by SEC), and a comparison of experimental rheology and a theoretical prediction of that rheology. The agreement between the prediction and the experimental rheology was extraordinarily good, but there is one more piece to add to this interdisciplinary jigsaw. The use of chlorosilane coupling chemistry for the synthesis of the intermediate star was an attempt to accelerate the synthetic process, but as we would

eventually come to learn, these modifications resulted in the production of defect structures. We too (initially) made the assumption that a monomodal symmetrical peak in the SEC trace and narrow molecular weight distribution were indicative of perfect structural homogeneity, i.e., the presence of no defect structures. However, it was around the time that we were carrying out these experiments that we became aware of TGIC and in collaboration with Chang (who developed TGIC); we obtained TGIC analysis of this DendriMac and discovered that this “perfect” DendriMac was not quite as perfect as we had hoped.

3. THE CASE FOR TEMPERATURE GRADIENT INTERACTION CHROMATOGRAPHY As mentioned above, for decades SEC has been the primary method for the characterization of polymer molecular weight and molecular weight distribution; however, by virtue of the mode of separation (by molecular size rather than molecular weight) SEC has an intrinsic limitationnamely, that it is incapable of separating polymers with identical or nearly identical hydrodynamic volumes, and such a limitation is a particular concern for the characterization of model complex branched polymers of the types discussed here. As we have shown, even with the most sophisticated synthetic strategy, the potential for defect byproducts arising from incomplete branching and premature termination of living systems, without exception means that it is practically impossible to synthesize complex branched polymers with a perfectly uniform chain structure. While SEC results may suggest a high degree of molecular homogeneityand the literature is full of reports of monomodal distributions with low dispersityin reality the detection of significant levels of branched byproducts by SEC is often impossible given the very small differences in hydrodynamic volume. The presence of even small quantities of defects and byproducts can (but does not always) have significant implications when relating the polymer structure to experimental rheology whereby discrepancies make the 5631


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Figure 6. SEC (right) and TGIC (left) data for a series of fractionated H-shaped polybutadienes.88 In each case, sample code HAxBy, x is the molar mass of the arm and y the molar mass of the backbone. Reproduced with permission from ref 88.

laboratories) suggested that the products were in fact a binary blend of the desired H-shaped polymer and three-arm star the precursor to the formation of the H-shaped polymer (see Scheme 1)while TGIC revealed a greater degree of structural heterogeneity. Each sample was shown to contain multiple low molecular weight byproducts and one particular case (HA12B40; Figure 6), at least six peaks were resolved, and it was estimated that the desired product accounted for as little as 30 wt % of the mixture. Each component of the mixture was isolated by TGIC fractionation and then analyzed by laser light scattering. From the resulting molecular weights and knowledge of the synthetic methodology the authors were able to postulate the structure of each component in the mixture from at least nine (or more) possible byproducts. The nine most likely byproducts along with the desired H-shaped polymer are shown in Figure 7. The desired H-shaped polymer (10) is obtained by coupling together two (living) 3-arm stars (5) hence, the backbone in structures 8−10 is shown as a longer blue segment formed by coupling the shorter blue segments as shown in 5. It is clear that the authors revealed a whole zoo of possible structures which can be formed during the synthesis as a result of premature termination reactions, incomplete linking

validation and subsequent modification of theoretical models impossible. TGIC separation is believed to be driven by enthalpic interactions between the solute molecules and the stationary phase and these interactions, which can be controlled by temperature variation during the elution, are to a first approximation proportional to the molecular weight, NOT the hydrodynamic volume,71,72 and TGIC has emerged in recent years as a particularly valuable tool (complementary to SEC) for the analysis of complex model-branched polymers. In the following section we will review how TGIC has recently been exploited to analyze the products of some of the syntheses described above and how TGIC has shone a light on structural heterogeneity. 3.1. TGIC Analysis of H-Shaped Polymers. The earliest example of TGIC analysis of an H-shaped (polybutadiene) polymer dates back to 2001.28 In this work, the synthesis of the branched polymer followed the commonly used chlorosilane route (as described above), and following fractionation, SEC indicated an apparently pure product. However, TGIC analysis (Figure 5) revealed the presence of a number of species which had not been removed by fractionation and showed that only ∼70% of the product was the desired H-shaped polymer which was contaminated with 25% of low molecular weight byproducts and 5% high molecular weight byproducts. Online light scattering analysis of the TGIC data suggested that the structures of the byproducts are those shown in Figure 5, and a look at the synthetic method would suggest that the low molecular weight defects (in green, Figure 5) arise from incomplete linking of the arms to the backbone, possibly due to steric hindrance, and the high molecular weight defects (in blue, Figure 5) arise from backbone−backbone coupling upon addition of the chlorosilane to the living difunctional chain. It is likely that the presence of such defect structures, especially the low molecular weight defects, would dramatically alter the rheological properties of this sample. In order to avoid the production of such defects Mays took an alternative approach for the synthesis of H-shaped polybutadiene26 (described above), and once again SEC analysis of the fractionated product indicated a narrow molecular weight distribution and a high degree of structural homogeneity. However, subsequent studies88 involving further SEC analysis and TGIC analysis showed that a number of byproducts had been formed. Additional SEC studies (carried out in three separate

Figure 7. The most probable products and byproducts arising from the synthesis of H-shaped polymer using DCMSDPE as linking agent.26,88 5632


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Figure 8. SEC (left) and TGIC (right) of H-shaped polymer (a, in red) and H-shaped polymer with additional identical arm pendant to the backbone (b, in black). Reproduced with permission from ref 89.

Figure 9. Master curves for the storage and loss moduli and loss angle (tan δ) as a function of angular frequency (aTω) for the original (solvent fractionated) samples and the IC fractionated samples. Sample a: (○) original sample, (red ●) IC fractionated sample. Sample b: (○) original sample, (blue ●) IC fractionated sample. Reproduced with permission from ref 89.

reactions, and the coupling of byproductsa consequence of the use of “living coupling reactions” which are extremely sensitive to the presence of traces of environmental impuritiesand the need to precisely control the stoichiometry of reactants. In the case of HA12B40, the authors associated peaks 1−6 in the TGIC trace (Figure 6) with structures 2, 4, (7 or 5), 6, 9 ,and 10 (the perfect H) in Figure 7. The increased sensitivity of TGIC over SEC in this case is remarkable. In a further paper29 by Larson and Mays, a new H-shaped polymer (HA20B40) and a symmetric star-shaped synthetic precursor of HA20B40 were prepared by the same methodology. These polymers were submitted to characterization by TGIC and SEC, analyzed by rheometry, and the resulting characterization data were used to test advanced tube models for long-chain branched polymers. The structural detail/heterogeneity revealed by TGIC prompted these workers to probe the rheological properties of blends of well-characterized linear and star-branched polymers with HA20B40, thereby mimicking the effect of structural byproducts in the sample and allowing them to test the ability of the hierarchical model to account for the effect of such impurities. Modeling predictions for HA20B40 and its blends with star and linear polymers showed good agreement with experimental rheological data, indicating that the modeling validation is successful for the symmetric Hshaped polymers. This work further demonstrates the value of TGIC characterization and emphasizes the fact that well-

characterized, but structurally heterogeneous, branched polymers can provide data which is every bit as valuable in the attempt to validate models of the tube theory. Recently, Hadjichristidis et al.74 synthesized H-polymers and H-polymers with a single branch pendant to the backbone (see Figure 2), following an almost identical method to that of Mays.26 In a subsequent study,89 when they too were characterized by TGIC analysis, they were found to be contaminated with a similar selection of byproducts to those indicated in Figure 7. Adopting a similar philosophy to that of Larson and Mays,29 TGIC was used to identify and quantify the amount of each byproduct, and the impact of these byproducts on the rheological properties was evaluated. In this study, two of three branched polymers discussed earlier were investigatedsamples a and b in Figure 8. The original samples had been purified by solvent fractionation (toluene/methanol) to yield products with dispersity values of 1.04 and 1.09 for (a) and (b), respectively; however, even SEC analysis of the two original samples suggests some structural dispersityespecially in the case of sample b which has a clear shoulder to lower retention times (high molecular weight) and some tailing to higher retention times (Figure 8). TGIC analysis of these samples once again reveals the presence of byproducts, with these being far more prevalent in the case of (b), not surprising given a Đ index value (from SEC) of 1.09. Moreover, TGIC indicates that the main peak of samples a and bin each case 5633


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the target structurerepresents about 85% and 60%, respectively, by mass of the original sample. A combination of theoretical analysis and TGIC indicated the presence of more than ten separate structures in the original solution fractionated samples, with molecular weights both higher and lower than the target structure. In an attempt to further purify these branched polymers, they were fractionated by interaction chromatography (IC) using the same solvent, column, and flow rate as the TGIC analysis. However, the fractionation was carried out under isothermal conditions. Repeated fractionation using an automated system yielded sufficient material to allow subsequent chromatographic characterization and rheological measurements. IC fractionation appeared to deliver a highly purified sample of (a), but sample b, even after IC fractionation, appeared to be contaminated with byproducts. The authors suggest that sample b proved difficult to purify by IC fractionation due to the presence of byproducts having a similar molecular weight to the target structure. A comparison of the rheological response of the original solution fractionated samples and the samples subjected to further IC fractionation threw up some interesting and alarming (sic) results. Briefly, the additional IC fractionation step made little or no difference to the viscoelastic response (see Figure 9). While it might initially be assumed that architectural dispersity therefore has little consequence upon the rheology, this is probably not the case. In this case the authors attribute the negligible difference between the master curves of the samples of differing purity to the presence of fast relaxing byproducts and slow relaxing byproducts which effectively cancel each other out. This conclusion is entirely feasible given the wide range of defect structures produced in the described synthesis but presents something of a problem. I will argue in my concluding remarks that the real power of TGIC is in the elimination of uncertainty. TGIC in many cases offers a far more detailed examination of the structural dispersity than could ever be possible with SEC and allows not only for identification of possible defect structures but also quantitative data about how much of each defect structure is present. In the case of the H-shaped polymers produced by Hadjichristidis et al., TGIC coupled with statistical analysis suggests a plethora of potential defect structures arising from the synthetic strategy, many of which have such similar structures that they may not be resolved by SEC or TGIC and cannot be removed by IC fractionation. The conclusion by the authors that unidentified byproducts probably cancel each other out in terms of their rheological response is probably correct but does leave us with a degree of uncertainty. One other possible consequence of this particular synthetic strategy that is not considered in this analysis is dispersity/errors in the molecular weight of the linear segments. If one revisits the synthetic strategy exploited by both Mays and Hadjichristidis (Scheme 1), one can see that the synthesis of the crossbar/backbone involves the reaction of butyllithium with the double bond of a DPE moiety to which have been coupled two chains, followed by the addition of butadiene monomer (see Scheme 10). It is clear that should any defects or imperfections arise during the preceding steps, should the stoichiometry be less than perfect or should any impurities be introduced during the addition of the butadiene monomer, then errors may arise in the molecular weight of the resulting polybutadiene chain. The molecular weight is likely to be different to that which was intended, and unfortunately it is not possible to independently verify the molecular weight of this linear segmentthe molecular weight can only be obtained by

Scheme 10. Part of the Synthetic Methodology Exploited for the Synthesis of H-Shaped Polymers by Both Mays26 and Hadjichristidis74 (Reproduced with Permission from Ref 26)

subtraction. It is clear from the discussion above that defect structures do arise during the early stages of this synthesis and therefore the potential error in the molecular weight of the cross-bar does not seem to have been given sufficient consideration. 3.2. TGIC Analysis of Comb Polymers. We described above the challenges associated with the synthesis of perfect model comb polymers in which all of the molecular variables are controlled. Although these challenges seem to have been met first in preliminary work by Hadjichristidis77 and subsequently in a more developed approach by Hirao78,79 by the adoption of a careful, iterative approach, in neither case, to the best of this author's knowledge, have these materials been subjected to TGIC. However, there is one notable report of TGIC analysis of comb polymers. In this case, Fernyhough et al.75 (described above) prepared initially disperse polymers in which the molecular weight of the backbone and arms can be controlled but the number of arms is less well controlled. The samples were fractionated to produce samples of combs with a (narrower) dispersity index of 1.1 and an average of about 4 arms per backbone. SEC analysis indicated a single peak with no detail relating to the distribution of number of arms per comb, only the average number of arms. However, TGIC is capable of revealing far more detail of the distribution of structures present. Two polystyrene combs were analyzed, PSC2 and PSC3.75 PSC2 had a backbone molecular weight of 176 000 g mol−1 and arms of 63 000 g mol−1, and PSC3 had a backbone molecular weight of 190 000 g mol−1 and arms of 83 000 g mol−1. TGIC analysis of both combs (Figure 10) clearly indicates a distribution of structures of increasing molecular weight which elute with increasing time and temperature. The results for PSC3 are particularly striking, and peaks or shoulders corresponding to combs with 1 to 8 arms are clearly visible. This direct information on the distribution of the number of arms on the comb molecules was used for modeling the linear rheology of an entangled melt of such combs. A successful prediction of the linear rheology was obtained using a previously published algorithm8 which compared favorably with the experimental linear rheology. However, once again it is worth noting that even TGIC has its limitations. In a subsequent paper90 2D-LC analysis of PSC3 revealed further structural detail, specifically the presence of a second series of combs (at low concentration) formed from a coupled backbone. This additional level of detail was in part revealed due to the different retention properties of deuterated and hydrogenous polystyrenethe backbone was prepared using hydrogenous PS, and the arms of the comb were deuterated. 3.3. TGIC Analysis of Dendritically Branched Polymers. The benefits of TGIC analysis of complex branched 5634


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Figure 10. Normal phase TGIC chromatograms of PS combs (a) PSC2 and (b) PSC3 recorded by RI (solid curves) and LS (dashed curves) detectors. The molecular weight distribution curves were obtained from the LS detection and are shown in the plots. Reproduced with permission from ref 75.

Figure 11. Chromatograms of polybutadiene G1 DendriMac obtained by size exclusion chromatography (a) and temperature gradient interaction chromatography (b). The dotted lines are an estimate of the relative concentrations of each species obtained by deconvolution of the full chromatogram using a Guassian model distribution function. Reproduced with permission from ref 87.

3.1. Moreover, it is likely that these syntheses will produce a similar range of possible defect structures as produced in the synthesis of H-shaped polymers (see Figure 7), many of which could not be resolved by TGIC or removed even by IC fractionation. It would be wrong to imply that the previously reported investigations into the rheological properties of dendritically branched polymers have not made a significant contribution to the understanding of branched polymer rheology; however, one suspects that TGIC analysis of the dendritically branched polymers produced by the same synthetic methodology would reveal valuable additional information about the extent of structural dispersity, the distribution, and quantity of defect byproducts, thereby minimizing uncertainty in comparing experimental rheology and theoretical predictions of rheology. We have previously collaborated with Chang (who developed TGIC as a technique) and recently established a TGIC system in Durham with the primary aim of enhancing the characterization capability for branched polymers. In a recently published paper87 we described the synthesis and characterization of a two-level polybutadiene DendriMac according to the methodology described above and shown in

polymers has been amply demonstrated by its use in characterizing both H-shaped polymers and combs. The additional information gained is beyond the scope of SEC alone due to the inherent limitations of SEC, and the presence of defect byproducts has been revealed by TGIC where the presumption of purity had previously existed. A monomodal peak with a dispersity index of 1.1 or lower can no longer be taken as evidence of structural homogeneity. Despite the fact that this has become increasingly apparent in recent years, to date only one group (ourselves) have subjected dendritically branched polymers to TGIC. We have discussed above the synthetic approaches taken by a number of groups to prepare such polymers, and the dendritically branched polymers synthesized by Hadjichristidis46,81 have been those most thoroughly investigated by rheometry, as model polymers for structure−property correlation studies.11,47 The strategy adopted for the synthesis of these dendritically branched polymers is identical to that used by the same group for the synthesis of H-shaped polymers as described above (see Schemes 1 and 6), and it is probably safe to assume that the synthesis of the dendritic polymers will be subject to the same issues described above for the H-shaped polymers in section 5635


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if such a defect was present at this low concentration, its impact upon the rheology would be immeasurably small. What was maybe more surprising (initially) was that the impact of the low molecular weight defect structure also had precious little impact upon the rheology. It can be seen in Figure 12 that the

Scheme 9. This slightly modified method to include chlorosilane coupling in the initial stages did expedite the synthesis but was not without its problems as revealed by TGIC. SEC analysis of the two-level DendriMac (following fractionation), in common with many of the other studies described here, initially suggested a high degree of structural homogeneity, as evidenced by a monomodal peak with a narrow dispersity index (1.05) (Figure 11a). However, TGIC analysis revealed the presence of a lower molecular weight byproduct (at lower retention volumes) in the form of a shoulder on the TGIC chromatogram (Figure 11b). Online molecular weight analysis allowed an estimation of the molecular weight of the main peak and shoulder on the TGIC chromatogram, and it was concluded that the most likely structure of the lower molecular weight byproduct is that inset in Figure 11b, that is, a G1 DendriMac with one of the outer arms missing. One obvious possible source of this defect is incomplete linking of the outer arms in the synthesis of the intermediate asymmetric 3-arm star (Scheme 9). Unfortunately, TGIC analysis of the intermediate star was not carried out so we cannot verify that possibility. However, a second possible explanation lies in the susceptibility of the branch point linkage to cleavage under the conditions (mild acid hydrolysis) used to deprotect the primary alcohol functionality. Quirk et al.91 have previously reported arm cleavage when hydrolysis was carried out for prolonged reaction times in an identical deprotection reactionan observation that we also initially made during the synthesis of such DendriMacs.42 Attempts were made to minimize arm cleavage by carrying out the deprotection reaction under high vacuum for only 30 min in presence of BHT (0.1% w/w) at 70 °C, but it is possible that arm cleavage was not completely eliminated. Although the intermediate star was purified by solvent fractionation to give a monomodal, narrow dispersity peak, it is unlikely that SEC would have been able to resolve the star from traces of the defect structure with one of the shorter arms missing. Clearly it is disappointing for us, the synthetic chemists, to discover such impurities in our products; however, in this case it is worth noting that the desired structure was present as the major product (estimated at 83%), and the byproduct was present at a total weight percent of 14%. This in fact means that only 5% of the compound arms have a missing outer arm or (equivalently) that only 2.5% of the outer arms are missing. We believe that these results demonstrate an unprecedented level of both structural characterization and structural homogeneity for a Cayley tree type polymer. It is also worth noting that there was also some suggestion from the TGIC data (Figure 11b) of slight broadening to higher retention volumes, possibly suggesting the presence of a trace of a high molecular weight defect structure. Deconvolution of the TGIC data suggests if such a defect is present, it is present at a very low concentrationless than 4%. Although during this study we could not be certain that a high molecular weight byproduct was present (due to the low concentration), we did consider its possible origin and its potential impact upon the rheological properties of the DendriMac. One possible origin of the high molecular weight defect is the formation of small amount of the intermediate asymmetric star polymer with two long arms and one short arm (Scheme 9) which in turn could result in a DendriMac in which one of the compound arms has one long outer arm and one short outer arm. It is conceivable that such a defect could arise as a result of the synthetic methodology although theoretical predictions of the rheology suggested that

Figure 12. Rheological moduli of G1 DendriMac from small amplitude shear experiments (symbols) and the computational prediction of the same with segment lengths chosen from the SEC measurements (lines). The dashed red line considers perfect DendriMacs, while the solid black line considers 14% by mass of the material having one of the outer segments missing (as suggested from the TGIC measurements; Figure 11b). Reproduced with permission from ref 87.

agreement between the experimental rheology (the open symbols) and the theoretical predictions of the rheology (lines), calculated using BoB, a previously published algorithm,8 is extraordinarily good. Moreover, the prediction of the rheology of perfect DendriMacs (red dashed line) is almost indistinguishable from a comparable prediction in which the DendriMac is contaminated with 14% by mass of the low molecular weight defect (solid black line) as suggested by TGIC analysis. This observation however is less surprising when one considers that the presence of this defect at 14% corresponds to only 2.5% of the outer arms being missing. So it is clear that this synthetic methodology is not without problems and could result in the production of defects which are both high and low molecular weight with respect to the target structure, resulting from incomplete/imperfect linking reactions, and the susceptibility of the resulting linkages to cleavage being a clear weakness. However, we believe this strategy probably produces a narrower range of defect structures than some of the other synthetic approaches described here, a belief reinforced by TGIC which did allow us to identify and quantify the presence of defect byproducts and therefore consider their impact on rheology.

4. CONCLUSIONS AND PERSPECTIVE The synthesis of all of the described model complex branched polymers has made an invaluable contribution to, and have made possible significant advances in, understanding and predicting the rheology of branched polymersan exercise that has been ongoing for decades with direct relevance to industrial polymer processing. Moreover, the contribution of 5636


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it generally minimizes the range of possible defect structures, and in theory, the macromonomer approach should only produce defects with molecular weights lower than the target structure as a result of incomplete coupling reactions. In practice, this has not always been the case, and in particular the modified approach adopted to synthesize two-level polybutadiene DendriMacs42,87 did result in some high molecular weight defects in the crude product. This may be the direct result of adopting the chlorosilane route in the initial stages and that, combined with the fragility (under hydrolysis conditions) of the linkage introduced through chlorosilane coupling, has led us to conclude that this approach is not optimal despite being less time-consuming and having delivered a nearly perfect DendriMac. The Durham macromonomer approach has not been adopted yet by other groups preparing model polymers for the study of rheological properties but has been exploited by many others.68 It is possible that the reason this approach has not been more widely adopted or recognized is due to the fact that it is a very much slower process, and as a direct consequence, materials and results have been slower coming to press. As alluded to above, we believe the condensation macromonomer approach offers further scope for improvement. The current disadvantages of this approach are interconnected, namely that the process is extraordinarily slowit may take many months to synthesize (and purify) a single sample from start to finish, and much of this time is consumed in purifying intermediate and final products by solution fractionation. This problem is exacerbated by the fact that the Williamson coupling reactions are not always terribly efficient or quantitative. In our initial proof of concept43 study whereby polystyrene macromonomers were coupled via a Williamson coupling reaction between an alkyl halide and a phenolic alcohol, the reactions were seriously hampered by side reactions, and highly disperse products were formed as a result of incomplete linking reactions. This made fractionation a lengthy and tedious process always resulting in the loss of substantial amounts of material. The efficiency of the coupling reactions was improved somewhat in a parallel study,85 but there is still much room for improvement. We are currently investigating the use of “click” coupling reactions which are often very efficient and may be carried out under more benign conditions. We hope to report our initial results in the near future, but early indications are that click coupling will offer substantial improvements in the efficiency and extent of coupling. So, this author still believes the chemist has a significant role to play in delivering cleaner, purer, complex branched polymers with a significantly lower degree of structural dispersity. However, there is no doubt that one of the most significant breakthroughs in this field in recent years is the development and adoption of TGIC as a characterization technique to complement SEC. It is clear from the examples described above that TGIC is capable (in the majority of cases) of delivering a level of structural characterization way beyond that possible with SEC alone. In every case where TGIC has been used to characterize complex branched architectures, including our own, structural heterogeneity has been revealed, which had hitherto been hidden from sight. Although for many years many of us had suspected that a dispersity index of 1.05 may not be indicative of perfect structural homogeneity, there had also been a certain amount of “out of sight, out of mind” attitude. It is clear that TGIC must inevitably become a fundamental part of the jigsaw, alongside careful (improved) synthesis, SEC, and

the polymer chemists whose work is discussed in this Perspective is prodigious. However, without exception, the synthesis of such polymers, described herein, is timeconsuming, complicated, and despite the best efforts of careful chemists can (and usually does) result in defect byproducts as a result of a reliance (to a greater or lesser extent) on reactions that are sensitive to traces of environmental impurities, incomplete linking reactions, imperfect control of stoichiometry, and in some cases the inability to isolate intermediates for purification. Further uncertainty arises from the inability to directly characterize the molecular weight of linear segments in some cases. In the opinion of this author, the approach that relies upon the selective coupling of living chains to DCMSDPE and subsequent reinitiation of living chains with a macroinitiator suffers more than most from these issues. However, I am not so pessimistic about the future contribution of the chemist as some89 who posed the question “how model are model polymers?” and responded with the answer “as model as you can get”. The pervading view appears to be that as far as the chemistry is concerned, the current state-of-the-art is as good as it is likely to get, and even complex branched polymers produced by living anionic polymerization are never going to be clean enough or sufficiently free from structural heterogeneity to make the impact of defect structures upon rheological properties negligible. There are some elements of truth in this pessimistic view, and it is undoubtedly the case that relying solely upon anionic polymerization/living coupling reactions is unlikely to result in significant advances in the development of new synthetic methodologies. However, I believe our recent paper87 shows that it is possible to prepare complex branched polymers with very low levels of structural defect, and we found that as a result of the particular structure of the defect and its low concentration, the impact upon the rheological properties was immeasurable. While the DendriMac reported in this paper was, following solution fractionation, as pure as (or purer than) comparable dendritically branched polymers, I believe the approach developed in Durham in recent yearsthe condensation macromonomer approach still offers room for improvement. The basis of this approach is to synthesize the linear segments by living anionic polymerization, thereby controlling molecular weight and minimizing dispersity of the linear segments but also allowing the (quantitative) introduction of reactive functionalities at each chain end through the use of functionalized initiators and endcapping with diphenylethylene derivatives.43 The molecular weight and dispersity of each linear segment can be obtained directly by SEC, thereby removing one possible source of uncertainty that arises in alternative methods. The subsequent coupling of chains to build up branched architectures occurs via condensation reactions which no longer rely on high-vacuum techniques to maintain the rigorously pure environment required to successfully carry out anionic polymerization. This modular approach allows us to isolate and characterize the product of each coupling reaction and to purify intermediates by fractionation before proceeding with the next step. This step is key in both establishing the molecular weight of the intermediate products but also, in removing defect structures at this intermediate stage, it is possible to minimize the scope for making further defect structures in subsequent steps. Recent studies have shown that in some cases it is possible to generate a very wide range of possible defect structures accompanying the synthesis of complex branched polymers (see Figure 7); thus, another advantage of the macromonomer approach is that 5637


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experimental rheology if, as a community we are to validate existing theories of rheology and develop improved models. It is obvious that in many cases structural heterogeneity will have a significant impact upon rheology, and there may be some merit in revisiting previous studies in which apparently homogeneous model polymers were used to test the theories of branched polymer rheology, with the assumption that the tested polymers were probably contaminated with undetected defect byproducts. One final point. In one of the recently published reports89 exploring the rheological consequences of structural dispersity in branched polymers, TGIC revealed a zoo of possible byproducts, yet even IC chromatography was unable to remove some of these to generate a pure sample. The authors were certain that (low levels of) defects were still present in one particular IC fractionation purified sample and yet could not be certain what these defects were. The impact of these unidentified defects upon the experimental rheology was not significant, and the conclusion was that in a blend of the desired structure, fast relaxing defects and high molecular weight, slow relaxing defects, the defect structures effectively canceled each other out. This explanation is entirely plausible but presents a situation which is not entirely satisfactory. The testing of current theories and models of rheology requires unambiguous experimental data. Overwhelmingly, the most significant contribution of TGIC is that in the majority of cases TGIC has delivered a level of structural analysis which has removed a great deal of uncertainty and ambiguity. However, the final case in point above shows that ultimately it may fall to the chemist to devise synthetic routes which produce complex branched architectures with a much narrower range of defect structures which can either be readily removed by fractionation or at least be readily identified and quantified by TGIC.

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a group of synthetic polymer chemists who exploit controlled/living polymerization mechanisms to make polymers which are well-defined in terms of molecular weight, polydispersity, and chemical functionality. Of particular interest are the synthesis of complex branched architectures and functional polymers to modify surface and interfacial properties. He is the deputy director of the Durham Centre for Soft Matter, is a member of the editorial advisory board of Macromolecules, and has published more than 60 articles in peer reviewed journals and in 2008 received the Arthur Doolittle award from the PMSE division of the ACS.

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ACKNOWLEDGMENTS The author acknowledges the financial support of the Engineering and Physical Sciences Research Council for financial support across a number of years. Much of the author's work described here was carried out as part of the MuPP project (GR/T11838/01). Moreover, I acknowledge the ongoing support of the Department of Chemistry, Durham University, and colleagues across the UK, who have contributed directly or indirectly to this work. Finally, special thanks must be made to Professor Taihyun Chang of Pohang University of Science and Technology, Republic of Korea, for his collaboration and support in helping us establish TGIC in Durham.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography

Dr. Lian Hutchings is currently a Reader in Polymer Chemistry at Durham University in the United Kingdom. He obtained his degree (1989) and Ph.D. (1993) from the University of Leeds before moving to the Chemistry Department at Durham University in 1993 as a research assistant working with Randal Richards. He has been promoted through the ranks, becoming a Reader in 2011. He leads 5638


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Complex Branched Polymers for StructureâProperty Correlation

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