Anionic polymerization branched polystyrene

Unlike radical polymerization, branched polystyrenes having a variety of controlled structures have been synthesized (Figure 24.10). This is because termination can be precisely controlled. The branched polystyrenes synthesized using anionic chemistry have been used to study the effect of branch structure on rheology [15]. As will be discussed in the next section, branch architecture (like those presented in Figure 24.10) can influence the rheological properties of polystyrene resins. 

Fig.46. Dependence of [r ] on theMn of polymers prepared by anionic polymerization of 1,4-DVB in THF. The symbols represent linear ( ) branched (V) and microgel ( ) structures. The dashed line represents the [iq]/Mn relationship of anionically prepared polystyrene. [Reproduced from Ref. 231 with permission, Hiithig Wepf Publ., Zug, Switzerland].

The anionic homopolymerization of polystyrene macromonomers was carried out successfully. The methacrylic ester sites at the chain end do not require very strong nucleophiles to be initiated diphenylmethylpotassium was used, and the process was carried out at — 70 °C in THF solution24). The products are comparable with those obtained by free-radical polymerization. The molecular weight distribution should be narrower but this cannot be easily checked because these polymer species are highly branched and compact as already mentioned. 

Polymers are produced by one of two processes, addition or condensation polymerization. Addition polymerization occurs by one of three mechanisms, radical (e.g., low density branched polyethylene), cationic (e.g., butyl rubbers), or anionic (e.g., polystyrene). Condensation polymerization is used to produce Nylon 6,6 from adipic acid and hexamethylenediamine with the elimination of water. Industrially,... 

The major forte of anionic polymerization has been the ability to prepare polydlene and polystyrene polymers and copolymers with control over the major variables affecting polymer properties. Researchers continue to exploit this method for the preparation of model block copolymers, graft copolymers, and branched copolymers, and homopolymers with controlled, well-defined structures. The ability to prepare well-defined polymers and copolymers with functionalized end groups, especially ionic or ionizable groups, is also generating considerable current interest. Methods are also being explored to anionically pol)rmerize and copolymerized a variety of polar monomers with controlled structures. The current interest in blends and... 

Developments in the anionic polymerization of butadiene were adopted for manufacture of solution SBR. While the emulsion process gave primarily 1,4-cis microstructure in the final product, the solution process gave a lower level of 1,4-cis level, typically around 45%. Furthermore the cis content as well as 1,2-vinyl content could be modified. In addition, better control of branching and molecular weight distribution attainable with anionic process made solution SBR suitable for tire applications, challenging the established use of cold SBR. Developments in the anionic process also led to new copolymer structures in which blocks of polybutadiene can be coupled to blocks of polystyrene, generating a imique class of polymers. Developments in SB block copolymers led to new materials which were thermoplastic in character, unlike SBR which is an elastomer. Solution-processes-based thermoplastic SB block copolymers form the basis of the transparent impact polystyrene (TIPS) as well as the other block copolymers used in plastics modification. The block copolymers of styrene and butadiene are the subject of the second part of this article. 

The list of monomers compatible with anionic polymerization overlaps the radical list (Figure 13.16) considerably, but the unique features of anionic polymerization mean that the same polymer can be a different material. For example, one important feature of anionic polymerizations is that they show a tendency to produce stereoregular polymers, in contrast to radical polymerizations. As such, polystyrene produced by anionic polymerization is more crystalline than polystyrene produced by radical polymerization. This is just another example of how one polymer— polystyrene—can represent several different materials. Note also that because carbanions generally do not abstract protons from C-H bonds, chain transfer (and branching) is typically not a problem in anionic polymerizations. 

Hirao, A. and Ryu, S.W. (2(X)3) Synthesis of teanched polymers by means of living anionic polymerization, 11. Anionic synthesis of densely branched polystyrenes carrying double branches in each repeating unit. Macromolecular Symposia, 192,31 2. 

Knauss, D.M. and Huang, T. (2003) ((PS)nPS)n, star-shaped polystyrene with star-shaped branches at the terminal chain ends by convergent living anionic polymerization. Macromolecules, 36,6036-6042. 

Yoo, H.-S., Watanabe, T., and Hirao, A. (2009) Precise synthesis of dendrimer-like star-branched polystyrenes and block copolymers composed of polystyrene and poly(methyl methacrylate) segments by an iterative methodology using hving anionic polymerization. Macromolecules, 42,4558 570. 

Star polymers may be considered to be highly branched polymers that have linear chains radiating out from a central area. This area may be one atom, a small molecule, or a "core". The "core" is a quasi-spherical structure as opposed to a linear structure that would be present in a conventional comb or branched polymer. An early example of stars made from small molecules is the star polymer of Schaefgren and Flory (1) who polymerized E-caprolactam in the presence of a tetrafunctional or octafunctional carboxylic acid to produce polymers that have 4 or 8 arms radiating out from a central molecule. Other examples use the coupling of "living" anionically polymerized polystyrene with silicone tetrachloride (2) or chloromethyl-benzene (3). Recent work in this area includes that of Fetters (4) who has made 12 and 18 arm stars with this general technique. 

By means of anionic polymerization, it is possible to produce in the laboratory linear polymers that are nearly monodisperse and have many types of branching such as multi-armed stars and combs and H-shaped molecules. For example, there have been reports of studies of anionically polymerized polystyrene, polybutadiene, and polyisoprene. An example of the anionic polymerization of a branched polymer is the technique of Roovers and Toporowski [22] for making comb polystyrenes. The varieties of model branched polymer that can be produced today by means of block polymerization and coupling chemistries include stars, H-shaped molecules, super-H molecules (multi-armed stars at both ends of a backbone segment), and combs of various types [23]. So-called pom-pom polymers are of special interest, because their rheological behavior has been modeled by McLeish and Larson [24]. These molecules have several arms at each end of a central crossbar, and polybutadienes having this structure have been synthesized [25,26]. 

Styrene polymerizes spontaneously on heating, but samples for use in research are made by anionic polymerization. Of particular interest are structures having well-defined branching structures [28-31]. Several rheological studies of branched polystyrenes are discussed in later chapters. 

Star polymers exhibiting three different branches (PS, PDMS, and poly( er -butyl methacrylate) were prepared in a similar way, the first being the preparation of PDMS macromonomers containing a terminal nonpolymeriz-able DPE entity. Thus, a living PS is reacted with the PDMS macromonomers to create active sites on DPE. These sites served subsequently as initiators for the anionic polymerization of tert- mXy methacrylate [89]. Stadler and coworkers have used that reaction to generate polystyrene-arm-polybutadiene-arm-poly(methyl methacrylate) [90]. 

Comb-shaped polymers are derived from polymerizing or copolymerizing macromonomers. Macromonomers can be synthesized by a variety of synthetic techniques. Asami and co-workers prepared a methacrylate-terminated polystyrene by anionic polymerization. The macromonomer was then polymerized using GTP [41] to yield an oligomer with a polystyrene backbone and PMMA grafts. McGrath and co-workers prepared a poly(dimethyl siloxane) macromonomer end-capped with a methacrylate group. This macromonomer was polymerized by GTP to yield a comb-shaped polymer with PDMS branches [19]. 

Star polymers are the simplest of the branched polymers they represent polymers wherein several linear chains are linked to a single multivalent molecular core 3-arm, 4-arm, and even 12-arm star polymers have been synthesized [43]. Typically, the most efficient process to prepare such polymers is to use a living polymerization, such as anionic polymerization, and terminate the process by a nucleophilic substitution reaction onto a multifunctional core. With the advent of CRPs and the CuAAC reaction, Gao and Matyjaszewski utilized, for the first time, a combination of ATRP and click reaction to prepare star-branched polymers [44] as described earlier, ATRP can be readily used to prepare azide-terminated polystyrene by transforming the terminal bromide, which is typically installed at one chain end during ATRP, to an azide. The PS-azide was then reacted with different core molecules bearing multiple propargyl groups an example is the... 

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

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