Radical 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. 

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. 

Clearly the benzylic H-atoms attached to the polystyrene backbone are not as labile as in cumene. This is likely due to the steric effect of the coil configuration of the polymer chain which blocks access of the /er/-butoxy radicals. Nonetheless, some backbone H-atom abstraction from the polystyrene backbone does occur during radical polymerization of styrene. The extent of abstraction is proportional to the concentration of peroxide initiator added to the process. Typically, in commercial continuous bulk polymerization processes the concentration of peroxide initiator is kept below 500 ppm. Also a few percent of a solvent having some chain transfer activity (ethylbenzene) is added to the styrene feed. This is done so that the extent of branching is small. If the concentration of initiator is increased to >500 ppm and/or the chain transfer solvent falls below a certain level, the extent of branching can increase to a level where gels began to appear in the product. The mechanism of... 

Many researchers have attempted to make branched polystyrene in continuous bulk radical polymerization processes. Approaches involving the addition of additives to the polymerization process which lead to branching inside the polymerization reactor always lead to gel problems. Examples include addition of divinylmonomer [4], vinyl peroxides (e.g. I) [5,6], branched peroxides (e.g. II) [7], vinyl-functional chain transfer agents (III) [8], and the use of addition-fragmentation chain transfer agents that lead to the formation of polystyrene macromonomers (Figure 24.3) [9]. 

A similar well-defined graft copolymer consisting of polystyrene main chain and branches (G-7) can be prepared simply via repetition of copper-catalyzed living radical polymerizations.209 Thus, the synthesis starts with the copolymerization of styrene and />(acetoxymethy 1)styrene or />(methoxymethyl)sty-rene, followed by bromination of the substituent into the benzyl bromide moiety, which then initiates the copper-catalyzed radical polymerization of styrene to give graft polymers with 8—14 branches. 

When styrene free-radical polymerization was carried out in the presence of a polymerizable dithioester, benzyl 4-vinyldithiobenzoate (Wang etal, 2003), branched polystyrene... 

A specific case of branched radical polymerization is the graft polymerization of styrene on rubber. This reaction is the basis for the manufacture of some industrially important composite plastics, such as shock-proof polystyrene, ABC plastic, and similar materials. 

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. 

Many commercial polymers, including polystyrene, PMMA, and LDPE, are synthesized via homogeneous free-radical polymerization of a single monomer. Homopolymer properties are controlled by average chain length and chain-length distribution as well as, in some cases, structural characteristics such as branching level. 

Radical polymerization forms a long chain of polystyrene with phenyl groups bonded to every other carbon. To form branches on this polystyrene chain, a radical on a second polymer chain abstracts a H atom. Abstraction of Ha forms a resonance-stabilized radical A. The 2° radical B (without added resonance stabilization) is formed by abstraction of Hb. Abstraction of Ha is favored, therefore, and this radical goes on to form products with 4° C s (A). 

Such hydrophilic macromonomers (DPn=7-9) were radically homopolymer-ized and copolymerized with styrene [78] using AIBN as an initiator at 60 °C in deuterated DMSO in order to follow the kinetics directly by NMR analysis. The macromonomer was found to be less reactive than styrene (rM=0.9 for the macromonomer and rs=1.3 for styrene). Polymerization led to amphiphilic graft copolymers with a polystyrene backbone and poly(vinyl alcohol) branches. The hydrophilic macromonomer was also used in emulsion polymerization and copolymerized onto seed polystyrene particles in order to incorporate it at the interface. 

A linear polymer is one in which each repealing unit is linked only to two others. Polystyrene (1-1), poly(methyl methacrylate) (1-34), and poly(4-methyl pentene-1) (1-35) are called linear polymers although they contain short branches which arc part of the monomer structure. By conirast, when vinyl acetate is polymerized by free-radical initiation, the polymer produced contains branches which were not present in the monomers. Some repeating units in these species are linked to three or four other monomer residues, and such polymers would therefore be classified as branched. 

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,... 

---