Styrene, stereoregular polymerization

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

Wullf and Hohn recently described several new stereochemical results (93). They reported the synthesis of a copolymer between a substituted styrene (M ) and methyl methaciylate (M2) having, at least in part, regular. . . M,M M2M MiM2. . . sequences. Polymerization involves the use of a chiral template to which the styrene monomer is loosely bound. After elimination of the template, the polymer shows notable optical activity that must be ascribed to the presence of a chiral stmcture similar to that shown in 53 (here and in other formulas methylene groups are omitted when unnecessaiy for stereochemical information). This constitutes the first stereoregular macromolecular compound having a three monomer unit periodicity. 

In polymers that exhibit tacticity, the extent of the stereoregularity determines the crystallinity and the physical properties of the polymers. The placement of the monomer units in the polymer is controlled first by the steric and electronic characteristics of the monomer. However, the presence or absence of tacticity, as well as the type of tacticity, is controlled by the catalyst employed in the polymerization reaction. Some common polymers, which can be prepared in specific configuration, include poly(olefins), poly(styrene), poly(methyl methacrylate), and poly(butadiene). 

The anionic polymerization of styrene to produce stereoregular polymers has been studied by Kern and co-workers (23). They found that the polymerization of styrene by n-amylsodium at low temperatures produced crystalline polystyrene. Braun, HerNER and Kern (24) reported also that the potassium polymerization of styrene produced polymers with increased crystallinity as the temperature was decreased. 

Polymer stereoregularity obtained with styrene and butadiene is particularly significant. In the polymerization of styrene with the three cocatalyst-dependent systems... 

Monocyclopentadienyl complexes of titaninm (Cp TtXs) perform poorly as catalysts for ethylene or propylene polymerization, bnt in the presence of MAO, they polymerize styrene to stereo- and regioregnlar syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a Ti complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

Polymerization Reactions. The enantioselective co-polymerization of styrenes and carbon monoxide has been achieved by the use of a palladium catalyst based on the (5,5)-r-Bu-box ligand. Copolymerization of p-/er/-butylstyrene (TBS) and carbon monoxide in the presence of 0.1 mol % chiral catalyst afforded the alternating co-polymer with a highly isotactic microstructure and excellent optical purity (eq 22). The stereoregularity of the polymer is >98% and the polymer exhibits high molar rotation. ... 

An optically active polystyrene derivative, 40 ([a]25365 -224° to -283°), was prepared by anionic and radical catalyses.113 The one synthesized through the anionic polymerization of the corresponding styrene derivative using BuLi in toluene seemed to have a high stereoregularity and showed an intense CD spectrum whose pattern was different from those of the monomer and a model compound of monomeric unit 41. In contrast, polymer 42 and a model compound, 43,... 

The apparent propagation rate constant for polymerization of styrene in THF at 25°C using sodium naphthalene as initiator is 550 L mol s . If the initial concentration of styrene is 156 g/L and that of sodium naphthalene is 0.03 g/L, calculate the initial rate of polymerization and, for complete conversion of the styrene, the number average molecular weight of the polystyrene formed. Comment upon the expected value of the polydispersity index (M /M ) and the stereoregularity of the polystyrene produced. 

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