Coordination polymerization syndiotactic polystyrene

We can employ coordination polymerization to produce stereoregular polystyrene. By performing this type of reaction at low temperatures, using Ziegler-Natta or single-site catalysts, we can prepare isotactic and syndiotactic versions of polystyrene. 

Although PS is largely commercially produced using free radical polymerization, it can be produced by all four major techniques—anionic, cationic, free radical, and coordination-type systems. All of the tactic forms can be formed employing these systems. The most important of the tactic forms is syndiotactic polystyrene (sPS). Metallocene-produced sPS is a semicrystalline material with a of 270°C. It was initially produced by Dow in 1997 under the trade name Questra. It has good chemical and solvent resistance in contrast to regular PS that has generally poor chemical and solvent resistance because of the presence of voids that are exploited by the solvents and chemicals. 

Titanium compounds with MAO or borate as co-catalysts effectively produce syndiotactic polystyrene from styrene monomer. The design of high-performance catalyst systems is now well demonstrated. The basic structure of the active site, the mechanism of coordination and insertion and the kinetics are also now well understood for this new polymerization. 

Syndiotactic polystyrene is produced by well stereo-controlled coordination polymerization by titanium compounds. The polymer has a high percentage of mr pentad structure. The 13C NMR chemical shift for the phenyl-1 carbon and backbone methylene carbon are approximately 145.3 and 44.9 ppm, respectively [1]. In general, these polymers are found to be >99% pure in syndiotactic structure as defined by NMR. The most stereoselective catalysts produce a >99.6% pure syndiotactic structure (Figure 18.2)... 

Althought this decomposition mechanism is not yet well-defined (homolytic cleavage, reductive dimerization. hydride formation, or other mechanisms), it is practically meaningful. Styrene polymerization by CpTiMea/FAB carried out in the low-temperature regime (below the decomposition temperature) produces only atactic polystyrene, possibly via a car-bocationic polyaddition. However, at higher polymerization temperatures (above the decomposition temperature), highly syndiotactic polystyrene can be obtained by coordinative 2,1-polyinsertion. ... 

Syndiotactic polystyrene (sPS) is a relatively new material discovery in semicrystalline pol5nners with a high melting point and rapid crystallization rate, which makes it possible to injection mold the material. The stereospecific polymerization was made possible by the combination of a transition metal catalyst with weakly coordinating cocatalysts, such as methylaluminoxane. The excellent balance of mechanical, electrical, solvent resistance, and dimensional stability properties combined with a relatively low price (based on styrene monomer) have made this material a competitor to existing engineering plastics. The products also have excellent heat performance and are finding application in antomotive (under the hood), electrical, and electronic connector systems. 

Nanocomposites of syndiotactic polystyrene (sPS) employing MMT-hexadecyltributylphosphonium [40, 41] and high-impact polystyrene (H1PS)/MMT-hexadecyltriphenylphosphonium [42] were prepared by melt-blending and in situ coordination-insertion polymerization. Partially exfoliated or intercalated materials were obtained in all cases, and a decrease of crystallinity of sPS was observed. However, the presence of clay did not have a strong influence on the sPS thermal transitions. Thermal decomposition of the material was slowed and mechanieal properties were improved in the presence of low organoclay content. Intercalated HIPS nanocomposites were obtained, with improved thermal and flame retardant properties compared to pure HIPS (Figure 3.8). 

S. Bruzaud, Y. Grohens, S. Ilinca, and J.-F. Carpentier, Syndiotactic polystyrene/ organoclay nanocomposites Synthesis via in situ coordination-insertion polymerization and preliminary characterization. Macromolecular Materials and Engineering, 290 (2005), 1106-14. 

Industrialization of coordination polymerization of styrene using metallocenes and leading to syndiotactic polystyrene is in progress. 

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. 

Polystyrene is unusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. j Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers ... 

The presumed 14 electron Ti(C5Me5)(2,4-C7Hn), Ti(Ph2CH)2, and Ti(Ph2C-SiMc3)2 complexes (Section IIIA), as well as a variety of other open and half-open titanocenes, have been reported to yield, upon activation with MAO and/or poorly coordinating borate ions, catalysts for the polymerization of ethylene/a-olefin mixtures, styrene, vinylcyclohexane, vinylcyclohexene, butadiene, and related spedes." The polystyrene was isolated predominately in the syndiotactic form. 

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