Syndiotactic polystyrene styrene

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

Polystyrene (PS), 7 610t 23 326, 348, 358. See also Polystyrenes Styrene Styrene plastics biodegradation of, 23 376 brominated, 11 470-474 chain transfer to, 23 383 colloidal suspensions, 7 275 crystalline syndiotactic, 23 388 decomposition of, 14 109 effect of orientation on oxygen permeability, 3 393t... 

Xu S, Chen B, Tang T, Huang B. Syndiotactic polystyrene/thermoplastic polyurethane blends using poly(styrene-l)-4-vinylpyridine) diblock copolymer as a compatibilizer. Polymer 1999 40 3399-3406. 

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

Also, divalent TiPh2 activated with [Al(Me)0]x appeared to be a catalyst for syndiospecific styrene polymerisation [71]. Even 5-TiCh or (Acac)3TiCl3, when activated with [Al(Me)0]x, could yield a mixture of isotactic and syndiotactic polystyrenes. Some other catalysts, such as rare-earth coordination catalysts, have been successfully used to obtain syndiotactic-rich polystyrenes [72],... 

Methylaluminoxane-free catalysts, such as cationic complexes derived from TiBz4 and tris(pentafluorophenyl)borane [B(C6F5)3], appear to be much less active, poorly stereospecific catalysts for the polymerisation of styrene. Under analogous conditions, the use of N, yV-dimethylanilinium tetrakis(pentafluor-ophenyl)borate [Me2N(Ph)H]+[B(C6F5)4]- instead of B(C6F5)3 does not yield active catalysts only traces of syndiotactic polystyrene were obtained [70]. 

Although there is dispute about the exact oxidation state of titanium in the active species [Ti(III) or Ti(IV)], it was suggested, from the results of ESR measurements, that Ti(III) species form highly active sites for producing syndiotactic polystyrene in styrene polymerisation systems with the TiBz4—[Al(Me)0]x catalyst [50]. The moderately low catalyst activity is attributable to the stability of the benzyl transition metal derivatives towards reduction. 

Catalysts of the Ti(OR)4—[Al(Me)0]x type show greatly inferior activity and syndiospecificity in the polymerisation of styrene by comparison with catalysts of the CpTi(OR)3—[Al(Me)0]x type [54,70]. The activity and syndiospecificity of Ti(OR)4—[Al(Me)0]x catalysts increases when the Al/Ti molar ratio in the polymerisation system is increased. The maximum activity of Ti(OR)4—[A1 (Me)0]x catalysts is observed at an Al/Ti molar ratio of ca 100 [54,55]. It is worth mentioning that, under the same polymerisation conditions, these catalysts yield syndiotactic polystyrene with a higher molecular weight than does the CpTiCl3—[Al(Me)O]x catalyst [71],... 

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). 

Styrene is a very versatile monomer. It can be polymerized by most types of polymerization mechanisms, e.g. free radical (FR), Ziegler-Natta (ZN), anionic, and cationic. Classical ZN polymerization of styrene yields isotactic polystyrene. However, if methylalumoxane (MAO) is added as a co-catalyst, syndiotactic polystyrene is formed. The resulting polymers formed using the various mechanisms of polymerization are summarized in Scheme 8.1. 

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. 

Highly syndiotactic polystyrene (SPS) was synthesized using a homogeneous catalytic system using a titanium compound and methylaluminoxane or borate [1]. The detailed syndiospecific polymerization of styrene is described in the previous chapter. 

Metallocenes and methylalumoxanes can further be used to synthesize isotactic polypropylene [70, 71], syndiotactic polypropylene [38], other propylene polymers or oligomers [72], ethylene/cycloolefin copolymers [10-13], syndiotactic polystyrene [14, 61], and ethylene/styrene copolymers [64]. Cycloolefin copolymers are amorphous, with high glass transition temperatures [10-13]. The syndiotactic polystyrenes are semicrystalline polymers with a glass transition temperature around 100 °C and a melting point of 270 °C [14]. 

The tetrahydrofluorenyl trichloro titanium compound (Scheme 180) has been prepared by using the dehalosilylation procedure and has been evaluated as a catalyst for styrene polymerization in the presence of MAO as co-catalyst. Syndiotactic polystyrene is obtained with similar characteristics to those reported for other substituted indenyl trichloro titanium derivatives.403,412... 

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