Syndiotactic polystyrene system

The major features for advanced photo-systems relative to the base film are a small-size thrust cartridge and return inside the cartridge. These features require low core-set curl values and high mechanical strength. The cellulose triacetate (CTA), polycarbonate and PET materials currently used in the photographic industry are not able to satisfy these requirements. Some newly developed film materials, such as modified-PET and syndiotactic polystyrene, were also unable to satisfy such requirements. It is known that annealing films just below the Tg (BTA treatment) could reduce core-set curl tendencies. BTA treatments of... 

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. 

With conventional polymerization processes, atactic chains are predominantly formed for the formation of isotactic and syndiotactic chains a special catalyst system is required, e.g. Ziegler-Natta catalysts. Such a process is called stereospecific polymerization. It enables the manufacture of, i.a., technically usable PP and also unbranched PE (see 4.1). The newest development is the metallocene katalyst it enables the building-up of chains-to-measure with very high degrees of chain regularity also the manufacture of syndiotactic polystyrene is technically possible in this way (see Qu. 2.47). 

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],... 

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. 

Figure 18.2 13C NMR of syndiotactic polystyrene produced by highly stereocon-trolled catalyst system. Apparatus JEOL Lambda 500 (13C 125.65 MHz). Frequency 25000 Hz. pulse 9.0 xs (45° pulse). Repetition time 4 s. Scans 10000...

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

The system Cp2TiCl2/MAO is suggested to be less active than CpTiCl3/MAO and Cp2TiCl/MAO in the polymerization of 1,3-butadiene, 4-methyl-l,3-pentadiene, and styrene to give predominantly cis-, 4-polybutadiene, 1,2-syndiotactic poly(4-methyl-l,3-pentadiene), and syndiotactic polystyrene.1214... 

Metallocene catalysts were commercialized in 1991 by Exxon Chemical for the industrial production of ethylene-propylene (EP) elastomers in solution polymerization using zirconocene catalysts [37], As a result of extensive research of different metallocenes applied to the stereoregular control of polymeric materials, these systems were able to produce novel polymers such as syndiotactic polystyrene and ethylene-styrene copolymers, which were not possible to produce with traditional Ziegler-Natta catalysts. 

In 1985, Ishihara [10] at Indemitsu, Japan, reported that monocyclopentadienyl, indenyl, or fluorenyl titanium complexes, with different substituents on the Cp derivatives, were highly active for styrene polymerization, producing syndiotactic polystyrene with high melting temperature. Ishihara smdied early transition metals from group 3 or 4 systems based on late transition metals such as Ni were inactive for styrene homo- or copolymerization. Monocyclopentadienyl complexes, also... 

Metallocene catalysts (Fig. 4.3) were first discovered in the early 1950s by Natta and Breslow, and were first used to make polyethylene in 1957. These catalysts were used to produce syndiotactic polystyrene in 1984 and syndiotactic polypropylene (FINA) in 1986. However, commercialization for polyethylene did not come until the mid-1990s, since until that time the advantages the new catalyst systems offered were not fully appreciated. Metallocene catalysts employed today commonly contain a co-catalyst to increase the catalyst activity. 

Homogeneous catalysis can be effected vsrith alkyllithiums, but the results depend on the purity of the reaction system. On the one hand, Braun et al. (1960) and Kem (1960) reported that isotactic polymer was formed, supposedly by homogeneous catalysis. On the other hand, Worsfold and Bywater (1963) showed that pure butyllithium gave syndiotactic polystyrene. The formation of isotactic polymer was presumably caused by lithium hydroxide. This was shown by adding traces of moisture to react with part of the butyllithium. In previously reported examples, it appears that either the solvent or monomer was contaminated with moisture. It is not known whether the lithium hydroxide was dissolved or dispersed colloidally. If dispersed colloidally, it could exert stereocontrol through surface effects. If dissolved, the lithium hydroxide could effect stereocontrol as an ion-pair associated with growing chain end. 

Mahesh K P O, Tsujita Y, Yoshimizu H, Okamoto S, Mohan D J (2005) Study on 6-form complex in syndiotactic polystyrene-organic molecules systems. IV. Eormation of complexes with a mixture of solvents and structural changes during the sorption of solvents by syndiotactic polystyrene mesophase membranes. J. Polym. Sci. Part B Polym. Phys. 43 2380-2387... 

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. 

The catalyst systems consisting of MAO and titanium alkoxides, such as Ti(OMe)4, Ti(OEt)4, and Ti(0-n-Bu)4, catalyze syndioselective styrene polymerization with moderate activities," - but are less active than mono(cyclopentadienyl)titanium-based systems. Nevertheless, higher molecular weight syndiotactic polystyrenes are obtained from these catalyst systems (Table 14.1, entries 3-6). 

NMR m and r dyad analysis of syndiotactic polystyrenes prepared with MAO activated CpTiCb or Cp TiCl3 systems was in agreement with the Bernoullian symmetric statistical model for stereoselective propagation. The polymer microstracture is a long sequence of syndiotactic dyads with only isolated m defects and no consecutive isotactic dyads (Figure 14.17), which is consistent with a chain-end control mechanism directed by the configuration of the tertiary carbon of the last inserted styrene unit (unlike 1,3-asymmetric induction). Thus, the syndiotactic preference exerted by the chain end arises from the phenyl-phenyl repulsive interaction between the last inserted... 

Ti(0-n-Bu)4 systems, the tacticity of the obtained polystyrene strongly depends on the support material (i.e., the presence of Cl in the carrier). When MgCh-supported Ti(0-n-Bu)4 is activated with MAO, the soluble part of the catalyst in toluene can afford sPS, whereas the insoluble part of the catalyst gives iPS. This is also true for the Ti(0- -Bu)4/Mg(0H)Cl system when activated with MAO. For Ti(0-n-Bu)4/Mg(OH)2/MAO, which has no Cl in the carrier, the insoluble part gives syndiotactic polystyrene, and the soluble part is almost inactive for styrene polymerization. For the TiCls (AA)/MAO-system (AA = aluminum-activated), iPS (95% mmmm pentad, melting point 221 °C) and sPS are obtained from the insoluble and soluble part of the catalyst respectively. ... 

When carried out using a syndioselective catalyst system, such as CpTiCb/MAO, the copolymerization of the styrene-terminated macromonomer, M4, with styrene can lead to the formation of graft copolymers with highly syndiotactic polystyrene main chains. The melting point of the graft copolymers decreases with an increasing number of grafts per main polymer chain. 

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