Syndiotactic polystyrene commercial

Syndiotactic polystyrene commercialized Styrene-ethylene copolymers Nanocomposites... 

In the late 1990s a crystalline form of polystyrene, syndiotactic polystyrene became commercially available but unless otherwise stated references to polystyrene in this chapter will refer to the traditional amorphous polymer. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Small amounts of iso tactic polystyrene have been synthesized in the laboratory using noncommercial polymerization techniques. These polymers are capable of partially crystallizing, albeit at a very slow rate. Syndiotactic polystyrene was available commercially for several years, but its continued production proved unprofitable. 

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. 

The original, simplest polyolefins, polyethylene and polypropylene, continue to dominate the scene, even after two decades, to such an extent that no other polyolefin even appears on the production charts. Nevertheless, a great many (we may assume all) available olefins have been tested, and many have been found capable of being converted to stereoregular polymers. As was mentioned above, poly(l-butene) and poly(4-methy1-1-pentene) are being offered commercially and may be expected to achieve significant volume in the future. Isotactic and syndiotactic polystyrene are of much theoretical interest (26) but are not yet commercial products. 

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. 

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. 

Polystyrene that is manufactured by free-radical polymerization is atactic. Isotactic polystyrene formed with Ziegler-Natta catalysts was introduced commercially in the 1960s, but failed to gain acceptance. Syndiotactic polystyrene is now being produced commercially. 

Ishihara et al. reported in 1986 that syndiotactic polystyrene can be prepared with the aid of organic or inorganic titanium compounds activated with methylaluminoxane [177]. There is much greater incentive to commercialize syndiotactic polystyrene than the isotactic one. This is because isotactic polystyrene crystallizes at a slow rate. That makes it impractical for many industrial applications. Syndiotactic polystyrene, on the other hand, crystallizes at a fast rate, has a melting point of 275°C, compared to 240°C for the isotactic one, and is suitable for use as a strong structural material. 

Syndiotactic polystyrene is available commercially under the trade name of Questra. This material is produced with the aid of a metallocene catalyst and is sold in several grades [181]. 

How are atactic and syndiotactic polystyrenes prepared commercially Describe and explain. 

While the range of the new metallocene-based polymers includes such specialty polymers as cyclo-olelin copolymers (COC), syndiotactic polystyrene, ethylene/ styrene copolymers, which are stiU in the developmental stage, commercially, the most prominent candidates are the elhylene/a-olefin copolymers such as ethylene/ butylene or hexene copolymers (Exxon s Exact ) or ethylene/l-octene copolymers (Dow s Engage and Affinity ). Depending on the comonomer content, these copolymers have been classified as plastomers or elastomers. At comonomer levels of >25 %, the copolymers exhibit the characteristics of thermoplastic elastomers such as high softness, toughness, flexibility, and resilience and hence been referred to as polyolefin elastomers (POE). CompositionaUy, these POEs usually contain 65 % ethylene and 35 % octene-1, hexene-1, or butene-1 as comonomers. 

Syndiotactic polystyrene, once commercially available from Dow as Questra , continues to be available from Idemitsu Kosan (Japan)(Xarec ) and is available in fiberglass filled versions as well as impact modified versions for electrical, automotive and appliance applications. Blends with polystyrene are noted and presumably include impact polystyrene to achieve toughened versions. 

Many types of commercial styrene polymerization processes are applied. However, the process for the production of syndiotactic polystyrene (SPS) is completely different from those for atactic polystyrene polymerizations. Catalysts are sensitive to the impurities in styrene monomer and SPS is insoluble in aromatic solvents. 

---