Synthetic Polyisoprenes

As described in Chapters 2 and 3, the monomer can be inserted into the polyisoprene chain potentially in nine different ways. These are the three tactic forms of the 1,2-adducts, two 1,4-adducts, cis and transy and three tactic forms of 3,4-adducts. In addition, there is some possibility of head-to-head and tail-to-tail insertion, though the common addition is head-to-tail. Table 5.8 presents the various microstructuies that can be obtained in polymerizations of isoprene with different catalysts. 

Cationic polymerizations of isoprene proceed more readily than those of butadiene, though both yield low molecular weight liquid polymers. AICI3 and stannic chloride can also be used in chlorinated solvents at temperatures below 0 °C. Without chlorinated solvents, however, polymerizations of isoprene require temperatures above 0 °C. 

At high conversions, cationic polymerizations of isoprene result in formations of some crosslinked material. The soluble portions of the polymers are high in tram A A structures. Alfin catalysts yield polymers that are higher in trans-lA structures than free-radical emulsion polymerizations.  

Chronuum oxide catalysts on support polymerize isoprene like butadiene to solid polymers. Here too, however, during the polymerization process polymer particles cover the catalyst completely within a few hours from the start of the reaction and retard or stop further polymer formation. The polymerization conditions are the same as those used for butadiene. The reactions can be carried out over fixed bed catalysts containing 3% chronuum oxide on Si02-Al203. Conditions are 88 °C 

Pentane Pentane Pentane Pentane Pentane Pentane Pentane Pentane Pentane Pentane Pentane Ethyl ether Ethyl ether Ethyl ether Pentane 

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NBRin pLASTOMERS, SYNTHETIC - NITRILE RUBBER] (Vol 8) -cis-l,4-polyisoprene in pLASTOMERS SYNTHETIC - POLYISOPRENE] (Vol 9)... 

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

With the avadabihty of polymerization catalysts, extensive efforts were devoted to developing economical processes for manufacture of isoprene. Several synthetic routes have been commercialized. With natural mbber as an alternative, the ultimate value of the polymer was more or less dictated by that market. The first commercial use of isoprene in the United States started in 1940. It was used as a minor comonomer with isobutylene for the preparation of butyl mbber. Polyisoprene was commercialized extensively in the 1960s (6). In the 1990s isoprene is used almost exclusively as a monomer for polymerization (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE). 

Polymerization. Isoprene polymeri2ation can proceed by either 1,4- or l,2-(vinyl)addition (see Elastomers, synthetic-polyisoprene). 

Tires, natural mbber tubes, and butyl tubes are the main sources of scrap and reclaim (see Elastomers, synthetic-polyisoprene). Specialty reclaim materials are made from scrap siUcone, chloroprene (CR), nitrile— butadiene (NBR), and ethylene—propjlene—diene—terpolymer (EPDM) mbber scraps (see... 

Polyisoprene (Synthetic). Polyisoprene has four possible chain unit geometric isomers cis- and /n j -l,4-polyisoprene, 1,2-vinyl, and 3,4-vinyl. 

Actually, production of synthetic polyisoprene is relatively small because of the sufficient and increa sing supply of natural mbber. It is important, however, in ensuring that i7j -l,4-polyisoprene (natural mbber), as a strategic material, is less subject to pohtical uncertainties. 

Between the 1920s when the initial commercial development of mbbery elastomers based on 1,3-dienes began (5—7), and 1955 when transition metal catalysts were fkst used to prepare synthetic polyisoprene, researchers in the U.S. and Europe developed emulsion polybutadiene and styrene—butadiene copolymers as substitutes for natural mbber. However, the tire properties of these polymers were inferior to natural mbber compounds. In seeking to improve the synthetic material properties, research was conducted in many laboratories worldwide, especially in the U.S. under the Rubber Reserve Program. 

The discovery by Ziegler that ethylene and propylene can be polymerized with transition-metal salts reduced with trialkyl aluminum gave impetus to investigations of the polymerization of conjugated dienes (7—9). In 1955, synthetic polyisoprene (90—97% tij -l,4) was prepared using two new catalysts. A transition-metal catalyst was developed at B. E. Goodrich (10) and an alkaU metal catalyst was developed at the Ekestone Tke Rubber Co. (11). Both catalysts were used to prepare tij -l,4-polyisoprene on a commercial scale (9—19). 

The preparation of a synthetic polyisoprene was first reported in 1879 by Bouchardat (1), who treated isoprene [78-79-5] obtained from the destmctive distillation of natural mbber with hydrochloric acid. This discovery led to a search for a way of converting isoprene into a material dupHcating natural mbber (Hepea brasilienses). During World War II, scientists extensively studied the polymerization of isoprene with the hope of replicating natural mbber since the United States was temporarily cut off from sufficient natural mbber suppHes. These studies were not successful. PinaHy, in 1954 the B.E. 

In 1989, the estimated United States consumption of synthetic polyisoprene was 51,000 metric tons, and of natural mbber 890,000 metric tons (84). The price for synthetic i7j -l,4-polyisoprene, namely, Natsyn 2200, Natsyn 2205, or Natsyn 2210 in the United States in August, 1992, was 1.98/kg in carload or tmckload quantities fob at Beaumont, Texas (87). The price of synthetic /n j -l,4-polyisoprene from Kuraray Co., Ltd. in the United States was 21.80/kg in 545-kglots (88). 

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