Isoprene polyisoprene

NR, IR Natural rubber, Isoprene Polyisoprene Most moderate wet or dry chemicals, organic acids, alcohols, ketones, aldehydes Ozone, strong acids, fats, oils, greases, most hydrocarbons... 

Polymerization of isoprene by 1,4-addition produces polyisoprene that has a cis (or Z) configuration. 

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). 

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic mbbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high i i -polyisoprene which dupHcates the stmcture of natural mbber. In one method (3,4) a Ziegler-type catalyst of tri alkyl aluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all i7j -l,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

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). 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Table 3. Isoprene Specification for the Preparation of High c/s-Polyisoprene ...

The first successhil use of lithium metal for the preparation of a i7j -l,4-polyisoprene was aimounced in 1955 (50) however, lithium metal catalysis was quickly phased out in favor of hydrocarbon soluble organ olithium compounds. These initiators provide a homogeneous system with predictable results. Organ olithium initiators are used commercially in the production of i7j -l,4-polyisoprene, isoprene block polymers, and several other polymers. 

The usage of isoprene monomer is somewhat limited by price and availabiUty. The historical large usage has been in the production of i7j -l,4-polyisoprene... 

Table 4. World Isoprene Demand and Polyisoprene Consumption, 1(P t...

Includes isoprene—isobutyl mbber (HR), polyisoprene mbber (IR), and other synthetic mbbers (SR). ... 

Natural mbber (Hevea) is 100% i7j -l,4-polyisoprene, whereas another natural product, gutta-percha, a plastic, consists of the trans-1,4 isomer. Up until the mid-1900s, all attempts to polymerize isoprene led to polymers of mixed-chain stmcture. 

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. 

Alkali Metal Catalysts. The polymerization of isoprene with sodium metal was reported in 1911 (49,50). In hydrocarbon solvent or bulk, the polymerization of isoprene with alkaU metals occurs heterogeneously, whereas in highly polar solvents the polymerization is homogeneous (51—53). Of the alkah metals, only lithium in bulk or hydrocarbon solvent gives over 90% cis-1,4 microstmcture. Sodium or potassium metals in / -heptane give no cis-1,4 microstmcture, and 48—58 mol % /ram-1,4, 35—42% 3,4, and 7—10% 1,2 microstmcture (46). Alkali metals in benzene or tetrahydrofuran with crown ethers form solutions that readily polymerize isoprene however, the 1,4 content of the polyisoprene is low (54). For example, the polyisoprene formed with sodium metal and dicyclohexyl-18-crown-6 (crown ether) in benzene at 10°C contains 32% 1,4-, 44% 3,4-, and 24% 1,2-isoprene units (54). 

Free-Radical Polymerization. The best method for polymerising isoprene by a free-radical process is emulsion polymerisation. Using potassium persulfate [7727-21-1] as initiator at 50°C, a 75% conversion to polyisoprene in 15 h was obtained (76). A typical emulsion polymerisation recipe is given as follows (77). 

Kuraraj TP Technical Information No. 731, Kuraraj Trans Polyisoprene TP-301, Kuraray Isoprene Chemical Co., Ltd., Synthetic Rubber Dept., 3-8-2, Nihonbashi, Chuo-Ku, Tokyo, Japan, Oct. 1, 1982. 

Property polyisoprene) (GR-S) (polyisoprene) (nitrile) (neoprene) isoprene) Polybutadiene Polysulfide (polysiloxane)... 

One other variant in double-bond polymerisations may be mentioned here. Many conjugated dienes may be polymerised in such a way as to generate long chain molecules with residual double bonds in the chain. Well-known examples of such dienes are buta-1,3-diene and isoprene, which yield 1,4-polybutadiene and 1,4-polyisoprene respectively (Figure 2.4). Natural rubber has a formula corresponding to the 1,4-polyisoprene. 

Alternatively, thermolysis yields the terminal alkene RCH=CH2. Note that, if propene or higher alkenes are u.sed instead of ethene, then only single insertion into Al-C occurs. This has been commercially exploited in the catalytic dimerization of propene to 2-methylpentene-1, which can then be cracked to isoprene for the production of synthetic rubber (cu-1,4-polyisoprene) ... 

The main use of isoprene is the production of polyisoprene. It is also a comonomer with isobutene for butyl rubber production. 

Isoprene (2-methyl 1,3-butadiene) is the second most important conjugated diolefin after butadiene. Most isoprene production is used for the manufacture of cis-polyisoprene, which has a similar structure to natural rubber. It is also used as a copolymer in butyl rubber formulations. 

Standard butyl rubber, which is a copolymer of isobutylene with about 2% of isoprene vulcanises in the same manner as natural rubber but, as it only contains a small proportion of polyisoprene, the cross-link percentage is much reduced. It is therefore not possible to make ebonite from a butyl rubber. The same vulcanisation chemistry, with some modifications, applies to ethylene-propylene terpolymers and brominated butyl rubber. 

The observations discussed above suggest that the kinetic order of lithium poly-isoprene propagation should vary with the living polymer concentration. The effect is imperceptible in aliphatic hydrocarbons, but is observed in benzene solutions. The apparent propagation constants of lithium polyisoprene (MW 2 2 10 ) were determined in benzene and the results are displayed in Fig. 16 in the form of a plot of log kapp vs log c, c denoting the total living polymer concentration. 

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