Polyisoprene polymerization

Spirin and co-workers (98, 101) have reported the polymerization of isoprene, butadiene and styrene in triethylamine. With styrene at —34° the rate is first order in polystyryllithium in the concentration range 0.4 to 3.2 x 10 2 molar. The observed propagation constant, 2.5 x 10-2 litre/mole sec should refer to the ion-pair reaction. With isoprene and butadiene in triethylamine the plots of rate against polyalkenyllithium concentration curve somewhat at higher concentrations. This could be due to some self-association because the concentrations used were rather high. Some uncertainty applies to the data on polyisoprene polymerization since the experiments may have been carried out at +30° (101) or at —30° (05). 

Catalysts. Iodine and its compounds are very active catalysts for many reactions (133). The principal use is in the production of synthetic rubber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-834], are employed for producing stereospecific polymers, such as polybutadiene rubber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymerization (66) (see Rubber CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabilization of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

Significantly different seemed intiaUy the crystal morphology of polyethylene, polybutene-1, polypropylene, polystyrene, poly(4-methyl pen-tene-1), and polyisoprene polymerized with varying solvents and at varying temperatures (114, 123). Discrete hollow particles with a fibrous texture could be observed. The fibrils had an appearance similar to polyethylene crystallized from solution sheared by rapid stirring (118). A closer analysis of this similarity was carried out by Wikjord and Manley (124), Keller and Willmouth (117), and Ingram and Schindler (125) for polyethylene. 

Effect of Initiator Concentration and Solvents on Chain Structure of Lithium Polyisoprene (Polymerization temperature = 20°C)... 

Alkyl lithium polyisoprene ceased to be manufactured in the U.S. in the early 1970 s in favor of polyisoprene polymerized by Ziegler catalysis and because of the greatly improved domestic reserve of natural rubber,Nevertheless, the work that was carried out on the copolymer end-block tire rubbers is a worthwhile addition to our knowledge. Although the idea and the work with this polymer occurred over a decade ago, new insight into the nature of ordered and disordered states and their transition makes this subject relevant in todays world. 

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). 

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). 

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 revolutionary development of stereospecific polymerization by the Ziegler-Natta catalysts also resulted ia the accomplishment ia the 1950s of a 100-year-old goal, the synthesis of i7j -l,4-polyisoprene (natural mbber). This actually led to the immediate termination of the U.S. Government Synthetic Rubber Program ia 1956 because the technical problem of dupHcating the molecular stmcture of natural mbber was thereby solved, and also because the mbber plantations of the Far East were again available. 

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 Ekestone group also polymerized 1,3-butadiene to give an extremely high mol wt polybutadiene of 70% cis-1 4 stmcture. In thek research, they purposefully avoided the preparation of vinyl stmctures in both polyisoprene and polybutadiene since it was beheved that vinyl groups adversely affected tke performance. Since natural mbber was 99.9% cis-1 4 stmcture and had superior properties, they beheved that a 1,4 stmcture was necessary for acceptable physical properties. The addition of polar compounds to the hthium-catalyzed polymerization of butadiene changes the microstmcture from the 90% tij -l,4 stmcture to a mixed cis-1 4 and trans-1 4 microstmcture. 

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). 

Table 2. Effect of Polymerization Solvent on Polyisoprene Microstmcture...

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). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is a-TiCls cocatalyzed with aluminum alkyls. The polymerization rate and cis... 

A number of different synthetic rubbers are produced commercially by diene polymerization. Both cis- and frrms-polyisoprene can be made, and the synthetic rubber thus produced is similar to the natural material. Chloroprene (2-chloro-l,3-butadiene) is polymerized to yield neoprene, an excellent, although expensive, synthetic rubber with good weather resistance. Neoprene is used in the production of industrial hoses and gloves, among other things. 

Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

In a similar manner polyisoprene-polyethylene oxide block copolymers can prepared301. It is surprising that the poly(methyl methacrylate) anion can be successfully used for the polymerization of ethylene oxide without chain transfer302. Graft copolymers are also prepared by successive addition of ethylene oxide to the poly-... 

Closer examination of the data pertaining to polymerization of lithium polyisoprene in aliphatic hydrocarbons suggests that kpcl and kptt are negligible in comparison with k, and k, = k IS, i.e. the addition of isoprene whether to cis or trans active... 

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