Synthetic polyisoprene rubbers structure

The use of alkali 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. Independendy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-1,3-butadiene was described (17). Interest in alkali 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 t /s- 1,4-polyisoprene, similar in structure and properties to Hevea natural rubber (see Elastomers, synthetic-polyisoprene Rubber, natural). 

The infrared spectra of hevea (natural rubber), balata (or guttapercha), the latter both in the crystalline (a) and the amorphous forms, and of synthetic polyisoprene are compared in Fig. 32. The hevea and balata (amorphous) spectra offer calibrations for cfs-1,4 and irans-1,4 structures, respectively, in the synthetic polymer. Owing to the presence of the methyl substituent, however, the spectral difference between the as and trans forms is slight both absorb at about 840... 

The determination of the various types of geometric isomers associated with unsaturation in Polymer chains is of great importance, for example, in the study of the structure of modern synthetic rubbers. In table below are listed some of the important infrared absorption bands which arise from olefinic groups. In synthetic "natural" rubber, cis-1, 4-polyisoprene, relatively small amounts of 1, 2 and 3, 4-addition can easily be detected, though it is more difficult to distinguish between the cis and trans-configurations. Nuclear magnetic resonance spectroscopy is also useful for this analysis. 

The structural uniformity of synthetic polymers is in general not as perfect as in the case of their natural counterparts. However, using special initiators and optimized polymerization conditions, it is possible to prepare quite homogeneous CIS-1,4-polyisoprene ( synthetic natural rubber ). 

Natural rubber and guttapercha consist essentially of polyisoprene in a s-1,4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structures but reduced chain regularity, although... 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

Initial attempts to make synthetic rubber similar to NR date back to mid 1800s. Modern synthetic polyisoprene is designed to be similar to natural rubber in structure and properties. Although it has lower green strength, slower cure rates, lower hot tear, and... 

The polymers of rubber plastics have unsaturated hydrocarbon chain structure, since they are polymerized from alkadienes. The general formula of poly(l,3-butadiene) or butadiene rubber (BR) and polyisoprene or natural rubber (NR) is drawn in Scheme 12.5, where X is hydrogen in BR and methyl group in synthetic polyisoprene or NR. The free radical mechanism of thermal decomposition starts by homolytic scission of the alkyl C-C bonds. Two primary macroradicals (4 and 5) are formed for which the rearrangement... 

Synthetic natural rubber, cw-polyisoprene, is an example of a stereospecific polymer made possible by this means. There are five types of stereo specific (or stereoregular) structures cis, trans, isotactic, syndi-otactic, and tritactic. 

Synthetic polyisoprene, prepared by free-radical polymerization of isoprene monomer, is a copolymer of six structurally distinct kinds of isoprene chain units. Unlike natural rubber, which is a regularly repeating Class I structure (cis-1,4), such synthetic polyisoprene does not crystallize. On the other hand, by the use of the appropriate stereospecific catalyst, isoprene monomer can be converted to a regular Class I polymer with the same structure as natural rubber (. ... 

A true synthetic natural rubber was introduced in the mid-1960s with the exact same chemical structure as latex tapped from a tree. The difference is that natural rubber comes with a variety of other ingredients in the latex that can both add and detract from performance, while polyisoprene is considered relatively pure. In addition, there are some differences in molecular weight distribution that impact performance. Available in both latex and solid forms, this elastomer can be directly substituted for natural rubber in many applications. Adhesives which are not cured tend to have higher creep values than natural rubber, but also exhibit lower tack and green strength properties. Vulcanized adhesive products perform equal to cured natural rubber adhesive products. 

Dr. N.R. Legge, 1987 Charles Goodyear medalist and, at that time, director of the synthetic polyisoprene program at Shell Development Company, recalled that it was in attempting to provide a solution to this problem of poor "green strength in alkyl lithium polymerized polyisoprene that the styrenic block copolymers were first synthesized. The functional use of the first block copolymers was not as an identifiable monolithic rubber structure but provided a vital function to another identifiable material and lost its identity in this process. 

The main objective of this chapter is to inform rubber technologists how and when to use synthetic high cw-polyisoprene and to attempt, where possible, to link certain observed behaviour with studies made by rubber scientists concerning molecular structure. Emphasis is placed on situations where technical advantage is established for the synthetic polymer, since it is almost invariably sold at a premium to natural rubber and its value is appreciated by rubber manufacturers. Selective usage is the pattern that has developed and correctly so, since synthetic polyisoprene is unlikely to be a cheap replacement for natural rubber in the foreseeable future. 

The structural parameters, determined by infra-red spectroscopic measurements (Table 1) show that the synthetic polyisoprenes produced using the Ziegler catalyst systems are closely similar to each other and are almost as structurally pure as natural rubber. Recent studies using the NMR technique indicate that the natural rubber hydrocarbon is at least 99-6% c/.y-l,4-polyisoprene. The small stereo-irregularities present in the synthetic polymers are sufficient to cause a reduced tendency for the synthetic polymers to crystallise either at low temperature or induced by applied strains. This difference in the rate of crystallisation, or perhaps the magnitude of the crystallites formed, is suggested to influence both processing and vulcanisate properties. The alkyl/lithium catalysed rubbers... 

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

Finally, one last type of natural polymer is natural rubber, obtained from the rubber tree and having the all cw-l,4-polyisoprene structure. This structure has been duplicated in the laboratory and is called synthetic rubber, made with the use of Ziegler-Natta catalysis. 

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 rubbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high ar-polyisoprene which duplicates the structure of natural rubber. In one method (3,4) a Ziegler-type catalyst of trialkylaluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all t /s- 1,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

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