Isoprene polymers trans-1,4-polyisoprene

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

Some of the polybutadienes obtained with transition metal-based coordination catalysts have practical significance the most important is cA-1,4-polybutadiene, which exhibits excellent elastomeric properties. As regards isoprene polymers, two highly stereoregular polyisoprenes, a cA-1,4 polymer (very similar to natural rubber) and a trans- 1,4-polymer (of equal structure to that of gutta percha or balata) have been obtained with coordination catalysts. Various polymers of mixed 3,4 structure, amorphous by X-ray, were also obtained [7]. 

We have seen that the double bonds in a chain of polyisoprene can exist as cis and trans stereoisomers. Synthetic polyisoprene then has the added complexity of 1,2- versus 1,4-polymerization in addition to the possible existence of different stereoisomers about the double bond. As with butadiene, different coordination catalysts produce isoprene polymers with a preponderance of 1,2- or 1,4- polymer as well as different stereochemistry. Not unexpectedly, these different polymers possess strikingly different physical properties. 

The polymer of isoprene is called polyisoprene. It exists in two forms, cis- and frons-polyisoprene. The two forms are called geometric isomers. They have the same kind and number of atoms, but the atoms are arranged differently in the two forms. Natural rubber consists of trans-polyisoprene, while another product found in rubber plants, gutta percha, is made of c/s-polyisoprene. 

Monodispersed (polydispersity index = 1,04) polystyrene and polyisoprene with a molecular weight in the range of 2 x 10 were used as the carrier polymers by Bates and Baker [18,51]. The isoprene polymer was synthesized anionically at -78°C using toluene as the solvent. It was composed of approximately 80% cis 1,4, 15% trans-, A and 15% 3,4-disubstitutedrepeating units. A few percent (3%) of butadiene were randomly copolymerized with styrene anionically at 25°C in order to provide unsaturated moieties for the next modification step. Electrophilic sites were then introduced into the respective carrier polymers by either oxidation or epoxidation. It was expected that the sites consist mainly of aldehydes, ketones, and/or epoxides. ffj-Chloroperbenzoic acid (m-CPBA) was found to be effective in epoxidation of the unsaturated moieties in... 

Addition of butadiene to ethene polymerizations gives cross-linked material, but dienes are themselves important substrates for polymerization reactions. Natural rubber is an all-ds polymer of isoprene (Figure 21.10), which we encountered in Chapter 11, as an important precursor of the terpenes. Synthetic rubber made by radical polymerization is a mixture of cis- and trans-polyisoprene, (21.10). The material produced by metal-catalyzed polymerization is, however, all-ds and essentially identical to natural rubber. 

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). 

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. 

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. 

The marked variation in stereostructure of diene polymers caused by changes in the counter-ion and solvent when butadiene or isoprene are polymerized anionically, are as yet not fully explained. Much progress has been made on elucidating the causes of variations in the cis/trans ratio of the l h structures in these systems (], , ), but the causes of the change in the ratio of 1 2 to 1 U structures in butadiene for example has been left largely unresolved. In dioxane, for instance, the amount of 1 2 structure decreased from with Li counter-ion at 15°, to hl% with Cs (I4). Less variation is found in THF because a substantial part of the reaction is carried by the free ion. Changes are also observed in polyisoprene ( ). 

When all of the double bonds in the polymer molecule have the same configuration, the result is two different ordered polymer structures—transtactic and cistactic. Figure 8-5 shows the structures of the completely cis and completely trans polymers of isoprene. The stereochemistry of these polymers is indicated in their names. For example, the trans polymer (IX) is named as trans-1,4-polyisoprene or poly( -l-methylbut-l-ene-l,4-diyl). The first name is the IUPAC-recommended trivial name the second name is the IUPAC structure-based (Sec. l-2c) [IUPAC, 1966, 1981, 1996],... 

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. 

The various regular polymers that can be produced by polymerization of butadiene and isoprene are summarized in reactions (4-3) and (4-4). In addition to the structures shown in these reactions, it should be remembered that 1, 4 polymerization can incorporate the monomer with cis or trans geometry at the double bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2 and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybutadiene or polyisoprene in the latter cases. Further, these various monomer residues can alt appear in the same polymer molecule in regular or random sequence. It is remarkable that all these conceivable polymers can be synthesized with the use of suitable catalysts comprising transition metal compounds and appropriate ligands. 

Comparison of results from the polymerizations of butadiene and isoprene with an AlR3-TiCl4 catalyst system reveals some interesting features. The 1 to 1 Al/Ti ratio yields a cis-l,4-polyisoprene and a mns-1,4-polybutadiene. Kinetic studies have, in fact, indicated that in both cases, the rate of polymerization at this ratio is proportional to the first power of the monomer pressure 6, 21). At lower Al/Ti ratios, higher trans-l,A- content is observed in both polyisoprene and polybutadiene. At comparable Al/Ti ratios, lower temperatures increase the trans-l,A-structure in both polymers. Although essentially all-cis-1,4-poly isoprene and all-... 

Polymers of isoprene, too, can be made artificially they contain the same unsaturated chain and the same substituent (the —CHj group) as natural rubber. But polyisoprene made by the free-radical process we have been talking about was—in the properties that really matter—a far cry from natural rubber. It differed in stereochemistry natural rubber has the c/5-configuration at (nearly) every double bond the artificial material was a mixture of cis and trans. Not until 1955 could a true synthetic rubber be made what was needed was an entirely new kind of catalyst and an entirely new mechanism of polymerization (Sec. 32.6). With these, it became possible to carry out a stereoselective polymerization of isoprene to a material virtually identical with natural rubber cw-l,4-polyiso-prene. 

The chemical structure of naturally occurring ais polyisoprenes was determined by 13C NMR spectroscopy using acyclic terpenes and polyprenols as model compounds. The arrangement of the isoprene units along the polymer chain was estimated to be in the order dimethylallyl terminal unit, three trans units, a long block of ais units, and ais isoprenyl terminal unit. This result demonstrates that the biosynthesis of cis-polyisoprenes in higher plants starts from trans,trans,trarcs-geranylgeranyl pyrophosphate. ... 

---