Isoprene polymers 3,4-polyisoprene

As with many polymers, polyisoprene exhibits non-Newtonian flow behavior at shear rates normally used for processing. The double bond can undergo most of the typical reactions such as carbene additions, hydrogenation, epoxidation, ozonolysis, hydrohalogena-tion, and halogenation. As with the case of the other 1,4-diene monomers, many copolymers are derived from polyisoprene or isoprene itself. 

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. 

Kawai and Inoue (49) recently explored the mechanical blend of poly-styrene/polyisoprene/poly(styrene-b-isoprene) polymer 1 and polymer 2 appear as both homopolymers and block components. 

Like substituted ethylene, isoprene undergoes free radical polymerization. Polymerization of isoprene results in the polymer polyisoprene, which is a simple alkene having one double bond in each unit (Figure 9.3). Polymerization of isoprene may follow one of two pathways cii-polymerization or trans-polymerization. 

Block-Copolymer Rubbers. Block copolymer rubbers are thermoplastic elastomers that are the most widely used class of PSAs (see Elastomers, Thermoplastic). The most commonly used are ABA block copolymers, where A is polystyrene and B is a polydiene. Polyisoprene (R = CH3) and polybutadiene (R = H) are the most common B poljnners, giving SIS and SBS copolsnners, respectively (see Butadiene Polymers Isoprene Polymers). 

Polyisoprene has also been pyrolysed in an inert atmosphere and here the main products are isoprene and l-methyl-4-isoprenylcyclohexene. The latter compound can disproportionate to l-methyl-4-isopropyIbenzene and methyl-l-iso-propylcyclohexenes and this reaction is catalysed by Ziegler-Natta catalyst residues or by carbon black. The dominant initiation process is j9-chain scission with the formation of two allylic radicals. The kinetics of thermal decomposition have been studied for cis- and rraiw-1,4-polyisoprene and the copolymer of isoprene with 4-isopropyl-methyl styrene and also for isoprene polymers containing 4-CjH4—Z—4-C(H4— and —CjH4—Z—C5H4N=N— units, where Z may be O, CHj, SOj or a single bond. ... 

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... 

Rubber is an example of an elastomer, a polymer that stretches easily and returns to its original shape. Natural rubber comes from rubber trees and has the structure shown in Figure 15-32 it is called polyisoprene because it consists of repeating isoprene imits. Polyisoprene polymers form coils that stretch under ten-... 

A pyrogram of the copolymer (isoprene-styrene) resulting from a 10 s pyrolysis at 601°C yields product distributions similar to the sum of the two constituent product distributions. For example, when the polymer polyisoprene is pyrolyzed, C2, C3, C4, isoprene and Cjo dimers are produced. When polystyrene is pyrolyzed, styrene and aromatic hydrocarbons are the products. The copolymer product distribution and relative area basis resemble the two individual polymer product distributions. 

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). 

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). 

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. 

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 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. 

The catalyst activity is so high that uranium concentration lower than 0.1 millimoles per liter allows a complete conversion of butadiene to be obtained in a few hours, at 20°C, The transfer reaction of uranium based catalyst is similar to that of conventional 3d-block elements (titanium, cobalt, nickel) so that the molecular weight of the polymer is affected by polymerization temperature, polymerization time and monomer concentration in the customary way. This is in contrast, as we shall see later on, to some catalysts based on 4 f-block elements. Uranium based catalysts are able to polymerize isoprene and other dienes to high cis polymers the cis content of polyisoprene is 94%, somewhat inferior to titanium based catalysts. In contrast, with 3d-block elements an "all cis", random butadiene-isoprene... 

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. 

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