Polyisoprene chain

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

Successive 1,4 units in the synthetic polyisoprene chain evidently are preponderantly arranged in head-to-tail sequence, although an appreciable proportion of head-to-head and tail-to-tail junctions appears to be present as well. Apparently the growing radical adds preferentially to one of the two ends of the monomer. Which of the reactions (6) or (7) is the preferred process cannot be decided from these results alone, however. Positive identification of both 1,2 and 3,4 units in the infrared spectrum shows that both addition reactions take place during the polymerization of isoprene. The relative contributions of the alternative addition processes cannot be ascertained from the proportions of these two units, however, inasmuch as the product radicals formed in reactions (6) and (7), may differ markedly in their preference for addition in one or the other of the two resonance forms available to each. We may conclude merely that structural evidence indicates a preference for oriented (i.e., head-to-tail) additions but that the 1,4 units of synthetic polyisoprene are by no means as consistently arranged in head-to-tail sequence as in the naturally occurring poly-isoprenes. 

One of the most sophisticated architectures prepared by this method has the general formula (AB)2(BA)2, where A and B are polystyrene and polyisoprene chains, respectively two arms are linked by the styrene block, the other two arms are linked via the isoprene block to the central core. Moreover, the ratio of inner and outer blocks has been kept constant [65],... 

Figure 5. Effect of organolithium concentration on polyisoprene chain structure undiluted ( ) 0.5M in n-hexane (A) 0.5M in benzene (O ...

The metalation of naphthalene 8-substituted by both an ethyl group or polyisoprene chain (PIP) is completely similar as established by the titration of the carbanions formed and the UV analysis of the reaction medium (21, 22, 25). Accordingly the naphthalene radical anion, naphthalene dianion and its further isomerization by hydrogen transfer sure successively observed and the final stage of the metalation can be represented by the following structure ... 

The characteristic ratios of stereoirregular 1,4-poiybutadiene and 1,4-polyisoprene chains are theoretically investigated by the Monte Carlo procedure in accordance with the model proposed by Mark (V 001 and V 003). It is pointed out that the presence of discrete cis units in frans-rich chains significantly reduces the characteristic ratio while that of discrete trans units in c/is-rich chains has little effect on the characteristic ratio. The characteristic ratio and its dependence on both the trans and cis contents and their sequence distribution is calculated for stereoirregular polymers in accordance with the interdependent RIS model proposed by Mark (V 001 and V 003), and Ishikawa and Nagai V 005 and V 007 . 

Vulcanized rubber has disulfide cross-links between the polyisoprene chains. Cross-linking forms a stronger, elastic material that does not pull apart when it is stretched. 

The most important of all of these types of reactions is the vulcanization of rubber. Originally, the raw rubber was just heated with sulfur (Sg) and cross-linking of the polyisoprene chains with short chains of sulfur atoms gave it extra strength without destroying the elasticity. Nowadays, a vulcanizing initiator, usually a thiol or a simple disulfide, is added as well. Some examples are... 

To illustrate the use of this model, consider a m-polyisoprene chain (1-18). The length of the isoprene unit is found to be 4.60 x 10" m, and the fully extended... 

The biodegradation of the cw-l,4-polyisoprene chain was achieved by Tsuchi, Suzuki, and Takeda. They used bacterium that belonged to the genus Nacardia and led to considerable weight loss of different soft-type NR-vulcanizates. The microbial... 

Fractionation of the resulting products allowed polyisoprene chains containing exactly one anthracene chromophore in the chain center to be isolated. The labeled polymer was Mn = 10,800 and M /Mn =... 

Figure 5. Time-dependent anisotropies for labeled polyisoprene chains in dilute 2-pentanone solutions. The smooth curves through the data points are the best fits to the Hall-Helfand model for 22.8 C, -8.6 C, and -26.5 °C (bottom to top). The data at 35.1 °C is omitted for clarity. Semilog plots of the best fit correlation functions are shown in the inset. Note that all the correlation functions are quite non-exponential.

Menaquinones are 3-methyln hthoquinones with a polyisoprene chain in position 2. 

In this connection, a recently proposed theory (16) to explain the effect of lithium concentration on polyisoprene chain structure deserves mention. This theory is based on a proposed competition between the rates of chain propagation and isomerization of the chain end, which presumably changes from the cis-1,4 to the trans-1,4 configuration. Although this theory may have some merit, it cannot account for the resiilts demonstrated in Tables II, III and V above, i.e., the absence of any effect of temperature or degree of conversion, both of which would strongly affect the propagation rate, but would not be expected to influence the chain-end isomerization rate. It is far more likely, therefore, that the effects on chain structure described above are due to subtle effects of these reaction parameters on the structure and reactivity of the carbon-lithium bond complex at the active chain end. 

The practice of cross-linking the polyisoprene chains in natural rubber to form a usable elastomer was discovered by Goodyear, in 1839. He heated natural rubber latex with sulphur, a process called vulcanisation. This transforms the sticky runny natural latex into a product in which the elastic... 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

It may be enquired why it is that natural rubber is highly stereoregular, and in this respect is ery different from butadiene polymers obtained by free-radical polymerization which contain a mixture of microstructures. The reason is that the polyisoprene produced by the Hevea brasiliensis tree is formed not by polymerization of isoprene, free-radical or otherwise, but by an enzyme-catalysed condensation of isopentenyl pyrophosphate (see Section 23.5.2). Natural rubber usually contains some crosslinked polyma- gel, at least after it has left the tree and become exposed to the atmospha-e. Crosslinking does not, however, occur by polymerization through the olefinic double bonds of the polyisoprene chain, but by reactions involving minor concentrations of other functional groups which are attached to the polyisoprene chain. 

Miyamoto, N., Yamauchi, K., Hasegawa, H., et al., 2006. Aggregation behavior of polyisoprene chain ends during living anionic polymerization as investigated by time-resolved small-angle neutron scattering. Physica B 385, 752-755. 

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