Molecular weight polyisoprene

A number of high molecular weight polyisoprenes occur in nature which differ from natural rubber in that they are essentially non-elastic. As with natural rubber they are obtained from the latex of certain plants but they differ in that they are either frani-l,4-polyisoprenes and/or are associated with large quantities of resinous matter. 

Fig. 10. The variation of 6, with E for three different molecular weight polyisoprene tethered chains in a cross-linked polyisoprene lens [23].

In summary, cationic polymerization gives polymers of structure P8a Ziegler-Natta complexes mainly lead to blocks of structure P8b. This study gives further evidence for cyclization reactions of high-molecular-weight polyisoprenes. This point of view has been confirmed by the study of the cyclization of model polyisoprene molecules with two, three, or four monomer units toward the catalysts able to initiate such a reaction (24). 

Similar results were obtained with 1-methyl cyclohexene which is a more accurate model of natural rubber than cyclohexene. However with low molecular weight polyisoprenes such as dihydromyrcene, geraniolene and squalene intramolecular reactions leading to the production of cyclic sulphides were also observed. 

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). 

In Table 1 some of the properties of raw synthetic hj -l,4-polyisoprene (Goodyear s Natsyn) and natural mbber (Hevea) are presented along with references that contain additional thermal, optical, electrical, and mechanical property data. Some properties of synthetic /n j -l,4-polyisoprene (Kuraray TP-301) are also given. Molecular weights and mol wt distribution are determined by gel-permeation chromatography (gpc) (11). 

Natural latex is polydisperse (size of individual particles may vary from 0.01 to 5 p.m). Flowever, synthetic latex has a relatively narrow particle size, and therefore the viscosity at a given rubber content is higher in synthetic rubber (polyisoprene) solutions. The average molecular weight is typically about I million g/mol, although it depends on the gel content. 

The influence of the gel content in polyisoprene-tackifier blends on creep resistance and peel behaviour have been recently studied [62]. The gel content was achieved by cross-linking the adhesives with electron beam irradiation. The molecular weight of the soluble fraction in the blend was always dominated by that of the initial elastomer. Creep resistance was achieved either through molecular weight increases or gel content increases. However, the peel strength is strongly... 

To confirm these phenomena the UV absorption spectra of lithium polyisoprene of high and of low molecular weights dissolved in cyclohexane were re-examined over... 

Fig. 24a and b. Differential molecular weight distribution curves of polyisoprene prepared, a) with oligoisoprenyl-Li b) with oligoisoprenyl-Li in the presence of NNN N tetramethyl ethylenediamine [TMDA]/[OLi] = 0.01) (A. Davidjan, N. N. Lolaev, V. Sgonnik, B. Belenkii, V. Nesterov, B. Erus-salimsky, Ref. 1531... 

In the low molecular weight fraction a relatively high content of the 3,4-linkages is formed, characteristic of the polymers produced in the presence of TMEDA. On this basis it was concluded that the unassociated polymers coordinated with TMEDA propagate more slowly than those unassociated and non-coordinated with the diamine, a conclusion concordant with the previously discussed findings of Fontanille84) and confirmed by Schue, 55> for lithium polyisoprene. 

It has previously been found experimentally [22], albeit for a different elastomer (a synthetic polyisoprene), that to a good degree of approximation there is a linear relationship between tan A and average inter-cross-link molecular weight, IVE, such that tan A oc Me. We therefore write ... 

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