Cz-polyisoprene

From the data given above calculate the values of cr and Coo for cis- and tram-polyisoprene, given that the length of an isoprene repeating unit is 4.60xl0 °ra. [Ans. Czs-polyisoprene cr = 1.2 Coo = 3.1. Trans-polyisoprene cr = 1.8 Coo = 6.4]... 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

Conventional prefixes indicating cis and trans isomers are placed in front of the polymer name. An example is cZs-1,4-polybutadiene, or in frans-l,4-polyisoprene. 

In contrast to the behavior of butadiene stand the reactions of the substituted butadienes. As many workers have shown, iso-prene is converted to an essentially all czs-l,4-polyisoprene at RsAl/TiCb = 1.0. Likewise, 2,3-dimethyl-l,3-butadiene (Yen, 1959a), and 2-iert-butyl-l,3-butadiene (Marconi et al., 1964) are polymerized to predominantly m-1,4 polymers under the same conditions. At other ratios (Al/Ti > 1.0) isoprene is still converted to a predominantly cis-1,4 polymer, though in de-... 

The ratio MwjMn approaches unity asymptotically as Vm increases. Narrow molecular weight distributions should thus be obtained in living ionic polymerizations with fast initiation in the absence of depropagation, termination, and chain transfer reactions. Values of polydispersity index (PDI) below 1.1-1.2 are indeed found for many living polymerizations. Molecular weight standards for polystyrene, polyisoprene, poly(a-methylstyrene), and poly(methyl methacrylate) are thus synthesized by living anionic polymerizations. However, the termination reactions in methyl methacrylate polymerizations and depropagation in cz-methylstyrene polymerizations tend to broaden the PDI in these systems. 

As a consequence of the lower molecular weight of high cz y-polyisoprene and probably reduced branching compared to natural rubber, dissolution in common solvents is both fast and provides solutions having significantly lower viscosities at the same concentrations (Fig. 1). This phenomenon has been put to useful purpose by some manufacturers of pressure sensitive adhesive tape by reducing the amount of solvent required in the adhesive... 

The nature of the vulcanisation of trans-1,4-polyisoprene (TPI) and the effect of the degree of vulcanisation on the mechanical properties of vulcanised TPI were studied. The results showed that among three conventional types of accelerator used, (M, CZ, and TT), the vulcanisation curve of TPI with CZ as accelerator was found to be the most suitable. Vulcanised TPI was found to change suddenly from a hard material to a soft elastic rubber when the sulphur content increased from 4 phr to 5 phr. In addition, the addition of carbon black influenced the tensile strength advantageously. 5 refs. 

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