Polyisoprene tacticity

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

So Cataldo et al. [101] have focused their studies on polymers like 1-2 polybutadiene, 3-4 polyisoprene and poly(4-methyl-l,3 pentadiene). These polymers are very sensitive to ozone, and their own tacticity seems to have a very low influence on their reactivity. In a usual way, substituents make... 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg= — 108°C) and trans(T = — 18°C) or polyisoprene cis Tg = —73°C) and trans T = —53°C). An example of tacticity variation is polyfmethyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

The principles apphed in the previous section to essentially polar monomers can be extended to the stereoregnlar polymerization of dienes by alkali metals and metal alkyls. We have already seen that the cis-trans isomerism presents a variety of possible structures for the polydiene to adopt and complicates the preparation of a sample containing only one form rather than a mixture. Thus polyisoprene may contain units in the 1,2 or 3,4, or cis-1,4 or trans-1,4 configuration without even considering the tacticity of the 1,2 or 3,4 monomer sequences in the chain. 

IV to VIII metals and base metal alkyls of Group II or III metals (Penczek and Premia, 2012 Boor, 1979 Ciardelli, 1992). It arose from the spectacular discovery of Ziegler et al. (1955) that mixtures of titanium tetrachloride and aluminum alkyls polymerize ethylene at low pressures and temperatures and from the equally spectacular discovery by Natta (1955) that the Ziegler catalysts can stereospecifically polymerize monoolefins to produce tactic, crystalline polymers. As can be imagined, these systems can involve many combinations of catalyst components, not all of which are catalytically active or stereospecific. However, we shall be concerned here only with polymerizations involving the commercial elastomers, principally polyisoprene, polybutadiene (Duck and Locke, 1977 Zohuri et al., 2012 Teyssie et al., 1988), and the ethylene-propylene copolymers (Schobel et al., 2012 Ver Strate, 1986 Davis et al., 1996 Noordermeer, 2003 Baldwin and Strate, 1972). 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

As described in Chapters 2 and 3, the monomer can be inserted into the polyisoprene chain potentially in nine different ways. These are the three tactic forms of the 1,2-adducts, two 1,4-adducts, cis and transy and three tactic forms of 3,4-adducts. In addition, there is some possibility of head-to-head and tail-to-tail insertion, though the common addition is head-to-tail. Table 5.8 presents the various microstructuies that can be obtained in polymerizations of isoprene with different catalysts. 

During the period covered by this article a number of books and review articles have been published. Some of these are fairly general. Others deal with one polymer, for example polyisoprene, or one aspect of the subject such as the effects of tacticity in polymer reactions or pKjlymer modifications with polymerizable monomers. ... 

Infrared spectroscopy also provides information on molecular microstructure, e.g. the repeat units resulting from addition polymerization of dienes. For example, polyisoprenes (Fig. 2.9) can be distinguished, based on differences in absorption between C-H out-of-plane bending vibrations. The infrared spectra of stereoregular polymers are also distinct from those of their less regular counterparts, but these differences do not arise directly from tacticity but indirectly due to its effect on chain conformation. 

If all the units along the chain are trans or if all the units along the chain are ois the polymer is called tactic in the first case trans-tactic, in the second case cis-tactic. It may be noted that, in the case of 1,4 polyisoprene, these two possibilities correspond to guttapercha and natural rubber respectively tactic polymers of isoprene are naturally occurring polymers. 

---