Isomerization polyisoprenes

The chemical shifts of the characteristic carbon signals in acyclic terpenes, polyprenols, and cis-trans isomerized poly-isoprenes are plotted in Fig. 3. Here, the chemical shifts are correlated using the w C-5 methyl carbon signal at 17.66 ppm as an internal standard (except for isomerized polyisoprenes) in order to compensate for the effect of solution concentration. It is clear that these chemical shifts are independent of the chain length of the compounds and can be used for the determination of the arrangement of isoprene units as well as the terminal units in various isoprenoid compounds (8). 

Fig. 7.1. NMR spectra of hevea (A), balata (C) and isomerized polyisoprene (B) at 100 Me. (Reprinted with permission from J. Am. Chem. Soc., 84, 4981 (1962). Copyright by the American Chemical Society.)...

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

Fig. 2. To and We for isomers of polybutadiene and polyisoprene, and for typical average isomeric compositions in styrenic block copolymers.

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. 

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 reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

The sulfonation of low molecular weight model olefins was undertaken to determine the feasibility of this approach. Competitive sulfonations using acetyl sulfate were carried out on the model compounds below, representing the repeat structures of cis-l,4-polyisoprene (PIP), cw-l,4-polybutadiene (c-PBD), and trans-l,4-polybutadiene (Z-PBD), respectively. It was necessary to model both the cis and trans isomeric forms of 1,4-polybutadiene, since ttey have a nearly equal probability of occurrence when the anionic polymerization (Ii counterion) is conducted in a nonpolar hydrocarbon medium... 

The simplest unit is 1,4-isoprene, although there are two distinct cis and trans isomeric configurations of 1,4-polyisoprene. 

Effect of Temperature. Table III shows the effect of polymerization temperature on the chain structure of lithium polyisoprene, both in the case of undiluted monomer and in the presence of n-hexane as solvent. Within the ranges shown, there does not appear to be any influence of temperature on the placement of the various isomeric chain unit structures. 

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. 

A third backbone index trans was defined by Equation 12.24, as the number of carbon-carbon double bonds along the chain backbone which are in a trans configuration divided by the number NBB of backbone atoms. If there is a mixture of cis and trans isomerization around the C=C bonds along the chain backbone, only the trans double bonds are counted. For example, all-trans polyisoprene has one trans C=C backbone bond per repeat unit with NBB=4, so that... 

In later studies, Golub and Danon [329, 330] reported that, in addition to loss of unsaturation, polybutadiene and polyisoprene films and solutions undergo radiation-induced cis-trans isomerization. 

Problem 2.9 Polymers of dienes (hydrocarbons containing two carbon-carbon double bonds), such as butadiene and isoprene, have the potential for head-to-tail and head-to-head isomerism and variations in double-bond position as well. How many constitutional isomers can form in the polymerization of (a) polybutadiene and (b) polyisoprene ... 

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

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. 

Gutta-percha. The name, derived from Malayan ge-tah pertcha=latex of the percha tree, for a natural rubber (structure, see there) from the gutta-percha trees Palaquium gutta and P. oblongifolia, Sapotaceae) with properties similar to those of balata. In Sumatra, Java, and south east India, the rapidly coagulating latex of incised trees is collected, rapidly kneaded, and marketed as raw G. Pure G. is the all-trans-isorntr of polyisoprene, related to balata molecular mass ca. 100000. In contrast to the cis-isomeric natural rubber, G. is hard and less elastic but not brittle, it softens at 25-30°C, becomes plastic at 60 °C, and melts at >100°C with decomposition and formation of a sticky mass. For uses, see literature. 

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. 

As shown in Figure 1.7, the polymerization of monomers with two double bonds (e.g., butadiene and isoprene) leads to polymer chains with a residual double bond per monomer unit cis-trans isomerism is possible. Two important polymers that show this type of isomerism are 1,4-polybutadiene and 1,4-polyisoprene. The regularity of the trans configuration makes this type of isomer more crystalline, with a higher melting point compared to the cis configuration. 

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