Preparation of 3,4-Polyisoprene

Monomeric isoprene is purified as described for styrene in Example 3-1 before use it is run under nitrogen through a 30-cm column packed with neutral aluminum oxide cyclohexane and 1,2-dimethoxyethane are refluxed over sodium for 6 h and distilled off under nitrogen. 

The type of arrangement of the monomeric units in polymeric dienes can be determined qualitatively and quantitatively by IR spectroscopy. For this purpose thin films are prepared by dropping an approximately 2% solution in carbon disulfide (spectroscopically pure) on to suitable rock salt plates and allowing the solvent to evaporate at room temperature.The plates are placed in the spectrometer beam and the IR spectrum is recorded.The different types of chemical linkages are associated with characteristic IR bands as summarized in Table 3.9. 

The spectra may first be evaluated qualitatively.The polyisoprene prepared in solution shows a pronounced band at 888 cm which indicates a high proportion of 3,4-link-ages. For the product of bulk polymerization this band is much reduced in favor of absorptions at 1127 and 1315 cm indicating predominantly c/s-1,4-linkage of the monomeric units in this case.The polymer made by radical polymerization in emulsion (see Example 3-12) shows the presence of all possible structural units,although the proportion of c/s-1,4-linkages is low. 

A distinction between 1,2- and 1,4-arrangement is possible through selective epoxida-tion of the olefinic components using rrt-chloroperbenzoic acid. Double bonds in the 

Polymerization by opening of C=0 bonds was investigated many years ago in the case of formaldehyde. As may be expected from its polar mesomeric structure formaldehyde can polymerize both anionically and cationically  

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Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. (a) Preparation of 3,4-Polyisoprene... 

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