Special Polymers from Dienes
There are many reports in the literature of preparations of polymers from various other substituted dienes. Most have no commercial significance. Some are, however, interesting materials. An example is a polymer of 2-t-butyl-1,3-butadiene formed with TiCl4 and either alkylaluminum or aluminum hydride catalysts [132]. The polymer is crystalline and melts at 106°C. It can be dissolved in common solvents. Based on X-ray data, the monomer placement is high cis-1,4. Poly(carboxybutadiene)s also forms with coordination catalysts [133-135]:
where R = CH3; R′ = CH3, C2H5, C4H9, or C6H5.
X-ray crystallography [133-135] showed that the placement is trans-isotactic. Based on the mode of packing of the chains in the crystalline regions and from the encumbrance of the side groups in relationship to the main chain, an erythro configuration can be assigned [134]. The polymers, therefore, are trans-erythro-isotactic. Polymerization of 1,3-pentadiene can potentially result in five different insertions of the monomers. These are 1,4-cis, 1,4-trans, 1,2-cis, 1,2-trans, and 3,4. In addition, there are potentially 3-cis-1,4 and 3 trans-1,4 structures (isotactic, syndiotactic, and atactic). Formations of trans-1,4 isotactic, cis-1,4 isotactic, and cis-1,4 syndiotactic polymers are possible with Ziegler-Natta catalysts [136-138]. Amorphous polymers also form that are predominantly cis-1,4 or trans-1,4, but lack tactic order. Stereospecificity in poly(1,3-pentadiene) is strongly dependent upon the solvent used during the polymerization. Thus, cis-1,4 syndiotactic polymers form in aromatic solvents and trans-1,2 in aliphatic ones. The preparations require cobalt halide/aluminum alkyl dichloride (or dialkyl chloride) catalysts in combinations with Lewis bases. To form a trans-1,4 structure, a catalyst containing aluminum to titanium ratio close to 5 must be used [139].
