Cyclopolymerization of Conjugated Dienes
Conjugated dienes like isoprene, butadiene, and chloroprene cyclopolymerize with catalysts consisting of aluminum alkyls, like ethylaluminum dichloride, and titanium tetrachloride [141]. The ladder polymers that form contain fused cyclic structures. The products prepared in hexane are generally insoluble powders, while those prepared in aromatic solvents are soluble even when the molecular weights are high [142]. A high ratio of the transition metal halide to that of the aluminum alkyl must be used. Such a ratio might, conceivably, mean that the mechanism of polymerization is cationic. Also, conventional cationic initiators can be used to yield similar products. The cyclization occurs during propagation. Unsaturation in the products can vary from none to as high as 80%, depending upon the initiator used [142]. Different mechanisms were offered to explain the cyclization of 1,3 dienes [142, 143]. The cyclization might conceivably occur by a sequential process:
or, perhaps from attacks by the propagating carbon cation on trans-1,4 double bonds:
where R* can represent either a propagating carbon cation or an initiating species. The extensive cyclization may be a result of a sequential process [142, 143]. Cyclopolymerizations typically result in low conversions and dormant reaction mixtures. When additional monomer is added, the dormant mixtures reinitiate polymerizations that again proceed to some limited conversions. If the original dormant mixtures are allowed to stand for a long time the unreacted monomers are slowly consumed [142].Polymerization of 2,3-dimethylbutadiene-1,3 with Ziegler-Natta catalysts consisting of Al(i-C4H9)3-TiCl4 yields cis-1,4-polydimethylbutadiene as described earlier. This, however, takes place when the aluminum alkyl is in excess. If, on the other hand, the ratio of Al to Ti is 1 or less, cyclic polymer forms instead. The product has reduced unsaturation and some trans-1,4 units in the chain [144]. A complex catalyst, consisting of Al(i-C4H9)3-CoCl2, yields polymers that are predomi- nantly cis-1,4 with about 20% of 1,2 units. On the other hand, acid catalysts, like Al(C2H5) Cl2, yield cyclic polymers [143, 144]. A polymer formed with the aid of X-ray radiation at low temperatures also contains cyclic units and some trans-1,4 [145]. Butadiene and isoprene also form this type of polymer at the same conditions [145].
