Enhancement of stereospecificity in isoprene polymerization with alkylaluminum-titanium chloride catalysts through use of carbon disulfide

The effect of CS2 on isoprene polymerization with triisobutylaluminum‐titanium tetrachloride catalysts was studied at Al/Ti ratios of optimum (0.9) and higher values. In the absence of CS2, appreciable amounts of low molecular weight oils (“extractables”) were formed at the expense of cis‐1,4‐polyisoprene with higher than optimum Al/Ti ratios. Small amounts of CS2 were found to prevent extractables formation and allow attainment of higher yields of cis‐1,4‐polyisoprene. The optimum CS2/Ti chloride molar ratio (0.1) was independent of the Al/Ti ratio of the catalyst. Polymer microstructure and dilute solution viscosity were unaffected by CS2. The results support the theory that the catalyst surfaces hold two types of active sites: p‐sites, which initiate polymerization, and o‐sites, which lead to oligomerization. CS2 appears to enhance polymerization by coordinating selectively at the o‐sites. The predominance of oligomerization at the higher Al/Ti ratios was attributed to a destruction of p‐sites by excess trialkyl‐aluminum.