Carbocationic Copolymerization of Isobutylene and 2,4-Dimethyl-1,3-Pentadiene

The copolymerization behavior of isobutylene (IB) and 2,4-dimethyl-1,3-pentadiene (DMPD) was studied with the intent of producing random copolymers of these two monomers. Copolymerizations were conducted between −76 °C and −40 °C using either TiCl4 or ethylaluminum dichloride (EADC) catalysis in solvent mixtures ranging from pure methyl chloride to a 60/40 mixture of hexane and methyl chloride. Contrary to expectations based on prior literature reports, we observed overwhelmingly greater reactivity of DMPD relative to IB. NMR analysis gave evidence that the products of attempted copolymerizations were poly­(DMPD-b-IB) block copolymers under all conditions employed, and monomer conversion data from FTIR revealed that a portion of the DMPD was lost to side products, likely cyclic and/or straight-chain DMPD oligomers. Furthermore, we observed a minor fraction of 1,2-additions of DMPD, which is typically not observed under these conditions for isoprene. Homopolymers of DMPD were synthesized to aid NMR structural assignments in the copolymer. Chain-transfer dominated polymerizations of DMPD occurred under the same conditions that produce living polymerization of isobutylene, further suggesting the higher relative reactivity of DMPD and the unsuitability of this comonomer pair toward the synthesis of random copolymers. The fraction of 1,2 additions in DMPD homopolymers averaged about 0.13