Oligomerization and Polymerization of Isoprene Catalyzed by Alkylaluminium with Different Structures

Alkylaluminium (AlR3), as co-catalyst component in Ziegler-Natta catalytic system, plays important roles in the alkylation, forming and changing the structure and concentration of active centers through the reduction and reversible adsorption-desorption reactions with the metal compound of the catalyst, acting as chain transfer agent, etc. However, the alkylaluminium itself do have the catalytic effect on the conjugated diene monomers. In this article, alkylaluminium with different structures such as triethylaluminium (AlEt3), triisobutylaluminium (Al(i-Bu)(3)), diisobutylaluminium hydride (AlH(i-Bu)(2)), diethylaluminium chloride (AlEt2Cl), ethylaluminium dichloride (AlEtCl2) were used to catalyze isoprene oligomerization and polymerization. The effects of the structure and concentration of alkylaluminiums (n(Al)/n(M)=7 x 10(-5), 35 x 10(-5), 350 x 10(-5), 1050 x 10(-5)) on the catalytic behaviors of isoprene were studied. The microstructure (trans-1,4 and cis-1,4), molecular weight and molecular weight distribution of the products were characterized by H-1 nuclear magnetic resonance spectroscopy (H-1 NMR), gel permeation chromatography (GPC) and gas chromatography-mass spectrometry (GC-MS). It was found that alkylaluminium could initiate oligomerization and cationic polymerization of isoprene under the minor incorporation of H2O, which were affected greatly by the structure and concentration of alkylaluminium. Using AIEtCl(2) led to the highest catalytic activity and produced products containing more linear polymers with mixed cis-1,4/trans-1,4 structures when n(Al)/n(M)=1050 x 10(-5). The Al(i-Bu)(3) and AlH(i-Bu)(2) didn't have basically cation initiation ability, which led to isoprene oligomerization. The alkylaluminium with n(Al)/n(M)<= 350 x10(-5) had negligible influence on the isoprene polymerization and oligomerization. And lower or higher alkylaluminium concentration were not beneficial to obtain polyisoprene with high molecular weight. The catalytic mechanism of alkylaluminium on isoprene was discussed, which provided a further understanding on the catalytic behavior of alkylaluminium components in Ziegler-Natta catalyst and the effect of alkylaluminium on polymers.