Oligomerization of dicyclopentadiene catalyzed by TiCl4/Et2AlCl

Norbornene and its derivatives vinyl-type polymers possess high glass transition and decomposition temperatures, a small optical birefringence and dielectric loss, which are advantageous for technical application in microelectronics. This application is however limited by the poor solubility and hard processability of the polymers. These drawbacks can be overcome by using functional norbornenes such as 5-vinyl-2-norbornene and dicyclopentadiene (DCPD). In addition to the ready availability of these monomers, the resulting polymers are expected to have double bonds which might be useful for further chemical functionalization. Recently we reported on the polymerization of DCPD (the mixture of endo/exo isomers) catalyzed by TiCl4/Et2AlCl. A crystalline tetramer with a 2,3-exo disyndiotactic compact structure with the cyclopentene in the endo conformation was obtained. As a continuation of this work, we have now examined the polymerization of both the isomers with the aim to i) study the influence of the starting monomer (single isomer or mixture of isomers) on the polymerization mechanism and stereospecificity and ii) prepare new functional materials. Exo- and endo- DCPD were polymerized with TiCl4/Et2AlCl in heptane at 0°C at different DCPD/Ti mole ratio. We found that the polymerization of the endo isomer, at low DCPD/Ti ratio (11-15), gave the crystalline DCPD tetramer as revealed by the XRD of the crude product. In contrast, the polymerization of the exo isomer, as well as the fractionation of the resulting polymers, gave only amourphous materials. To gain more insight on the formation of the crystalline tetramer, we polymerized, under the same reaction conditions, the hydrogenated-DCPD (H-DCPD). The XRD measurements showed that both the crude product and all the boiling solvent extracted fractions were amourphous. The results allowed us to support some of the hypotheses previously formulated on the DCPD oligomerization and to highlight some new features about the reaction mechanism. Indeed, we found that the stereochemisty of the oligomerization/polymerization is determined by two main factors: the exo or endo conformation of the cyclopentene ring and the monomer chemical nature (DCPD vs H-DCPD). Moreover, the availability of the DCPD tetramer has allowed us to have access, via hydrogenation reaction with p-toluenesulfonyl hydrazide, to an hydrogenated tetramer with a crystalline structure different from the starting material, which we were not able to obtain from the stereospecific polymerization of the corresponding H-DCPD monomer. Further experiments and computational studies are in progress in order to better clarify the effective role of the monomer structure in determining the polymerization selectivity.