Helical Structure of 2,3-exo-Disyndiotactic Polynorbornene

Norbornene (NB) has been polymerized by many catalysts and different types of polymers have been obtained. Of particular interest is the vinyl-type polynorbornene (PNB), which has industrial relevance for optical and electronic applications. Most of the catalysts so far identified for vinyltype polymerization of norbornene give polymers that are amorphous by X-ray examination because of their low tacticity[1]. Crystalline polymers, however, have been obtained with some catalysts: Al(iBu)3-TiCl4 [2]; ansa-zirconocene-MAO (MAO = methylaluminossane)[3]; CrCl2(L)2- MAO (L = bidentate phosphine)[4]. The polymerization with metallocene proceeds by means of a cis-exo monomer insertion, but a C7 linkage is formed as a result of a -bond methatesis reaction. Due to the formation of the C7 linkage, metallocene PNBs cannot be considered of vinyl–type. The crystalline PNB obtained with the chromium system MAO-CrCl2(dppa) has a 2,3-exo-diheterotactic structure, consisting of stereoregular sequences of mr triads. As mentioned, a crystalline PNB was obtained in 1963 with Al(iBu)3-TiCl4, but its structure was not investigated. Metathesis reactions as those observed with zirconocene catalysys are less common with Ti catalysts, and therefore it appeared of interest to examine the structure of the TiCl4-PNB. In a previous Note we reported on the oligomerization of norbornene (NB) using the catalyst system AlEt2Cl-TiCl4 at a monomer/titanium molar ratio of about 11. In addition to other oligomers, a crystalline heptamer was isolated having a stereoregular 2,3-exo-disyndiotactic structure[5]. As a continuation of this work we have now examined the polymerization of NB at higher monomer/titanium molar ratios, using the same catalyst, under the same conditions. Products of high molecular weight have been obtained, all consisting of an amorphous fraction soluble in diethyl ether and a crystalline fraction, part of which is soluble in boiling toluene. In this poster communication we report that the crystalline fractions obtained at high monomer/TiCl4 ratio consist of polymers having a 2,3-exo-disyndiotactic structure as the heptamer.