Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Ring-opened metathesis polymers of 7-methylnorbornadiene (1) have been synthesized with cis main-chain double bond contents ranging from 20 to 97% using several initiators; their microstructures have been examined in detail using 13C NMR spectroscopy. The result show that polymerization from the anti-exo orientation is greatly favoured over the syn-exo mode. A nonbonded repulsion energy of 6-8 kJ · mol-1 between the syn-7-methyl group and the double bond is estimated. The microstructural features also resemble those for polymers of the anti- and syn-7-methylnorbornenes, in contrast to poly(norbornadiene) itself and to poly(norbornene), in that the splitting of a given resonance due to tacticity effects is of the same order as splittings due to neighbouring double bonds. Cis double bond contents and tacticities using different catalysts agree well with those expected from previous work with norbornadiene and several methyl-substituted norbornenes, respectively, as monomers. The previous surprising observation that there is significantly less or no discrimination between the syn- and anti-orientations in polymerizing 7-tert-butoxy- and 7-acetoxynorbornadiene is discussed and is attributed to a pseudo [3 + 2] transition state replacing the normal one for [2 + 2] metallacyclobutanation. In the [3 + 2] mode a lone pair of electrons on the oxygen acts in conjunction with the π pairs of the C=C and [Mt]=C moieties. The net effect is to lower the energy barriers for the [2 + 2] cycloaddition and the reverse step. The same explanation, rather than chelation, is also believed to be correct for the beneficial effect on the metathesis polymerization of the 7-oxa group in several substituted 7-oxanorbornenes and 7-oxanorbornadienes when compared to the corresponding 7-methylene monomers.
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