Endo/exo reactivity ratios in living vinyl addition polymerization of substituted norbornenes

Abstract

Substituted norbornenes are an attractive platform for material synthesis: they can be enchained by either living vinyl addition polymerization (VAP) or by ring-opening polymerization (ROMP). The Diels-Alder route to produce substituted norbornenes naturally leads to a mixture of endo and exo isomers, and the two isomers can differ widely in polymerization reactivity. Thus, a gradient polymer can be obtained during a living polymerization, as the polymerization of the “monomer” is a copolymerization of the two isomers. This project aimed to determine the reactivity ratios for endo/exo isomers of several C-5 mono-substituted norbornene monomers with a particular Pd-based VAP proinitiator, denoted Pd343. The following reactivity ratios were determined for endo/exo isomers (± one standard deviation): butyl (0.32 ± 0.02 / 2.64 ± 0.26), methylhexafluoroisopropanol (0.12 ± 0.01 / 3.99 ± 0.08), norbornyl (0.27 ± 0.02 / 2.38 ± 0.28), and pentamethyldisiloxane (0.001 ± 0.01 / 7.51 ± 0.03). The reactivity ratios of these monomers indicate that the copolymerization tends towards a gradient copolymer. If the exo and endo monomers yield polymers with different physical properties, then the gradient copolymers could behave differently from a truly random copolymer, particularly if the ends of the copolymer are collocated, as in a block copolymer, or if grafted to a solid surface.These reactivity ratios were found using the Skeist model rather than the analytically simpler Beckingham model since the Beckingham model was found to not capture the copolymerizations of substituted norbornenes accurately. The terminal unit of the polymer chain plays a significant role in the selection of the next monomer during VAP, as the product of the reactivity ratios is not found to be close to unity. With respect to kinetics, a polymerization induction time is present with the Pd343 proinitiator but not the related Pd359 proinitiator, reflecting the importance of proinitiator selection.Given that the product of the reactivity ratios is not unity, batch copolymerizations of substituted norbornenes are not expected to precisely follow first-order kinetics. For pentamethyldisiloxane, severe departures from first-order kinetics were observed, due to the extremely low value of the endo reactivity ratio. For this monomer, an increase in exo content would be necessary to preclude excessively long polymerization times, which are accompanied by active site termination and/or chain transfer.These findings are crucial for permitting the synthesis of vinyl addition polymers of these substituted norbornenes, in terms of achieving the desired content and distribution of endo and exo monomers in the polymer chains, which in turn may affect both the polymer’s properties, as well as whether a polymerization may be easily run to completion.