Endo/Exo Reactivity Ratios in Living Vinyl Addition Polymerization of Substituted Norbornenes

Substituted norbornenes may be enchained by vinyl addition polymerization (VAP), through the norbornene double bond. Such monomers are generally prepared by Diels–Alder cycloaddition of cyclopentadiene and the corresponding olefin, which leads to a mixture of endo and exo stereoisomers with potentially very different reactivity, but these differences have not previously been quantified (e.g., as copolymerization reactivity ratios r_endo and r_exo). A living Pd-based VAP initiator is employed and the consumption of endo and exo monomers to high conversions is measured to determine r_endo and r_exo; since the polymerization is living, the products are gradient copolymers. For norbornenes bearing n-butyl, norbornyl, or methylhexafluoroisopropanol substituents, r_exo = 1–4, and r_exo r_endo = 0.2–0.8, indicating only moderate deviations from ideal copolymerization behavior. By contrast, for norbornene bearing a pentamethyldisiloxane substituent, r_endo is indistinguishable from zero, indicating that the endo isomer is effectively incapable of homopropagation. However, since the polymerization is living, propagation resumes (and chain extension proceeds) when additional exo monomer is charged.