Monomer Discrimination in the Enantioselective Living Coordinative Chain Transfer Polymerization of α,ω-Nonconjugated Dienes vs α‑Olefins

The stereoselective living coordinative polymerization
(LCP) of
α-olefins and α,ω-nonconjugated dienes can be achieved
using initiators derived from the homochiral, configurationally stable,
monocyclopentadienyl, 7- and 9-membered-ring cycloamidinate group 4 metal complexes, 1–3, respectively,
as preinitiators in combination with borate B1 as activating
co-initiator. However, yield, molar mass (Mn), and degree of stereoregularity in these LCPs were found to be
highly dependent upon initial monomer concentration due to decreased
magnitudes for monomer complexation and site isomerization, KM, and KSI, respectively.
The enantioselective living coordinative chain transfer
polymerization (LCCTP) of 1,5-hexadiene using the initiator derived
from 1/B1 and excess equivalents of ZnEt2 as chain transfer agent (CTA) provides isotactic cis/trans poly(methylene-1,3-cyclopentane)
(PMCP) in high yield. In contrast, under identical conditions, all
attempts to carry out the enantioselective LCCTP of 1-hexene failed.
It is proposed that this unprecedented discrimination of polymerizability
between the two monomer types for LCCTP is due to stark differences
in conformational freedom of the growing polyolefin chains. More specifically,
in the case of the rigid-rod nature of growing PMCP, reversible chain
transfer is not sterically impeded and can occur as required, whereas
the unrestricted conformational flexibility of side chains in poly(1-hexene)
creates a steric barrier that prohibits chain transfer to occur after
just a few monomer insertions. Collectively, these results provide
insights for designs of the next generation of enantiopure and configurationally
stable cycloamidinate preinitiators that can potentially lead to the
successful enantioselective LCCTP of both α,ω-nonconjugated
dienes and α-olefins with high stereoregularity.