Abstract:
The bimetallic cluster Na[W-2(mu-Cl)(3)Cl-4(THF)(2)].(THF)(3)
(W-2, W-3-W(6+), a’(2)e’(4)), which features a triple
metal-metal bond, is a highly efficient room-temperature initiator for
ring opening metathesis polymerization (ROMP) of norbornene (NBE) and
norbornadiene (NBD), providing high-cis polymers. In this work, W-2
was used for the copolymerization of the aforementioned monomers,
yielding statistical poly(norbornene)/poly(norbornadiene) PNBE/PNBD
copolymers of high molecular weight and high-cis content. The
composition of the polymer chain was estimated by C-13 CPMAS NMR data
and it was found that the ratio of PNBE/PNBD segments in the polymer
chain was relative to the monomer molar ratio in the reaction mixture.
The thermal properties of all copolymers were similar, resembled the
properties of PNBD homopolymer and indicated a high degree of
cross-linking. The morphology of all materials in this study was smooth
and non-porous; copolymers with higher PNBE content featured a
corrugated morphology. Glass transition temperatures were lower for the
copolymers than for the homopolymers, providing a strong indication that
those materials featured a branched-shaped structure. This conclusion
was further supported by viscosity measurements of copolymers solutions
in THF. The molecular structure of those materials can be controlled,
potentially leading to well-defined star polymers via the
“core-first” synthesis method. Therefore, W-2 is not only a
cost-efficient, practical, highly active, and cis-stereoselective
ROMP-initiator, but it can also be used for the synthesis of more
complex macromolecular structures.
