Chemoselective, Stereospecific, and Living Polymerization of Polar Divinyl Monomers by Chiral Zirconocenium Catalysts

This contribution reports the first
chemoselective, stereospecific,
and living polymerization of polar divinyl monomers, enabled by chiral ansa-zirconocenium catalysts through an enantiomorphic-site
controlled coordination–addition polymerization mechanism.
Silyl-bridged-ansa-zirconocenium ester enolate 2 has been synthesized and structurally characterized, but
it exhibits low to negligible activity and stereospecificity in the
polymerization of polar divinyl monomers including vinyl methacrylate
(VMA), allyl methacrylate (AMA), 4-vinylbenzyl methacrylate (VBMA),
and N,N-diallyl acrylamide (DAA).
In contrast, ethylene-bridged-ansa-zirconocenium
ester enolate 1 is highly active and stereospecific in
the polymerization of such monomers including AMA, VBMA, and DAA.
The polymerization by 1 is perfectly chemoselective for
all four polar divinyl monomers, proceeding exclusively through conjugate
addition across the methacrylic CC bond, while leaving the
pendant CC bonds intact. The polymerization of DAA is most
stereospecific and controlled, producing essentially stereoperfect
isotactic PDAA with [mmmm] > 99%, Mn matching the theoretical value (thus a quantitative
initiation efficiency), and a narrow molecular weight distribution
(Đ = 1.06–1.16). The
stereospecificity is slightly lower for the AMA polymerization but
still leading to highly isotactic poly­(allyl methacrylate) (PAMA)
with 95–97% [mm]. The polymerization of VBMA
is further less stereospecific, affording PVBMA with 90–94%
[mm], while the polymerization VMA is least stereospecific.
Several lines of evidence from both homo- and block copolymerization
results have demonstrated living characteristics of the AMA polymerization
by 1. Mechanistic studies of this polymerization have
yielded a monometallic coordination–addition polymerization
mechanism involving the eight-membered chelating intermediate. Post-functionalization
of isotactic polymers bearing the pendant vinyl group on every repeating
unit via the thiol–ene “click” reaction achieves
a full conversion of all the pendant double bonds to the corresponding
thioether bonds. Photocuring of such isotactic polymers is also successful,
producing an elastic material readily characterizable by dynamic mechanical
analysis.