Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers

Regioselective carbyne-transfer reagents
derived from (3,3,3-trifluoroprop-1-yn-1-yl)­benzene
give access to functionalized ring-opening alkyne metathesis polymerization
(ROAMP) initiators [R-C6H4CMo­(OC­(CH3)­(CF3)2)3] featuring electron-donating
or -withdrawing substituents on the benzylidyne. Kinetic studies and
linear free-energy relationships reveal that the initiation step of
the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo­[a,e]­[8]­annulene exhibits a moderate positive
Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring
electron-withdrawing benzylidynes not only selectively increase the
rate of initiation (ki) over the rate
of propagation (kp) but also prevent undesired
intra- and intermolecular chain-transfer processes, giving access
to linear poly-(o-phenylene ethynylene)
with narrow molecular weight distribution. The regioselective carbyne
transfer methodology and the detailed mechanistic insight enabled
the design of a bifunctional ROAMP-reversible addition–fragmentation
chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization
yields hybrid poly-(o-phenylene
ethynylene)-block-poly-(methyl acrylate)
block copolymers.