4 research outputs found
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Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization
Alkyne
cross-metathesis of molybdenum carbyne complex [TolCMo(OCCH<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>]·DME with 2
equiv of functional ynamines or ynamides yields the primary cross-metathesis
product with high regioselectivity (>98%) along with a molybdenum
metallacyclobutadiene complex. NMR and X-ray crystal structure analysis
reveals that ynamides derived from 1-(phenylethynyl)pyrrolidin-2-one
selectively cleave the propagating molybdenum species in the ring-opening
alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[<i>a</i>,<i>e</i>][8]annulene and irreversibly deactivate
the diamagnetic molybdenum metallacyclobutadiene complex through a
multidentate chelate binding mode. The chain termination of living
ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers
a functional end-group to the polymer chain, giving access to telechelic
polymers. This regioselective carbyne transfer strategy gives access
to amphiphilic block copolymers through synthetic cascades of ROAMP
followed by ring-opening polymerization of strained ε-caprolactone
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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-C<sub>6</sub>H<sub>4</sub>CMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] 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[<i>a</i>,<i>e</i>][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 (<i>k</i><sub>i</sub>) over the rate
of propagation (<i>k</i><sub>p</sub>) but also prevent undesired
intra- and intermolecular chain-transfer processes, giving access
to linear <i>poly</i>-(<i>o</i>-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 <i>poly</i>-(<i>o</i>-phenylene
ethynylene)-<i>block</i>-<i>poly</i>-(methyl acrylate)
block copolymers
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Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization
Molybdenum
carbyne complexes [RCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] featuring a mesityl (R
= Mes) or an ethyl (R = Et) substituent initiate the living ring-opening
alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[<i>a</i>,<i>e</i>][8]annulene, to yield fully conjugated
poly(<i>o</i>-phenylene ethynylene). The difference in the
steric demand of the polymer end-group (Mes vs Et) transferred during
the initiation step determines the topology of the resulting polymer
chain. While [MesCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear poly(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] results in cyclic
polymers ranging in size from <i>n</i> = 5 to 20 monomer
units. Kinetic studies reveal that the propagating species emerging
from [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] undergoes a highly selective intramolecular backbiting
into the butynyl end-group
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Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization
Molybdenum
carbyne complexes [RCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] featuring a mesityl (R
= Mes) or an ethyl (R = Et) substituent initiate the living ring-opening
alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[<i>a</i>,<i>e</i>][8]annulene, to yield fully conjugated
poly(<i>o</i>-phenylene ethynylene). The difference in the
steric demand of the polymer end-group (Mes vs Et) transferred during
the initiation step determines the topology of the resulting polymer
chain. While [MesCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] exclusively yields linear poly(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] results in cyclic
polymers ranging in size from <i>n</i> = 5 to 20 monomer
units. Kinetic studies reveal that the propagating species emerging
from [EtCMo(OC(CH<sub>3</sub>)(CF<sub>3</sub>)<sub>2</sub>)<sub>3</sub>] undergoes a highly selective intramolecular backbiting
into the butynyl end-group