15 research outputs found
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Ring-Opening Alkyne Metathesis Methods For Functional Conjugated Polymer Synthesis
Since its discovery in the mid 20th century, most applications of alkyne metathesis have relied on thermodynamics to control product distributions. Ring-opening alkyne metathe- sis polymerization (ROAMP), in contrast, requires the kinetic product of metathesis of a strained, cyclic alkyne monomer to give a living, chain-growth polymerization (Chapter 1, Introduction). This living polymerization of conjugated alkyne-containing monomers has the potential to access a wide range of functional poly(arylene ethynylene) materials with excep- tional control over length, dispersity, topology, and endgroups. To this end, we demonstrate the first ROAMP synthesis of conjugated poly(ortho-phenylene ethynylene) and elucidate a mechanistic description of the reaction to understand the enabling catalyst selectivity and unexpectely find that initiator sterics dictate endgroup fidelity and polymer topology (Chap- ter 2). To disentangle the role of steric and electronic factors in initiator performance, we describe a novel synthetic method that gives a series of isosteric benzylidyne catalysts which exhibit a strong, deterministic electronic effect on both ROAMP initiation rates and polymer endgroup fidelity (Chapter 3). Finally, we develop an extension of this methodology to lever- age the alkynes from these living polymers to template the synthesis of telechelic graphene nanoribbons (GNRs) (Chapter 4). This work has not only uncovered mechanistic insights and design principles to improve ROAMP catalysts and facilitate access to hybrid poly- mer materials, but also demonstrated the potential of ROAMP to access other conjugated materials via post-polymerization modification of precision polymer templates
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Ring-Opening Alkyne Metathesis Methods for Functional Conjugated Polymer Synthesis
Since its discovery in the mid 20th century, most applications of alkyne metathesis have relied on thermodynamics to control product distributions. Ring-opening alkyne metathe- sis polymerization (ROAMP), in contrast, requires the kinetic product of metathesis of a strained, cyclic alkyne monomer to give a living, chain-growth polymerization (Chapter 1, Introduction). This living polymerization of conjugated alkyne-containing monomers has the potential to access a wide range of functional poly(arylene ethynylene) materials with excep- tional control over length, dispersity, topology, and endgroups. To this end, we demonstrate the first ROAMP synthesis of conjugated poly(ortho-phenylene ethynylene) and elucidate a mechanistic description of the reaction to understand the enabling catalyst selectivity and unexpectely find that initiator sterics dictate endgroup fidelity and polymer topology (Chap- ter 2). To disentangle the role of steric and electronic factors in initiator performance, we describe a novel synthetic method that gives a series of isosteric benzylidyne catalysts which exhibit a strong, deterministic electronic effect on both ROAMP initiation rates and polymer endgroup fidelity (Chapter 3). Finally, we develop an extension of this methodology to lever- age the alkynes from these living polymers to template the synthesis of telechelic graphene nanoribbons (GNRs) (Chapter 4). This work has not only uncovered mechanistic insights and design principles to improve ROAMP catalysts and facilitate access to hybrid poly- mer materials, but also demonstrated the potential of ROAMP to access other conjugated materials via post-polymerization modification of precision polymer templates
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Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization.
Alkyne cross-metathesis of molybdenum carbyne complex [TolC≡Mo(OCCH3(CF3)2)3]·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[a,e][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
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
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 [RC≡Mo(OC(CH3)(CF3)2)3] 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[a,e][8]annulene, to yield fully conjugated poly(o-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 [MesC≡Mo(OC(CH3)(CF3)2)3] exclusively yields linear poly(o-phenylene ethynylene), polymerization initiated by [EtC≡Mo(OC(CH3)(CF3)2)3] results in cyclic polymers ranging in size from n = 5 to 20 monomer units. Kinetic studies reveal that the propagating species emerging from [EtC≡Mo(OC(CH3)(CF3)2)3] undergoes a highly selective intramolecular backbiting into the butynyl end-group
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Templated Synthesis of End-Functionalized Graphene Nanoribbons through Living Ring-Opening Alkyne Metathesis Polymerization.
Atomically precise bottom-up synthesized graphene nanoribbons (GNRs) are promising candidates for next-generation electronic materials. The incorporation of these highly tunable semiconductors into complex device architectures requires the development of synthetic tools that provide control over the absolute length, the sequence, and the end groups of GNRs. Here, we report the living chain-growth synthesis of chevron-type GNRs (cGNRs) templated by a poly-(arylene ethynylene) precursor prepared through ring-opening alkyne metathesis polymerization (ROAMP). The strained triple bonds of a macrocyclic monomer serve both as the site of polymerization and the reaction center for an annulation reaction that laterally extends the conjugated backbone to give cGNRs with predetermined lengths and end groups. The structural control provided by a living polymer-templated synthesis of GNRs paves the way for their future integration into hierarchical assemblies, sequence-defined heterojunctions, and well-defined single-GNR transistors via block copolymer templates