Yungwan Kwak

Understanding Atom Transfer Radical Polymerization: Effect of Ligand and Initiator Structures on the Equilibrium Constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.

Synergistic Interaction Between ATRP and RAFT: Taking the Best of Each World*

This review covers recent developments on the combination of atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer (RAFT) polymerization to produce well controlled (co)polymers. This review discusses the relative reactivity of the R group in ATRP and RAFT, provides a comparison of dithiocarbamate (DC), trithiocarbonate (TTC), dithioester (DTE), and xanthate versus bromine or chlorine, and an optimization of catalyst/ligand selection. The level of control in iniferter polymerization with DC was greatly improved by the addition of a copper complex. New TTC inifers with bromopropionate and bromoisobutyrate groups have been prepared to conduct, concurrently or sequentially, ATRP from Br-end groups, ATRP from the TTC moiety, and RAFT polymerization from the TTC moiety, depending on the combination of monomer and catalyst employed in the reaction. The use of concurrent ATRP/RAFT (or copper-catalyzed RAFT polymerization or ATRP with dithioester leaving groups), resulted in improved control over the synthesis of homo- and block (co)polymers and allowed preparation of well-defined high-molecular-weight polymers exceeding 1 million. Block copolymers that could not be prepared previously have been synthesized by sequential ATRP and RAFT polymerization using a bromoxanthate inifer. A simple, versatile, and one-step method involving atom-transfer radical addition–fragmentation (ATRAF) for the preparation of various chain transfer agents (including DC, DTE, and xanthate) in high purity is discussed and a one-pot, two-step polymerization starting with a RAFT agent synthesized by ATRAF, followed by polymerization, is demonstrated.

Photo-Cross-Linkable Thermoresponsive Star Polymers Designed for Control of Cell-Surface Interactions

Star polymers with thermoresponsive arms, consisting of 2-(2-methoxyethoxy)ethyl methacrylate (MEO₂MA) and oligo(ethylene glycol) methacrylate with ~4 ethylene oxide units (OEOMA₃₀₀, M(n) = 300), were synthesized via atom transfer radical polymerization (ATRP). 25% of the arms contained benzophenone chain-end functionality at the star periphery. A mixture of linear poly(MEO₂MA-co-OEOMA₃₀₀)-Br macroinitiators without and with benzophenone end-group macroinitiators were (MI and Bzp-MI, respectively) cross-linked with ethylene glycol dimethacrylate to form star polymers. Formation of star polymers was monitored by GPC, and the presence of benzophenone functionality in the stars was confirmed by ¹H NMR. The UV-vis spectroscopy revealed that the star polymers exhibit the low critical solution temperature (LCST) at 27 °C, slightly lower than LCST of either MI or Bzp-MI. Commercially available tissue culture grade polystyrene surface was modified by depositing a thin film of functionalized stars and UV cross-linking (λ = 365 nm). The star polymers covalently attached onto surfaces allowed a control of cell shrinkage and attachment in response to temperature changes.

Penultimate unit effects in free radical copolymerization

The penultimate unit effects (PUEs) on the propagation, termination, and reversible addition-fragmentation chain transfer (RAFT) processes in free radical copolymerization are discussed on the basis of recent publications. The propriety of the implicit and explicit PUE models in propagation and chain transfer processes is commented. The penultimate termination model with the geometric-mean approximation and the related rate equation are highlighted.

Mechanism and kinetics of organostibine-mediated living radical polymerization of styrene

Abstract The polymerization of styrene with a polystyrene-dimethylstibanyl (PS-SbMe2) adduct as a mediator and azobis(isobutyronitrile) (AIBN) as a conventional radical initiator was kinetically studied. PS-SbMe2 had no detectable effect on the polymerization rate Rp, which was virtually the same as that of the conventional (stibanyl-free) system. The pseudo-first-order activation rate constant kact was proportional to Rp, meaning that degenerative (exchange) chain transfer is the only important activation mechanism, in the examined range of temperature (40–100 °C). The exchange rate constant kex was sufficiently large at these temperatures, explaining why this polymerization can afford low-polydispersity polymers. The kex was about twice as large as that for polystyrene-methyltellanyl (PS-TeMe). This rationalizes the better polydispersity controllability in the PS-SbMe2 system than in the PS-TeMe system. The activation energy of kex was 22.6 kJ mol−1, which is smaller than that (27.8 kJ mol−1) for polystyrene-iodide (PS-I) and slightly larger than that (21.0 kJ mol−1) for polystyrene-dithioacetate (PS-SCSMe).

Critical evaluation of the microwave effect on radical (co)polymerizations.

Critical evaluations of the microwave effect on initiation, propagation, and termination during conventional radical polymerizations (RPs) of methyl methacrylate (MMA) and random copolymerization of styrene (St) with (meth)acrylates are examined by comparing microwave heating (MWH) and conventional heating (CH). Poly(methyl methacrylate) with similar

Role of Cu0 in Controlled/“Living” Radical Polymerization

\overline M_{\rm n}

A kinetic study on the rate retardation in radical polymerization of styrene with addition-fragmentation chain transfer

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Highly versatile organostibine mediators for living radical polymerization.

\overline M_{\rm w}

Mechanism-based invention of high-speed living radical polymerization using organotellurium compounds and azo-initiators.

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