Abstract
The kinetics of reversible addition−fragmentation chain transfer (RAFT) polymerization of butyl acrylate with ethyl S-thiobenzoyl-2-thiopropionate (ETTP) as the RAFT agent has been studied. The concentrations of propagating (P•) and intermediate radicals (INT•) were measured via highly time-resolved EPR spectroscopy after initiation by a laser single pulse. Predici simulation of experimental data results in rate coefficients for −40 °C of kad = (1.4 ± 0.4) × 106 L mol−1 s−1, kβ = (4.7 ± 1.5) s−1, and ktcross = 0.25 × kt, where kt is the rate coefficient for termination of two P• species. Fast fragmentation of the intermediate radical is thus observed at this low temperature. Measuring the ratio of INT• to P• concentrations during stationary polymerization at −40 and 70 °C yields Keq = kad/kβ values of (2.3 ± 0.6) × 105 and 75 ± 15 L mol−1, respectively, which correspond to an apparent activation energy difference, EA(kad/kβ), of −49.5 kJ mol−1.
Highlights
Since its invention in 1998, reversible addition-fragmentation chain transfer (RAFT)[1] polymerization has proven to be one of the most versatile techniques for producing polymers of defined size and shape
cumyl dithiobenzoate (CDB)-mediated polymerization at higher RAFT agent concentration exhibits an extended induction period, which is assigned to resonance stabilization of the cumyl radical. This effect is considerably larger than in case of ethyl acrylate and cyanoisopropyl radicals, which are released from the primary intermediate radical species with ethyl S-thiobenzoyl-2-thiopropionate (ETTP) and cyanoisopropyl dithiobenzoate (CPDB), respectively
No pre-equilibrium situation is observed in systems where the leaving radical moiety is of similar structure as the propagating radicals, as is the case with ETTP/BA
Summary
Since its invention in 1998, reversible addition-fragmentation chain transfer (RAFT)[1] polymerization has proven to be one of the most versatile techniques for producing polymers of defined size and shape. Within a second EPR-based method, the mRAeaFsTureeqdurilaitbioriuomf IcNoTnstatontP, K sepq e(c=ieksadv/iakβa)siisndgeledufacsetdEfProRmscthane during (pseudo)stationary photoinitiated polymerization.[16,17,18] Under conditions of cross-termination between INT and P occurring at negligible rate, compared to addition and fragmentation, INT and P coexist in a quasi-equilibrium state, and Keq may be estimated from cINT/cP at given RAFT agent concentration, cRAFT, via eq 1.19. Applying eq 1, Kwak et al.[19] reported Keq = 55 L mol-1 for polystyryl dithiobenzoate-mediated RAFT polymerization of styrene at 60 °C These authors estimated cP from the rate of Received: November 2, 2010 Revised: January 28, 2011 Published: March 30, 2011 r 2011 American Chemical Society. To compare rate retardation for RAFT agents of similar structure, BA was polymerized in bulk at 70 °C and different concentrations of CDB, CPDB, and ETTP using AIBN (1.5 Â 10-3 mol L-1) as the initiator.
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