Obtaining chain length dependent termination rate coefficients via thermally initiated reversible addition fragmentation chain transfer experiments current status and future challenges

The reversible addition fragmentation chain transfer (RAFT) process can be utilized in conjunction with rate of polymerization measurements to accurately map the chain length dependence of the termination rate coefficient. This novel approach was originally applied to styrene polymerization and has been termed the RAFT chain length dependent termination (RAFT-CLD-T) method. The RAFT-CLD-T technique is discussed in the context of the prerequisite analysis parameters as well as the choice of RAFT agent. In the present contribution we critically compare the data obtained via RAFT-CLD-T thus far for the monomers styrene (Sty), methyl methacrylate (MMA), methyl acrylate (MA), butyl acrylate (BA), dodecyl acrylate (DA), and vinyl acetate (VAc). For monomers with relatively low reactivity propagating radicals (MMA), a strong chain length dependence of kt in the small chain length regime was observed, indicated by a relatively high α value (in the frequently used expression k t i,i = kt 0·l -α). With increasing chain length, the α value is continuously decreasing, caused by a slow transition from translational diffusion to segmental diffusion as predicted by the composite model of chain length dependent termination. For monomers with higher reactivity propagating radicals (MA, VAc), a linear dependence of kt with chain length was observed (α = 0.36 for MA and 0.09 for VAc). Within the acrylate class, an interesting influence of the side chain was found. In the small chain length regime, α is increasing with increasing length of the side chain from 0.36 in case of MA to 1.2 in DA, which may be attributed to an increased shielding of the polymeric radical. At longer chain lengths, the α value of MA is significantly higher than those for BA and DA, where a is strongly decreasing with increasing chain length. This may indicate a different flexibility and coil structure of MA compared to BA and DA. In general, the acrylates display significantly higher a values in the long chain region than MMA and VAc, which we assign to the presence of mid-chain radicals. The data obtained via three dimensional simultaneous mapping of the chain length and conversion dependence of kt (3D-RAFT-CLD-T) for MA and VAc are also highlighted. © 2006 American Chemical Society