ABSTRACT:The reversible addition-fragmentation chain transfer-chain length dependent termination (RAFT-CLD-T) method has allowed us to answer a number of fundamental questions regarding the mechanism of diffusion-controlled bimolecular termination in free-radical polymerization (FRP). We carried out RAFTmediated polymerizations of methyl acrylate (MA) in the presence of a star matrix to develop an understanding of the effect of polymer matrix architecture on the termination of linear polyMA radicals and compared this to polystyrene, polymethyl methacrylate, and polyvinyl acetate systems. It was found that the matrix architecture had little or no influence on termination in the dilute regime.However, due to the smaller hydrodynamic volumes of the stars in solution compared to linear polymer of the same molecular weight, the gel onset point occurred at greater conversions, and supported the postulate that chain overlap (or c*) is the main cause for the observed autoacceleration observed in FRP. Other theories based on \short-long" termination or free-volume should be disregarded. Additionally, since our systems are well below the entanglement molecular weight, entanglements should also be disregarded as the cause of the gel onset. The semidilute regime occurs over a small conversion range and is difficult to quantify. However, we obtain accurate dependencies for termination in the concentrated regime, and observed that the star polymers (through the tethering of the arms) provided constriction points in the matrix that significantly slow the diffusion of linear polymeric radicals. Although, this could at first sight be postulated to be due to reptation, the dependencies showed that reptation could be considered only at very high conversions (close to the glass transition regime). In general, we find from our data that the polymer matrix is much more mobile than what is expected if reptation were to dominate.

Summary: The RAFT‐CLD‐T methodology is demonstrated to be not only applicable to 1‐substituted monomers such as styrene and acrylates, but also to 1,1‐disubstituted monomers such as MMA. The chain length of the terminating macromolecules is controlled by CPDB in MMA bulk free radical polymerization at 80 °C. The evolution of the chain length dependent termination rate coefficient, k, was constructed in a step‐wise fashion, since the MMA/CPDB system displays hybrid behavior (between conventional and living free radical polymerization) resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The obtained CLD of kt in MMA polymerizations is compatible with the composite model for chain length dependent termination. For the initial chain‐length regime, up to a degree of polymerization of 100, kt decreases with α (in the expression k = k · i−α) being close to 0.65 at 80 °C. At chain lengths exceeding 100, the decrease is less pronounced (affording an α of 0.15 at 80 °C). However, the data are best represented by a continuously decreasing non‐linear functionality implying a chain length dependent α. magnified image

The termination rate coefficient, k t i , i , for propagating chains of near equal length, i, was evaluated using the RAFT−CLD−T method over a wide range of chain lengths and up to a conversion of 70% for MMA polymerizations carried out in the presence of the RAFT agent, CPDB, at 80 °C. We found that the conversion for the gel onset corresponded to the conversion at which polymer chains begin to overlap (i.e., c*), and was found to range from 15 to 30% conversion depending on the M n. It was further shown that c* also corresponded with the gel onset conversions for vinyl acetate and methyl acrylate. The chain length dependence of k t in the gel regime scaled as αgel(x) = 1.8x + 0.056, suggesting that reptation alone does not play a role in our system. A composite model was then derived to accurately describe k t i , i for chain lengths up to 3200 and conversions up to 70%. The k t i , i profiles for well-known termination models were tested and most gave unsatisfactory agreement with our experiments. Our model can be readily applied to any monomer provided accurate k t i , i (x) data can be determined.

A universal methodology to obtain the termination rate coefficients in free radical polymerization as a function of conversion and chain length was used to describe the RAFT-mediated "living" radical polymerization (LRP) and conventional free-radical polymerization (FRP) of methyl methacrylate (MMA) up to the glass regime. A composite termination model determined previously using the reversible addition-fragmentation chain transfer-chain length dependent-termination (RAFT-CLD-T) method, based on conversion (x) and chain length (i) dependent termination rate coefficient, k t i,i (x), data, was the key parameter in obtaining accurate fits to the experimental rate and molecular weight data. Two kinetic modeling approaches were used in this study: (i) model 1: full chain length distributions; (ii) model 2: "method of moments". Both approaches gave excellent agreement with experimental results for the evolution of conversion and MWD for RAFT-mediated polymerizations and accurately described the kinetics after the gel onset conversion. The simulations were also used to validate the accuracy of the RAFT-CLD-T method up to the glass regime to obtain accurate k t i,i (x) data. Importantly, our simulations gave excellent fits to the rates of polymerization and MWDs for conventional FRP (with broad MWDs), especially the transition from dilute to the gel regime. There is little or no "short-long" termination in RAFTmediated polymerizations, and therefore the RAFT-CLD-T method provides an ideal approach to probe the mechanism for termination between chains of similar length and their interaction with the polymerizing matrix up to high conversion. We envisage this modeling framework can be easily applied to many other monomers or free radical systems (e.g., ATRP and NMP) and therefore allow highly accurate predictions of polymerization rates and MWDs.