Comment on “Aqueous SET-LRP catalyzed with in situ generated Cu(0) demonstrates surface mediated activation and bimolecular termination” by S. Samanta, et al., Polym. Chem., 2015, 6, 2084

A recent paper proposed a new polymerization mechanism for aqueous SET-LRP, but experimental results are consistent with conventional radical propagation and termination.

How are radicals (re)generated in photochemical ATRP?

The polymerization mechanism of photochemically mediated Cu-based atom-transfer radical polymerization (ATRP) was investigated using both experimental and kinetic modeling techniques. There are several distinct pathways that can lead to photochemical (re)generation of Cu(I) activator species or formation of radicals. These (re)generation pathways include direct photochemical reduction of the Cu(II) complexes by excess free amine moieties and unimolecular reduction of the Cu(II) complex, similar to activators regenerated by electron-transfer (ARGET) ATRP processes. Another pathway is photochemical radical generation either directly from the alkyl halide, ligand, or via interaction of ligand with either monomer or with alkyl halides. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes. A series of model experiments, ATRP reactions, and kinetic simulations were performed to evaluate the contribution of these reactions to the photochemical ATRP process. The results of these studies indicate that the dominant radical (re)generation reaction is the photochemical reduction of Cu(II) complexes by free amines moieties (from amine containing ligands). The unimolecular reduction of the Cu(II) deactivator complex is not significant, however, there is some contribution from ICAR ATRP reactions involving the interaction of alkyl halides and ligand, ligand with monomer, and the photochemical cleavage of the alkyl halide. Therefore, the mechanism of photochemically mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)generation.

In the (Very) Long Run We Are All Dead: Activation and Termination in SET-LRP/SARA-ATRP

The rate constants of activation and termination were determined for SET-LRP/SARA-ATRP polymerizations of methyl acrylate. Measurement of the rate of generation of CuBr₂ throughout the reaction (using data from Levere et al., Macromolecules 2012, 45, 8267-8274) allowed evaluation of the chain length dependence of the two rate constants, which were found to be 1.25(9) × 10^-4DP_n^-0.51(3) cm·s^-1 (activation) and 3.1(1) × 10⁹DP_n^-0.49(2) L·mol^-1·s^-1 (termination). Addition of the CuBr₂ deactivator at the beginning of the reaction is found to result in a higher proportion of dead chains due to rapid termination of short chains.

Obtaining kinetic information from the chain-length distribution of polymers produced by RAFT

We describe a simple model for the kinetics and chain-length distribution of polymers made by living radical techniques. Living radical methods give good control over the molecular weight of a linear polymer by capping the growing end and forming a dormant chain. The polymer is predominantly capped, and occasionally decaps to form a radical that propagates for a short period before recapping. Our model uses this mechanism to describe the chain-length distribution of polymers made by living radical methods. We focus on oligomers made by reversible addition-fragmentation chain transfer (RAFT) polymerization as model systems. Our model can determine optimal reaction conditions for desired polymer properties and test hypotheses about reaction schemes by using only two parameters, with each parameter related to the kinetics. The first parameter is the mean number of monomers added when a chain decaps. A broad distribution results if many monomers are added upon decapping. The second parameter is the mean number of times a polymer decaps. Many decapping events indicate high monomer conversion. Our model gives kinetic information by directly fitting to an experimental chain-length distribution, which is the reverse of other kinetic models that generate the distribution from rate coefficients. Our approach has also the advantage of being simpler than previously published kinetic schemes, which use many rate coefficients as inputs. Our model was tested against three monomers (acrylic acid, butyl acrylate, and styrene) and two RAFT agents. In each case, we successfully describe the chain-length distribution, and give information about the kinetics, especially the probability of propagation versus deactivation by the RAFT mechanism. This excellent agreement with a priori expectations and quantum calculations makes our model a powerful tool for predicting the structure of polymers obtained by living radical polymerization.