Diffusion Controlled Termination of Linear Polystyrene Radicals in Linear, 4-Arm, and 6-Arm Star Polymer Matrices in Dilute, Semidilute, and Concentrated Solution Conditions

Abstract

The effect of polymer matrix architecture (polystyrene stars) on the termination of linear polystyrene radical chains was studied in a continually evolving polymer-solvent mixture using the robust and accurate RAFT-CLD-T method. It was found that four distinct regions were observed for the stars analogous to the linear RAFT-mediated polymerizations with chain length dependencies (where k t i,i ∼i -R ) for the 4-arm and 6-arm stars in the dilute regime similar to that found for the linear polymer (with R S and R L equal to 0.53 and 0.15, respectively). However, the gel onset conversion (x gel ) increased with the greater number of arms on the star, in agreement with the Zimm-Stockmayer prediction and in excellent agreement with the theoretical overlap concentration c*. This supports our previous work that claimed chain overlap is the main cause of the onset of the gel effect in free-radical polymerizations. In concentrated solutions the chain length dependent exponents for linear radicals were much greater in solutions of 4-arm and 6-arm star polymers (with R gel close to 1.82x for both) when compared with linear polymer solutions (R gel ) 1.22x). Because of the increased topological constraints in star polymer solutions, termination was expected to be controlled by reptation (which expects dependencies of 1.5 or 2). However, the dependencies predicted by reptation will only be reached at high conversions (close to the glass transition), suggesting that although solutions of star polymers are more constrained than their linear counterparts, there is still a great deal of matrix mobility on the time scale required for diffusion. The RAFT-CLD-T method also provided a means to determine diffusion coefficients for chain length i using Smoluchowski's equation in the regions where translational diffusion is rate determining.

Bimolecular radical termination: New perspectives and insights

“…This finding is also in agreement with experimental data for poor leaving group effects on the determination of accurate k t data. 47 Thus, these results show that the RAFT-CLD-T method is accurate even when using a RAFT agent with a poor leaving group, and is an even more robust method than the CAMD team originally thought. However, as ref.…”

“…Figure 1C and 1D in ref. 47 shows with a decrease in C tr(Pre) , the M n increased to high initial values and then grew relatively linearly with conversion, coalescing with the M n profile for C tr(Pre) equals 1000 at high conversions ($50% for C tr(Pre) equals 5). The polydispersity index (PDI) values also show the same trend, with lower C tr(Pre) values resulting in higher initial PDIs.…”

“…However, should there be substantial \short-long" termination (such as in conventional FRP, or in the presence of an extremely poor RAFT agent) the RAFT-CLD-T Method is no longer valid. 8 We have previously published detailed simulations on the validity of the RAFT-CLD-T method 8,47 and found that under most conditions this was met for a typical RAFT polymerization between 5 and 80% conversion. Hence, experimental data is only used within this conversion range.…”

“…At very short chain lengths k i;i t scales with a power law exponent of a S $ 0.78 until the transition from short to long chain behavior i SL $ 15, after which the exponent decreased significantly, a L $ 0.18. These exponents are compared in Table 2 along with data for PSTY, 47 PMMA, 46 PVAc, 45 previously published results of Barner-Kowollik and coworkers 44 for PMA determined using the RAFT-CLD-T method at 80 8C, and results for PMA determined by Vana and coworkers 68 using the SP-PLP-NIR-RAFT method at 60 8C and 1000 bar. It is encouraging that the k 0 t , a S , i SL , and a L values determined at 50 8C, here in this work, are in good agreement with the results of Vana et al determined at 60 8C.…”

“…47 More recently, we also showed that this approach could be used to examine the effects of topological interactions on k i;i t by examining the termination of linear radicals in star polymer solutions. 47 These later studies have proven especially fruitful in clearing up some of the confusion regarding the cause of the gel effect, and in understanding the mechanism of termination in concentrated polymer solutions. In particular, we showed that the theoretical overlap concentration, c*, is able to explain the cause of the gel effect.…”