RAFT Polymerization Kinetics: Combination of Apparently Conflicting Models

Abstract

We propose a model for the kinetics of reversible addition-fragmentation chain transfer (RAFT) polymerization. The essence of this model is that the termination of the radical intermediate formed by the RAFT process occurs only with the shortest active radicals. This model accounts for the absence of 3-armed stars predicted by other cross-termination models since the short radical makes a negligible difference to the overall molecular weight. The model is tested against experiments on styrene at 60 °C with cyanoisopropyl dithiobezoate (CPDB) as the RAFT agent. The predicted rate coefficients are consistent with slow fragmentation of the RAFT intermediate, and the overall concentration of radicals is consistent with ESR experiments. Overall, it demonstrates that the two conflicting models that have been proposed so far can actually coexist.

Living Radical Polymerization by the RAFT Process - A Second Update

“…Mass spectrometry has been used to provide evidence to support the occurrence of intermediate radical termination during RAFT polymerization of acrylates, [44] styrene (St), [45,46] and N-acryloylmorpholine (NAM) [47] with dithiobenzoate RAFT agents. A recent paper [48] purports to suggest a possible compromise between the 'slow fragmentation' and 'intermediate radical termination' models for retardation in RAFT polymerization with dithiobenzoate RAFT agents. According to the proposed model intermediate radical termination occurs, but only for initiator-derived or oligomeric species (chain length <2).…”

“…According to the proposed model intermediate radical termination occurs, but only for initiator-derived or oligomeric species (chain length <2). [48,49] Further details of the so-called 'missing reaction step' mechanism for retardation in RAFT polymerization with dithiobenzoate RAFT agents have also been published. [50] Some support for this mechanism comes from studies on 2,2 -azoisobutyronitrile-α-13 C-initiated polymerization of St. [51] Kinetic analysis together with Monte Carlo simulation of the reaction of alkoxyamines with dithiobenzoates (22, 49) was investigated as a means of estimating rate constants for addition and fragmentation.…”

“…For an efficient RAFT polymerization (Scheme 1, Fig. 3 22* [86] S S MA [54] AAEMA [87] MAEP [87] MMA [88,89] AEMA [90] DMAEMA [91] (TPMMA) [92] TFPMA [93] St [48,94,95] 277 [96] 278 [96] 392 [97,98] 345 [99] 348 [100] 347 [101] St [51] 364 [102] 365 [102] NVP [103] 385 [104] 386 [98] 387 [104] 388 [104] 389 [104] 390 [104] BA/ 407 [105] St/MAH [53] AAEMA-b-AEP [87] AAEMA-b-MAEP [87] MAEP-b-AAEMA [87] BMA/TMSEMA [106] TFPMA-b-tBA [93] St-b-NIPAM [95] St-b-HEMA/DMAEMA [86] 364-b-384 [102] 365-b-384 [102] 386-b-392 [97] 23* [107] S CN S DEGMA [108] EGMA [108] MAA [108] MMA [109] PEGMA …”

“…The use of the corresponding trithiocarbonates 118-125 has also been promoted in this context. [27] Secondary aromatic dithioesters with R = -CHPh(CN) (42) and -CHPh(CO 2 R) (43)(44)(45)(46)(47)(48) and analogous trithiocarbonates 144, 145, and 146-148, respectively, have been shown to have utility in controlling 'R'-connected 79* [227] S S S S NIPAM [240] S S S S 80* [86,241,242] BMA [243] DMAEMA [240][241][242][243] AA [241] BA [243] St [241,243] DMAEMA-b-BMA [243] DMAEMA-b-BA [243] DMAEMA-b-St [243] St-b-HEMA/DMAEMA [86] 81 [244] CN S S…”

Use of Spin Trap Technique for Kinetic Investigation of Elementary Steps of RAFT-Polymerization

“…Moreover, the above-described reaction may occur only in the case of dithiobenzoates; in the case of trithiocarbonate-mediated polymerization, which is also characterized by retardation and in which inhibition phenomena are typical, this reaction is not possible. Another proposal is that there is a sharp dependence of the rate constant of termination of radical intermediates on the chain length (Konkolewicz et al, 2008). In this way, the majority of termination products would have low chain lengths and would not be detectable in the termination products of the polymer.…”