RAFT polymerization kinetics: How long are the cross‐terminating oligomers?

Abstract

ABSTRACT:We extend a new 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 very short oligomeric radicals. In this work, we consider cross-termination of oligomers up to two monomers and an initiator fragment. This model accounts for the absence of three-armed stars in the molecular weight distribution, which are predicted by other cross-termination models, since the short third arm makes a negligible difference to the polymer's molecular weight. The model is tested against experiments on styrene mediated by cyano-isopropyl dithiobenzoate, and ESR experiments of the intermediate radical concentration. By comparing our model to experiments, we may determine the significance of cross-termination in RAFT kinetics. Our model suggests that to agree with the known data on RAFT kinetics, the majority of cross-terminating chains are dimeric or shorter. If longer chains are considered in cross-termination reactions, then significant discrepancies with the experiments (distinguishable star polymers in the molecular weight distribution) and quantum calculations will result.

Living Radical Polymerization by the RAFT Process – A Third Update

“…While little retardation was observed when the combination of 5 and 6 (Chart 1) was used, neither was retardation observed with 11 as RAFT agent and AIBN as initiator for higher monomer conversions (.40 %). The latter finding is inconsistent with the short chain termination model of Konkolewicz et al [128] In very recent work, Meiser and Buback [130] isolated a missing-step product from the reaction of 2-cyanoprop-2-yl radicals with 2-cyanoprop-2-yl dithiobenzoate (12) thereby confirming the viability of this mechanism.…”

“…These involve the occurrence of (a) a missing step termination or (b) short chain termination. [128] Ting et al [129] have attempted to answer the question of whether crosstermination (between 4 and P n ) might involve short radicals only. They [129] compared the rates of polymerization when a macro-azo-initiator (5) and a polystyrene macro-RAFT agent (6) were used in a RAFT polymerization of styrene (St) (this system should involve no short chain radicals) to the situation where cumyl dithiobenzoate (11) was the RAFT agent and azoisobutyronitrile (AIBN) was used as initiator.…”

Living Radical Polymerization by the RAFT Process - A Second Update

“…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.…”

Synthesis of Glycopolymer Architectures by Reversible-Deactivation Radical Polymerization

“…was observed at the beginning of polymerization that was followed by pseudo-first order kinetics. This kind of behavior had already been observed in the CDB-mediated polymerization of methacrylates [214] and its origin is the subject of a lively debate [215][216][217][218]. Fairly uniform polymers (Đ  1.20) with M n up to 14,000 Da were obtained that were chain extended with 2-(dimethylamino)ethyl methacrylate M44 (Entry 193, Table 3) to give double hydrophilic-hydrophilic AB diblock glycopolymers after deprotection of the sugar moieties (TFA/H 2 O 5:1 v/v, RT, 1 h).…”