A Modeling Study on the RAFT Polymerization of Vinyl Monomers in Supercritical Carbon Dioxide

Abstract

synthesized by RAFT copolymerization in scCO 2 . [12] Homopolymerizations of methyl methacrylate (MMA), [13][14][15] styrene, [16] and methyl acrylate (MA) [17] have also been reported. Molar masses were linear with conversion; however, molar mass dispersity (Đ) values greater than 1.5 were experimentally measured in some cases, which is unexpected for controlled cases.Most polymerizations carried out in scCO 2 occur as heterogeneous processes. Since long-chain polymer molecules are insoluble in scCO 2 , two distinct phases are present, a CO 2 -rich phase (continuous) and a polymer-rich phase (dispersed). [9,18] In order to study the development of polymer properties, the kinetics of both phases should be considered. Mass transfer of low-and high-molar-mass species between the continuous and dispersed phases takes place during the polymerization. If the rate of mass transfer of the low-molar-mass species is sufficiently fast, such that their concentrations in both phases are close to their equilibrium values, thermodynamic models can be used to estimate their current values. Two-phase equilibrium problems involving polymer/solvent interactions have been addressed with several approaches: i) equations of state, ii) perturbation models, and iii) lattice-fluid models. [19] Oligomeric radicals produced in the continuous phase are either assumed to remain in that phase or migrate to the dispersed phase once they reach a critical chain length, P nc . [18,20,21] Some reports have focused on the influence of monomer-toinitiator ratio, initial monomer loading and pressure on P nc . [22] The modeling of RAFT polymerization has been performed via deterministic methods, [23][24][25] implementation of polymerization schemes within the commercial software Predici, [26][27][28] and stochastic methods. [29][30][31] It requires knowledge of the activity of the RAFT agent as well as appropriate models for diffusioncontrolled effects; however, there is no universal methodology to accurately determine the apparent addition and fragmentation kinetic rate constants. [32,33] Likewise, the so-called "slow fragmentation" and "intermediate radical termination" approaches have been proposed in order to describe experimental observations. [34] Predici has been used to describe the RAFT polymerization of MMA with 2,2′-Azobis(2-methylpropionitrile) (AIBN) as the initiator and S-thiobenzoyl thioglycolic acid (TBTGA) as the RAFT agent in scCO2. [27] The transport of low-molar-mass species was modeled by using the "phase-exchange" step while migration of radicals was addressed with the "k(s)-termination" step. Simpler expressions for partition of components have also RAFT Polymerization The kinetic modeling of reversible addition-fragmentation chain transfer (RAFT) dispersion homopolymerization of vinyl monomers in supercritical carbon dioxide is addressed. The model accounts for two reaction loci, a polymerrich phase (dispersed) and a solvent-rich phase (continuous). In one of the models, the partition of low molar mass components is estimated ...

Influence of structure and solubility of chain transfer agents on the RAFT control of dispersion polymerisation in scCO2

“…The initiator (AIBN) is known to have high solubility in scCO 2 ; DeSimone et al studied the decomposition rate and efficiency of AIBN. 38 Other studies also indicate that AIBN can equipartition between both phases, 48,49 or yet have a partition coefficient (K j ) ¼ 2, 50 or K j ¼ 0.5, 51 where K j is the ratio of the concentration in the dispersed phase by the concentration in the continuous phase.…”

“…Predici® software was used to study the RAFT polymerisation of styrene with AIBN initiator and S -thiobenzoyl thioglycolic acid (TBTGA) as the CTA in scCO 2 . 51 Both the continuous and the dispersed phases were considered in the simulations. When the CTA was present, the simulations reported large dormant polymer chains to be produced in the continuous phase at a significant level, while lower molecular weight chains were formed in the dispersed phase, where they assumed the CTA would mainly partition.…”

“…When the CTA was present, the simulations reported large dormant polymer chains to be produced in the continuous phase at a significant level, while lower molecular weight chains were formed in the dispersed phase, where they assumed the CTA would mainly partition. 51 In these simulations, all chains were considered to be solely initiated via RAFT. However, our data show that this is not the case in reality.…”

“…Even though, previous computational simulations assumed that mobility of the CTA was effectively the same as the monomer or that the CTA was completely and instantaneously transported into the dispersed phase. 51,[60][61][62] In the following section we try to identify the solvent factors that might tune the behaviour of the CTAs. Computational solvation models are used to probe the affinities of the different CTAs towards toluene and scCO 2 and we use the data to explain the disparity in control of our polymerisations.…”

“…67 Very few studies have tried to model RAFT dispersion polymerisation in scCO 2 . 51,60,62,68 In that study, the particles were assumed to be the main locus of the reaction as for conventional radical polymerisation. Predici® soware was used to study the RAFT polymerisation of styrene with AIBN initiator and S-thiobenzoyl thioglycolic acid (TBTGA) as the CTA in scCO 2 .…”