Styrene polymerization with RAFT

Moad et al. [291] showed that the type of RAFT agent is important. Using a very reactive RAFT agent (with a transfer constant of about 6000), similar to that used in the work of De Brouwer and Monteiro, resulted in a broad poly-dispersity in ab initio styrene polymerizations with ionic surfactant, which was ascribed to the fact that the RAFT agent was not uniformly dispersed in the polymerization medium. The use of less reactive RAFT agents (with transfer constants of 10-30) did not result in destabilization and the final polymer had a polydispersity close to 1.4. 

RAFT has also been used to produce graft polymers. The grafting from method is achieved by reacting a halogen-containing polymer such as styrene p-chloromethy I styrene copolymer with sodium dithiohenzoate to obtain a polymeric dithioester, which is an initiator in RAFT polymerization (Sec. 3-15d) [Quinn et al., 2002]. 

Also, Kerep and Ritter reported a radical chain transfer agent as a dual initiator, FRP-1 [45]. The first step builds on the fact that hydroxyl groups are much better nucleophiles in enzymatic ROP than thiols. Due to the chemoselectivity of the enzyme, PCLs with predominantly thiol endgroups were obtained, which were subsequently used as macroinitiator for styrene. The authors report that the reaction yield can be further increased by microwave irradiation. Although thiols provide less control over the radical polymerization than RAFT agents, the subsequent radical polymerization successfully leads to the synthesis of PCL-Z -PS. 

TABLE 3. Selected emulsion surfactant pairs used in the controlled RAFT polymerization of styrene monomer with dihenzyltrithiocarhonate as the chain transfer agent. 

Monteiro et al. [293] also studied the effect of xanthates (RAFT agents with low transfer constants) with styrene, in ab initio styrene polymerizations. Again rate retardation was observed throughout the polymerization. This is not surprising, since the low transfer constants of these RAFT agents mean that they are present during the whole polymerization, which results in an increased exit rate throughout the reaction. 

Fig. 28 Schematic presentation of complexation and RAFT polymerization of Me-fi-CD com-plexed styrene 18a with 3-benzylsulfanylthiocarbonylsulfanylpropionic acid (TTC) as RAFT agent...

Using NMP [114, 115] or reversible addition-fragmentation chain transfer (RAFT) [ 119,120,127], agents with ammonium groups for the ion exchange allowed the attachment of initiators on the clay surface for controlled radical polymerizations (NMP, RAFT). Samakande et al. investigated the kinetics of RAFT-mediated living polymerization of styrene [120] and styrene/BA [119] mixtures in miniemulsion. 

Investigating the use of functional surfactants for the modification of clay and the properties of the resultant polymer/clay nanocomposite, Hartmann s team [69, 70] used surface-active RAFT agents. The appropriate functionalization of these RAFT agents made their attachment to clay platelets possible. As a result, a controlled growth of polymer chains from the surface of the clay platelets was achievable. They extrapolated to miniemulsion for the first time [71] their studies on styrene polymerization performed in bulk [72] in the presence of Montmorilloniteclay modified with PCDBAB and DCTBAB (Fig. 6). 

RAFT polymerizations with this iniferter of iV-butyl acrylate and styrene yielded polymers with M. / Mn equal to 1.15 and 1.16 respectively. Polymerization of methyl methacrylate, however, yielded a polymer with a broad Mw/M ratio. On the other hand, polymerization in the presence of CuBr/TMPA by ATRP exclusively through the bromine chain ends yielded a polymer with narrow ratio [282],... 

CPDA mediated polymerization of MA behaves in a fashion somewhat similar to the situation of styrene polymerization mediated by dithiobenzoates as shown in Figure 1 and 2. The decrease of the RAFT-agent concentration with time is nearly linear, and growth of the polymer chains beyond single monomer adduct formation only commences after all RAFT-agent is converted. On the other hand, in the case of CDB-mediated polymerization... 

RAFT agent. The growing radicals produced from the fragmentation of the RAFT agent exit the particles and reenter into the continuous phase to form new particles before the precipitation of the existing particles, thus increased the exit rate coefficient with RAFT concentration. This induces the retardation of the polymerization due to the transfer of the RAFT agent to the particles and so the particle size decreases with the RAFT concentration. In the emulsion polymerization of styrene, the particle diameter decreased and the size distribution became narrower with the RAFT concentration. However, a partial... 

Problem 11 0 Kwak et al. (2004) presented experimental evidence to show that the rate retardation in the polymerization of styrene (St) with polystyryl dithiobenzoate [PSt—SC(=S)Ph] at 60°C is caused by the irreversible cross-termination between the polystyryl radical (PSt ) and the intermediate radical produced by the addition of PSt to PSt-SC(=S)Ph. The polymerization rate Rp was found to decrease with the increase of [PSt-SC(=S)Ph] such that a plot of l/P vs [PSt-SC(=S)Ph] was linear, (a) Show that this is in conformity with the scheme for RAFT polymerization shown in Fig. 11.36, taking into account termination of intermediate radicals and considering that cross-termination to form 3-arm star predominates, (b) What would be the corresponding rate behavior if termination between two intermediate radicals to form a 4-arm star predominates ... 

Postma et al. (2006) reported the synthesis of well-de ned polystyrene (PSt) with primary amine end groups through the use of phthalimide-functional RAFT agents. Styrene (St) polymerization with the RAFT agent, butyl phthalimidomethyl trithiocarbonate (P25-I), as shown in Scheme PI 1.25.1, was conducted at 110°C with thermal initiation in bulk and the following reaction conditions [RAFT]o = 0.0288 M and [St]o/[RAFT]o = 303, achieving 70% monomer conversion in 24 h to obtain RAFT PSt (P2S-II) with M = 22,400 g mol i and = 1.12. The polymerization was also successfully conducted at 60°C with AIBN... 

FIGURE 10.2 Dependence of initiated AIBN ([I] o=0.01 mol-L ) styrene polymerization reduced rate on motromer cotrversion at 60°C (1. experiment 2. estimation by introduced in this research mathematical model with temperature dependence of kp (see Eq. (18)) 3. estimation by introduced itr this research mathematical model with temperature dependence of kp (see Eq. (19)) [RAFT(0,0)]o = 0 mol L- (a), 0.007 (b)). 

RAFT polymerization has successfully synthesized a wide range of polymers with controlled molecular weight and low polydispersities (between 1.05 and 1.4 for many monomers). Some monomers capable of polymerizing by RAFT include styrenes, acrylates, acrylamides, and many vinyl monomers. Additionally, the RAFT process allows the synthesis of polymers with... 

RAFT polymerization of styrene or dimethylacrylamide with the same compounds in conjunction with AIBN as initiator leads to the corresponding polymers provided with end groups that are capable of initiating the MT polymerization with the formation of well-defined block copolymers (Scheme 33). 

Much has recently been published on radical ring-opening (co)polymerization with 5,6-benzo-2-methylene-l,3-dioxepan and derivatives. The monomer provides quantitative ring opening, copolymerizes readily with styrenic and acrylic mono-mers, and is compatible with RAFT and ATRP... 

On the basis of computational studies on RAFT agents, Z-C (=S)SR, a RAFT agent with Z=fluorine was proposed as a universal RAFT agent able to effidendy control the polymerization of both activated monomers (e.g., acrylates and styrene) and LAMs (e.g., VAc). The full utility of this dass of RAFT agent has yet to be demonstrated experimentally. The only example reported is the benzyl RAFT agent 173 and this has only been tested in styrene polymerization where limited control was observed (poor correspondence between fotmd and calculated molecular weights, slightly narrowed dispersity). ... 

---