Polymerization chain transfer agents

Control of radical poljmerization with the addition of thiocarbonylthio compounds that serve as reversible addition fragmentation chain transfer (RAFT) agents was first reported in 1998. Since that time much research carried out in these laboratories and elsewhere has demonstrated that RAFT polymerization is an extremely versatile process.f It can be applied to form narrow polydispersity poljmers or copolymers from most monomers amenable to radical poljmerization. It is possible to take RAFT poljmerizations to high conversion and achieve commercially acceptable polymerization rates. Polymerizations can be successfully carried out in heterogeneous media (emulsion, miniemulsion, suspen-... 

Mayadunne, R.T.A., et al. 2000. Living polymers by the use of trithiocaibonates as reversible addition— fragmentation chain transfer (RAFT) agents ABA triblock copolymers by radical polymerization in two steps. Macromolecules 33(2) 243-245. 

Controlled radical pol5nnerization can also be achieved by reversible addition-fragmentation chain transfer polymerization (RAFT) (79). In this technique, after the initiation, the RAFT agents reversibly deactivate the polymer chains as the rate constant of chain transfer is faster than the rate constant of propagation (Fig. 6). 

Table 10 Experimental results fra- rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO-catalyzed polymerization of propylene (100 psi) with St-Cl/H2, St-OSi/H2, and St-NSi2/H chain transfer (CT) agents...

Perrier S, Takolpuckdee P, Mars CA (2005) Reversible addition-fragmentation chain transfer polymerization end group modification for functionalized polymers and chain transfer agent recovery. Macromolecules 38 2033-2036... 

Perrier, S., Takolpuckdee, P, and Mars, C.A. 2005. Reversible addition-fragmentation chain transfer polymerization mediated by a solid supported chain transfer agent Macromolecules 38 6770-4. 

The microwave acceleration for the direct synthesis of N-phenylmaleimide from maleic anhydride and aniline based on specific microwave absorption of the ionic intermediates has been reported (Bezdushna and Ritter, 2005). Fischer et al. (2005) investigated the free radical polymerization of N-alkylacrylamides with 3-mercap-topropionic acid as chain transfer agent in methanol with thermal heating at ambient pressure and under superheated conditions as well as under microwave irradiation. While the chain transfer polymerization could be accelerated from 5 h to 1 h when going to superheated conditions with thermal heating, it was further accelerated down to several seconds under microwave irradiation. 

In a recent study, Tasdelen et al. synthesized a phenacyl morpholine-4-dithiocarbamate, which can act as both a photoiniferter and reversible addition fragmentation chain transfer (RAFT) agent. Polymerization of styrene was carried out in bulk under UV irradiation at above 300 nm at room temperature. The polymerization showed living characteristics up to 50% conversions and produced well-defined polymers with molecular weights close to those predicted from theory and relatively narrow poyldispersities (Mw/Mn 1.30). End group determination and block copolymerization... 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

The enthalpy of the copolymerization of trioxane is such that bulk polymerization is feasible. For production, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is in eluded if desired. Polymerization proceeds in bulk with precipitation of polymer and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. The mixing requirements for the bulk polymerization of trioxane have been reviewed (22). Raw copolymer is obtained as fine emmb or flake containing imbibed formaldehyde and trioxane which are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

In addition to the monomers, the polymerization ingredients include an emulsifier, a polymerization initiator, and usually a chain-transfer agent for molecular weight control. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

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