Radical Polymerization - Controlled Radical Polymerization (CRP)

Controlled Radical Polymerization (CRP)

Also known as living radical polymerization, this method relies on completely pure reactions so that no termination caused by impurities occurs. Because these polymerizations stop only when there is no more monomer and not when termination occurs, the polymerization can continue upon the addition of more monomer. Block copolymers can be made this way. Complete living radical polymerization allows for control of molecular weight and dispersity; however, this is very difficult to achieve and instead a pseudo-living polymerization occurs in which there is only partial control of molecular weight and dispersity. There are several types of CRPs. ATRP and RAFT are the main types.

• Atom Transfer Radical Polymerization (ATRP): based on the formation of a carbon-carbon bond by atom transfer radical addition. This method, developed by Polish chemist Krzysztof Matyjaszewski, requires reversible activation of a dormant species (such as an alkyl halide) and a transition metal halide catalyst (to activate dormant species).
• Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT): requires a compound that can act as a reversible chain transfer agent, such as dithio compounds.
• Stable Free Radical Polymerization (SFRP): used to synthesize linear or branched polymers with narrow molecular weight distributions and reactive end groups on each polymer chain. The process has also been used to create block co-polymers with unique properties. Conversion rates are about 100% using this process but require temperatures of about 135 °C. This process is most commonly used with acrylates, styrenes, and dienes. The reaction scheme in Figure 23 illustrates the SFRP process. Because the chain end is functionalized with the TEMPO molecule (Figure 24), premature termination by coupling is reduced. As with all living polymerizations, the polymer chain grows until all of the monomer is consumed.