Block Copolymers via the Macroinitiator Approach

The S-B block copolymer produced (XII) was characterized by a variety of techniques including NMR, gel permeation chromatography (GPC), thin-layer chromatography (TLC), and transmission electron microscopy (TEM). The results of these analyses clearly showed that the block copolymer was fairly pure with very little homopolymer contamination. 

Addition of the XIII as a macroinitiator to styrene polymerization resulted in the formation of S-MMA-S triblock copolymer. The same procedure was also used to make styrene-butyl acrylate block copolymers. 

The success of this chemistry versus a control was demonstrated by conducting a parallel experiment where PMMA was prepared under the same conditions as the preparation of XIII, except using azobisobutyronitrile (AIBN) as the initiator instead of the alkoxyamine functional azo initiator. The AIBN-and alkoxyamine functional azo-initiated PMMA were dissolved in styrene and heated at 130 °C. A film of the resulting block copolymer made using alkox-yamine-functionalized XIII was translucent and flexible whereas a film of the polymer formed by polymerizing styrene in the presence of unfunctionalized PMMA control was opaque and very brittle. 

ALKOXYAMINES AS CHAIN-STOPPERS IN STEP-GROWTH POLYMERIZATION 

The molecular weights of polymers made using step-growth polymerization are typically controlled by the addition of a chain-stopper to the process. The chain-stopper to monomer ratio determines the final molecular weight of the polymer. If functionalized chain-stoppers are used, functionalized polymers are produced. If macro chain-stoppers are used, triblock copolymers are formed during the step-growth polymerization. 

---