Addition polymerization group-transfer

Thiosilanes act as initiators through 1,4-addition for group transfer polymerization of acrylic acid esters (equation 19), and catalyse thecyclotrimerization of phenyl isocyanate through addition across the C=N bond (equation 20) to give first the thiocarbamate and thence the isocyanurate17. 

Abstract This chapter summarizes the properties and most representative applications of pH-responsive polymers in the biomedical field.The most common methodologies to synthesize pH-responsive polymers such as emulsion polymerization, group transfer polymerization, atom transfer radical polymerization and reversible addition-fragmentation chain transfer polymerization are described. This chapter also discusses the most important applications of pH-responsive polymers in drug and gene delivery and the use of these systems as biosensors, taking into account the chemical and physical properties of these smart polymer systems. 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

Webster OW, Hertler WR, Sogah DY, Eamham WB, Rajanbabu TV (1983) Group-transfer polymerization. 1. A New concept for addition polymerization with organo-silicon initiators. J Am Chem Soc 105 5706-5708... 

The controlled polymerization of (meth)acrylates was achieved by anionic polymerization. However, special bulky initiators and very low temperatures (- 78 °C) must be employed in order to avoid side reactions. An alternative procedure for achieving the same results by conducting the polymerization at room temperature was proposed by Webster and Sogah [84], The technique, called group transfer polymerization, involves a catalyzed silicon-mediated sequential Michael addition of a, /f-unsaluralcd esters using silyl ketene acetals as initiators. Nucleophilic (anionic) or Lewis acid catalysts are necessary for the polymerization. Nucleophilic catalysts activate the initiator and are usually employed for the polymerization of methacrylates, whereas Lewis acids activate the monomer and are more suitable for the polymerization of acrylates [85,86]. 

Group-transfer polymerizations make use of a silicon-mediated Michael addition reaction. They allow the synthesis of isolatable, well-characterized living polymers whose reactive end groups can be converted into other functional groups. It allows the polymerization of alpha, beta-unsaturated esters, ketones, amides, or nitriles through the use of silyl ketenes in the presence of suitable nucleophilic catalysts such as soluble Lewis acids, fluorides, cyanides, azides, and bifluorides, HF. ... 

Coordination polymerizations are becoming an inspiration source for further methodical development in addition polymerizations. The nearest aim could possibly be the insertion of polar monomers (as indicated by group transfer polymerization) and a deepening of our understanding of catalysis. Research in this field should lead to partial or even total replacement of catalysts by other means. I shall try to indicate one of the possibilities. 

Relative to living cationic polymerization, the structure of a-methylsty-rene is both advantageous and disadvantageous. Because of the additional methyl group on the a-carbon, the growing carbocation is tertiary and should be thermodynamically more stable, but it would also be prone to undergo j8-proton elimination (chain transfer) due to the increase in the number of abstractable protons. Another important aspect of this monomer is its low ceiling temperature that requires low temperatures for polymerization. 

The free-radical polymerization is accompanied by transfer reactions7). These reactions of transfer to polymer, usually followed by recombinations, can be avoided if the yield is kept low or if the polymerization takes place in a solvent. However, the solvent addition may promote transfer reactions to the solvent and modify the nature and the number of functional terminal groups. 

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