Solid-phase synthesis site isolation

It is apparent from the foregoing that complete and long-lasting site isolation within polymer supports has not been observed. Before complete use of the hyperentropic efficacy (the high dilution principle) (Crowley and Rapoport, 1976) of polymers can be made in solid-phase synthesis, more information about site isolation will be necessary, especially on those polymer supports designed to make use of this factor. 

The facilitation of this intrapeptide reaction has been explained on the basis of the possibility that in the rigid polymer matrix each peptide molecule is isolated from others and this would bring in the condition of infinite dilution necessary for the intramolecular cyclization (for a discussion of site-isolation in solid phase reactions see section 4.3). The method has been applied to the synthesis of several cyclic peptides in good yields. 

When solid polymer precipitates fiom bulk, solution, suspension, or emulsion polymerizations by a chain mechanism, the kinetics of the reaction and properties of the product may change sharply. If the solid phase is not swollen by the reaction mixture, the active site of the polymerization is isolated from possible termination reactions and may produce pronounced auto acceleration and increases in molecular weight of polymer formed. This behavior was quantified for one chain reaction polymer, polyvinyl chloride, covered in this book. Micldey (9) showed that polyvinyl chloride synthesis in bulk followed a two term kinetics equation with the second term representing incremental increases in polymerization rate due to accelerated polymerization in precipitated particles of polymer. 

The complexity of designing such catalysts arises because the C-H transformation function wanted is not the property of a bulk solid or a well-defined surface but of defects in phases. Such catalysts resemble their biological counterparts, the enzymes that consist also of a complex matrix holding the active site in place and in suitable electronic isolation. The ability to mimic this principle in heterogeneous catalysis with the concept of single site catalysts [45-47] has led to first academic success but is still far away from practical realization, because of the lack of suitable procedures for synthesis of bulk quantities of highly active material. 

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