Ring-closing polycondensations

The role of cyclization in polycondensations is discussed for two different scenarios: thermodynamically-controlled polycondensation (TCPs) on the one hand and kinetically-controlled polycondensations (KCPs) on the other. The classical Carothers-Flory theory of Step-growth polymerization does not in clude cyclization reactions. However, TCPs involve the formation of cycles via 'back-biting degradation', and when the ring-chain equilibrium is on th e side of the cycles the main reaction products of the TCP will be cyclic o ligomers. Two groups of examples are discussed: polycondensations of salicy clic acid derivatives (e.g. aspirin) and polycondensations of dibutyltin de rivatives with long alpha-, omega -diols or dicarboxylic acids. Furthermore , various kinetically-controlled syntheses of polyesters and polyamides wer e studied and carefully optimized in the direction of high molecular weight s. High fractions of cyclic oligomers and polymers were found by MALDI-TOF mass spectrometry, and their fractions increased with optimization of the p rocess for molecular weight, These results disagree with the Carothers-Flor y theory but agree with the theoretical background of the Ruggli-Ziegler di lution method (RZDM). When poly(ether-sulfone)s were prepared from 4,4 ' -d ifluorodiphenylsulfone and silylated bisphenol-A two different scenarios we re found. With CsF as catalyst at a temperature of more than 145 degreesC c yclic oligoethers were formed under thermodynamic control. When the polycon densation was promoted with K2CO3 in N-methylpyrolidone at less than or equ al to 145 degreesC the formation of cyclic oligoethers and polyethers occur red under kinetic control. A new mathematical formula is presented correlat ing the average degree of polymerization with the conversion and taking int o account the competition between cyclization and propagation.