TABLE OF CONTENTS The absence of end-groups and the equivalence of all monomer units in a cyclic polymer are predicted to lead to new insights in the fundamental properties of polymers. The synthesis of all-carbon main chain cyclic polymers, however, appears to be one of the great challenges in polymer chemistry. As this review will show, this challenge has not yet been met completely satisfactorily and newer synthetic and analytical techniques will be required in the future in order to make further rewarding progress.
The cyclization of a polymer chain with formation of a carbon-carbon bond is an irreversible process. It can involve either any two monomer units along the polymer chain or the reaction between two functional groups. The latter may be placed randomly along the chain or exclusively in specific positions. The cyclization is very often performed with two end-standing functional groups. Early attempts have been made to produce loops by intramolecular reactions of functional groups on linear polymers. The oldest study involved a copolymer of propylene and allylsilane (CH 2=CH-CH 2SiH 3) [1]. On warming the polymer in alkali/alcohol mixtures Si-O-Si bonds are formed. When the reaction is performed at high dilution, the intrinsic viscosity of the sample does not increase [1], indicating that no intermolecular linkages are produced. It was speculated that the loops that are formed are probably small, i.e. that nearest neighbours on the chain are favoured and that small loops have little effect...
