TABLE OF CONTENTS The idea of graft [259 261] or block [262] copolymerization probably first arose as a means of modifying naturally occurring polymers, such as cellulose (cotton), rubber, or wool. Graft copolymerization, by analogy to the botanical term, refers to the growth of a branch of different chemical composition on the backbone of a linear macromolecule. In contrast, the related term, block copolymerization, refers to the specific case of growth of a polymer chain from the end of a linear macromolecule, thus leading to a composite linear macro-molecule consisting of two or more blocks of different chemical structure. The importance of these types of polymer structures is basically due to the fact that polymer chains of different chemical structure, which are normally incompatible and form separate phases (because the small entropy of mixing is insufficient to overcome the mostly positive enthalpy of mixing), are chemically bonded to each other. This leads to the formation of microhetero-geneities, which can have a profound effect on the mechanical properties of these heterophase systems when compared with the two homopolymers or with a physical mixture of the two polymers [263].
As one might expect, graft and block copolymerization can be accomplished by means of each of the three known mechanisms, i.e, radical, cationic, and anionic, each of which shows its own special characteristics [264]. Hence these mechanisms have been used wherever appropriate for the polymer and monomer involved. The examples quoted in the following discussions will deal primarily with elastomers.
