| Popis: |
This report gives a classification of methods for the preparation of block and graft copolymers according to types of active centres. Particular attention is paid to methods comprising two stages of different types of polymerization with active centres generated by chemical means. Methods based of free radical polymerizations. The use of high viscosity chemically inert media to increase the life time of free radicals. Polymerization carried out in the presence of bifunctional radical initiators. The preparation of polymers containing functional end groups in the presence of radical initiators. Radical polymerization in the presence of ready made polymer as a means of synthesizing a second block or graft chains. The use of transfer steps by radicals as a means of carrying out polymerization in the presence of an efficient chain transfer agent. Methods based on free radical polymerization are reasonably efficient and are in use, although such methods do generally lead to the formation of block and graft copolymers that are difficult to identify, and contain the respective homopolymers. Anionic polymerization as a method of preparing block and graft copolymers. Block and graft copolymers may be prepared efficiently by sequential polymerization of two monomers in an aprotonic solvent in the presence of a suitable carbanionic initiator. Ternary block copolymers are obtainable through the use of bifunctional anionic initiators. This method has been used to prepare thermo-elasto-plastic materials of the type of polystyrene-polyisoprene-polystyrene copolymers as well as block copolymers wherein both blocks are approximately similar chemically. A study has been made of the use of terminationless anionic polymerization as a means to preparing copolymers based on heterocyclic monomers, and some examples have been examined where functional groups have been incorporated in polymers as a result of reactions between terminal carbanions and electrophilic compounds. These polymers may be used for the preparation of block copolymers. There are two ways in which graft copolymers are obtainable by anionic polymerization: either by fitting the main chain with metalorganic functional groups, which will then initiate polymerization of a second monomer, or by reacting “living” macrocarbanions with polymer whose backbone has been fitted with electrophilic functional groups. Cationic polymerization yielding graft and block copolymers. If cationic polymerization is to be used for the synthesis of graft and block copolymers it is essential that experimental conditions should be such as to exclude transfer steps, and that the initiators selected will react rapidly and quantitatively and form stable carbcations when added to monomers. Some novel examples of the synthesis of block and graft copolymers are given below. A study has been made of techniques for the synthesis of graft and block copolymers based on reactions of functional groups of polymers. |
