Copolymer using ionic techniques

Several methods can be used to synthesize block copolymers. Using living polymerization, monomer A is homopolymerized to form a block of A then monomer B is added and reacts with the active chain end of segment A to form a block of B. With careful control of the reaction conditions, this technique can produce a variety of well-defined block copolymers. This ionic technique is discussed in more detail in a later section. Mechanicochemical degradation provides a very useful and simple way to produce polymeric free radicals. When a rubber is mechanically sheared (Ceresa, 1965), as during mastication, a reduction in molecular weight occurs as a result of the physical pulling apart of macromolecules. This chain rupture forms radicals of A and B, which then recombine to form a block copolymer. This is not a preferred method because it usually leads to a mixture of poorly defined block copolymers. 

Several synthetic strategies are used to produce block copolymers containing a cationic block. Because charged monomers are not polymerizable by ionic techniques, the synthesis of the required block copolymers can be carried out by free radical polymerization of ionic vinyl monomers using macroinitiators, by modifying one block of a block copolymer and by coupling of two readily synthesized blocks. 

Richards has recently reported and speculated on some techniques using anionic and cationic living chains to produce polymers and copolymers possessing ionic groups, that is, quaternary ammonium bonds, at specified points along the backbone (53). 

CRP allows the polymerization of functional monomers for the coil block which is suitable to tune the materials properties, for example it is possible to obtained amphiphilic conjugated triblocks copolymers with neutral or ionic blocks. The used CRP technique depends on the functionalization of the endcapping conjugated polymer, indeed conjugated macro-initiators can be tuned... 

Since the preparation of the first identified block copolymer by Melville [26 ] a large variety of A-B and A-B-A block copolymers were prepared by free-radical polymerization by using as well macroinitiators with active chain ends, either peroxide or azo groups, as polyinitiators, for example polyazoesters [20]. These techniques are stiU used at present for the preparation of different types of polyelectrolyte block copolymers, because charge-carrying monomers are in general not directly polymerizable by ionic techniques. 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Cellulose can also be modified by introducing long-chain polymer(s) onto its main chain. The preparation of a graft copolymer requires the formation of a reactive site on cellulose in the presence of a polymerizable monomer. The principal techniques frequently used are (1) grafting initiated by free radical polymerization, (2) grafting initiated by ionic polymerization (3)... 

Copolymers can be made not just from two different monomers but from three, four, or even more. They can be made not only by free-radical chain reactions, but by any of the polymerization methods we shall take up ionic, coordination, or step-reaction. The monomer units may be distributed in various ways, depending on the technique used. As we have seen, they may alternate along a chain, either randomly or with varying degrees of regularity. In block copolymers sections made up of one monomer alternate with sections of another ... 

The ability of these methods in delivering block copolymer structures has been well demonstrated. The ATRP, RAFT, and SFRP methods could all be used to make diblock and triblock copolymers, as well as radial polymers using multiarm initiators. Because these methods are based on free-radical polymerization, they give access to a wider variety of monomer systems than are currently available through non-free-radical polymerization based techniques. They can also lead to controlled polymerization under more industrially practicable conditions as compared to ionic polymerization. 

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