Block copolymers involving same polymerization

A most useful and versatile approach to the preparation of block copolymers involves the use of anionic polymerization. This has the advantage that there is no spontaneous termination reaction so that although a polymer may appear dormant it may grow further, i.e. it can be activated, by introduction of a fresh monomer, which is not necessarily of the same species as used to produce the initial polymer. These so-called living polymers have a narrow molecular weight distribution and an easily varied and controlled average molecular weight. 

Instead of adding more styrene to the polymerization above, what if we were to add a second monomer say methyl methacrylate, after the first monomer had been totally consumed The result would be an AB block copolymer. Aw represents the first monomer, styrene in our example, and B the methyl methacrylate. The degrees of polymerization of the two blocks could be the same, or they could be different. The steps involved in constructing the methyl methacrylate block are shown in the following scheme ... 

Addition polymers can also be produced by mechanisms that involve ionic intermediates, reacting in much the same way as illustrated for free radical intermediates. These t)rpes of polymerization are used much less frequently than the free radical types, but are very important in some specialty reactions, such as preparation of block copolymers. 

In principle the composition of a copolymer may also be determined by analyzing the composition of the residual monomer by a suitable method after polymerization. It will usually be necessary first to separate the copolymer by precipitation, followed by careful recovery of the filtrate cmitaining the residual monomer, but the direct method of analysis of the copolymer will generally be preferred. Block and graft copolymers can be characterized in the same manner. However, consideration must be taken of the fact that they usually contain large amounts of the homopolymers which must first be removed. The more refined characterization of a statistic copolymer involves the determination of the reactivity ratios ri and 2 (copolymerization parameters), as well as the calculation of Q- and e-values (see Chap. 3.4, Example 3.36). 

Much work on the preparation of nonaqueous polymer dispersions has involved the radical polymerization of acrylic monomers in the presence of copolymers having the A block the same as the acrylic polymer in the particle core 2). The preparation of polymer dispersions other than polystyrene in the presence of a PS-PDMS diblock copolymer is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the PS anchor block and the polymer molecules in the particle core. The present paper describes the interpretation of experimental studies performed with the aim of determining the mode of anchoring of PS blocks to polystyrene, poly(methyl methacrylate), and poly(vinyl acetate) (PVA) particles. 

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