Block living polymerization

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

The second front originates in the polymer synthesis community. Efforts are mainly directed toward production of monodisperse block copolymers by living polymerizations. These stmctures typically result in microphase separated systems if one block is a high T material and the other is elastomeric in... 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

Graft copolymers made by living polymerization processes are often called polymer brushes because of the uniformity in graft length that is possible. The basic approaches to graft copolymers also have some analogies with those used in making block and star copolymers. 

Living polymerization of isobutylene (IB) by di- and trifunctional initiators to make the nearly uniform rubber mid-block... 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

The initiation of the cyclic siloxane monomers with a living polymeric lithium species such as polystyryl lithium leads to block copolymers, as outlined in Scheme 2, were also of interest. These styrenic-siloxane block copolymers were prepared with siloxane contents from 10 to 50 weight percent. 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

The advantages provided by the continuous reactor prompted us to explore CCTP in a reactor with two CSTRs connected in series [11]. This reaction scheme depicted in Scheme 6 provides a highly flexible process for production of a wide range of diblock OBC compositions. The block composition can easily be varied by changing the production rate in either reactor. The comonomer content of either block can also be independently tailored by varying the feed compositions because the process operates in two independent reactors. This CCTP scheme also produces multiple chains per catalyst, an advantage over stoichiometric living polymerization systems, but is necessarily stoichiometric in CSA. The reaction produces... 

The GPC traces in Fig. 24 reveal a broad molecular weight distribution, MJMn = 4.42, for the dual reactor blend sample. On the other hand, the diblock OBC displays an overall MJMn of 1.67. The narrowing of the distribution indicates that the polymerization has CCTP characteristics. The theoretical molecular weight distribution from an ideal living polymerization in a series of two CSTR reactors is given by the following equation, where/j and/2 are the mass fractions of polymer comprising the two blocks [11] ... 

Fig. 26 Comparison of block copolymer architectures from living polymerization, chain shuttling, and continuous CCTP...

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