Anionic chain polymerization block copolymer

Block copolymers have commercial potential for obtaining products that can incorporate the desirable properties of two or more homopolymers. This potential has led to an intense effort to find reaction systems that proceed as living polymerizations. Some anionic chain polymerizations proceed as living polymerizations under conditions where no viable chainbreaking reactions occur, and this has resulted in useful block copolymers (Sec. 5-4). 

Finkelmann H, Bohnert R. 1994. Liquid crystalline side chain AB block copolymers by direct anionic polymerization of a mesogenic methacrylate. Macromol Chem Phys 195 689 700. 

Thermoplastic elastomers have the important advantage over conventional elastomers that there is no need for the additional chemical crosslinking reaction and fabrication is achieved in the same way as for thermoplastics. However, only certain polymerization methods can be used to synthesis block copolymers — primarily living anionic chain polymerization and certain step polymerizations. Triblock copolymers are produced by living anionic polymerization by sequential addition of different momomer charges to a living anionic system, for example, a styrene-isoprene-styrene is synthesized by the sequence... 

Thus a polymer amine was available which in principle could be converted to the alkali metal salt, which in turn could be used to polymerize a second monomer anionically to create block copolymers (reaction 17). Only about 30% of the poly(THF) chains form a block copolymer. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

In a similar manner polyisoprene-polyethylene oxide block copolymers can prepared301. It is surprising that the poly(methyl methacrylate) anion can be successfully used for the polymerization of ethylene oxide without chain transfer302. Graft copolymers are also prepared by successive addition of ethylene oxide to the poly-... 

Anionic polymerizations are well suited for the synthesis of polymers fitted at chain end with reactive functions. Block copolymers can result from reactions between suitable functions carried by two different functional precursors. In some cases the carbanionic sites themselves are the reactive functions. In other cases, functional polymers (obtained anionically, or by other methods) can be reacted with low molecular weight coupling agents. Here are a few examples ... 

An alternate way to make block copolymers involving PDMS blocks 124,125) is to have these chains fitted with epoxide functions at chain end, and to react them with a vinylic or dienic polymer carrying terminal COOH functions. Sequential addition of monomers has also been used, the ring opening polymerization of the cyclic trimer (D3) being initiated by the anionic site of a living polymer126). 

In both anionic and cationic polymerization it is possible to create living polymers . In this process, we starve the reacting species of monomer. Once the monomer is exhausted, the terminal groups of the chains are still activated. If we add more monomer to the reaction vessel, chain groivth will restart. This technique provides us with a uniquely controllable system in which we can add different monomers to living chains to create block copolymers. 

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

Unlike in radical or anionic polymerizations, in ROMP with single-component metathesis catalysts the growing polymer chain remains able to further grow even after consumption of the monomer. This enables the manufacture of block copolymers with interesting physicochemical properties by sequential addition of different monomers to such living systems. 

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

Well developed is the anionic polymerization for the preparation of olefin/di-olefin - block copolymers using the techniques of living polymerization (see Sect. 3.2.1.2). One route makes use of the different reactivities of the two monomers in anionic polymerization with butyllithium as initiator. Thus, when butyl-lithium is added to a mixture of butadiene and styrene, the butadiene is first polymerized almost completely. After its consumption stryrene adds on to the living chain ends, which can be recognized by a color change from almost colorless to yellow to brown (depending on the initiator concentration). Thus, after the styrene has been used up and the chains are finally terminated, one obtains a two-block copolymer of butadiene and styrene ... 

Inaki (1992) synthesized a wide range of nucleobase-functionalized random and homopolymers. In addition, Inaki et al. (1980) synthesized block copolymers containing thymine and uracil groups in the main chain through ring-opening cationic and anionic polymerization of cychc derivatives of the nucleobases. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

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