Anionic block copolymers

The equivalence ratio can be calculated from the Mark-Houwink coefficient, K, of component homopolymers. The composition distribution in the chromatogram of a block copolymer is negligible. The peak point of a block copolymer chromatogram corresponds to the average structure of the polymer. Thus, analysis of block copolymers is reduced to analysis of the peak point. Analyses of anionic block copolymer structures have been successfully accomplished by this peak analysis technique with the aid of equivalence ratio. 

Alginate gels are not thermoreversible, in fact they appear heat stable up to >100 °C, and their formation can only be induced by certain, specifically divalent, cations. Alginates are anionic block copolymers composed of two... 

MAS Masci, G., Diociaiuti, M., and Crescenzi, V., ATRP synthesis and association properties of thermoresponsive anionic block copolymers, / Polym. Sci. Part A ... 

Kabanov et al. [59] reported nonstoichiometric complexes between cationic poly (V-ethyl-4-vinylpyridinium) and anionic block copolymer poly(ethyleneoxide-co-methacrylate). This system revealed highly soluble stoichiometric PEC particles. 

Furthermore, Yuan et al. [144] introduced spherical PEC particles obtained by complexation of the cationic graft copolymer PEI-gra/t-poly(A-vinylpyrrolidone) (PEI-g-PVP) with anionic block copolymer of PVP-h/oc -poly(2-acrylamido-2-methyl-l-propanesulfonic acid) (PVP-6-PAMPS) the particle size was around 140 nm. Folic acid was entrapped in the micelle core by electrostatic attraction and the release rate was strongly dependent on the pH of the release medium. 

Typical examples of well-defined amphiphilic anionic block copolymers are PS-PAA and PS-PMAA in the neutralized form (PS-PANa and PS-PMANa) which have been studied extensively over the last years by Tuzar [114,136] and by Eisenberg and co-workers [127, 178] who have also recently reviewed this topic of self-assembly of polyelectrolytes. These copolymers with a PAA or PMAA sequence have been examined in a very wide compositional and molar mass range. At high pH, the (meth)acryhc blocks are ionized resulting in stable micelles with extended shell regions, due to the electrostatic repulsion of the shellforming chains. 

Similar to their work on anionic block copolymers, Eisenberg and co-workers [178, 192] have shown that crew-cut micelles can be obtained with PS-PQ4VP of high PS content. 

A similar system has furthermore been investigated by Gohy etal. [215,245] in the case of complex formation between P2VP-PEO and PEO-PMAA diblock copolymers as well as for combination of a PS-P2VP-PEO triblock with a PAA-PEO comb-block copolymer. The same concept was applied by Tenhu and co-workers [273] involving PEO-PMANa anionic block copolymer and a cationic graft copolymer. 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Reaction Injection Molding. RIM uses the anionic polymeri2ation of nylon-6 to carry out polymeri2ation in the mold. A commercial process involves the production of block copolymers of nylon-6 and a polyether by mixing molten caprolactam, catalyst, and polyether prepolymer, and reacting in a mold (27,28). 

When the initial monomer supply is exhausted, the anionic chain ends retain their activity. Thus, these anionic chains have been termed living polymers. If more monomer is added, they resume propagation. If it is a second monomer, the result is a block copolymer. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

Styrene—butadiene block copolymers are made with anionic chain carriers, and low molecular weight PS is made by a cationic mechanism (110). Analytical standards are available for PS prepared by all four mechanisms (see Initiators). 

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. 

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

Commercially, anionic polymerization is limited to three monomers styrene, butadiene, and isoprene [78-79-5], therefore only two useful A—B—A block copolymers, S—B—S and S—I—S, can be produced direcdy. In both cases, the elastomer segments contain double bonds which are reactive and limit the stabhity of the product. To improve stabhity, the polybutadiene mid-segment can be polymerized as a random mixture of two stmctural forms, the 1,4 and 1,2 isomers, by addition of an inert polar material to the polymerization solvent ethers and amines have been suggested for this purpose (46). Upon hydrogenation, these isomers give a copolymer of ethylene and butylene. 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

Tbe system may be used for homopolymers and for block copolymers. Some commercial SBS triblock thermoplastic rubbers and the closely related K-resins produced by Phillips are of this type. Anionic polymerisation methods are of current interest in the preparation of certain diene rubbers. 

Closely related to these but thermoplastic rather than rubber-like in character are the K-resins developed hy Phillips. These resins comprise star-shaped butadiene-styrene block copolymers containing about 75% styrene and, like SBS thermoplastic elastomers, are produced by sequential anionic polymerisation (see Chapter 2). 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

Of the amorphous block copolymers, styrenic block copolymers are the vast majority. These are synthesized anionically in solution, with butyl lithium commonly employed as the initiator [4]. There are three processes for this polymerization ... 

Anionic Styrenic block copolymers (SBS, SIS, SEBS, SEES) 1.0-1.1 Excellent... 

Photoinitiators provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end groups. Under suitable conditions, and in the presence of a vinyl monomer, a block AB or ABA copolymer can be produced which would otherwise be difficult or impossible to produce by another polymerization method. Moreover, synthesis of block copolymers by this route is much more versatile than those based on anionic polymerization, since a wider range of a monomers can be incorporated into the blocks. 

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