Anionic polymerization star-shaped copolymers

The purpose of this review is to show how anionic polymerization techniques have successfully contributed to the synthesis of a great variety of tailor-made polymer species Homopolymers of controlled molecular weight, co-functional polymers including macromonomers, cyclic macromolecules, star-shaped polymers and model networks, block copolymers and graft copolymers. 

Yijin X. and Caiyaun P., Block and star-hlock copolymers by mechanism transformation. 3. S-(PTHF-PSt)4 and S-(PTHF-PSt-PMMA)4 from living CROP to ATRP, Macromolecules, 33, 4750, 2000. Feldthusen J., Ivan B., and Mueller A.H.E., Synthesis of linear and star-shaped block copolymers of isobutylene and methacrylates hy combination of living cationic and anionic polymerizations. Macromolecules, 31, 578, 1998. 

Living cationic polymerization techniques are also capable of producing well defined star-block copolymers. An approach similar to the DVB method described above for the case of anionic polymerization was employed in order to prepare amphiphilic star-block copolymers [20]. In one case, living diblock copolymers of vinyl ethers and ester-containing vinyl ethers, prepared by the initiating system Hl/Znh in toluene, were reacted with a small amount of a difunctional vinyl ether to produce star shaped block copolymers (Scheme 5). 

A comprehensive list of the grafting reactions of (a) and (b) onto different polymer backbones is given in Ref 543. The methods summarized there include radiation techniques [623], grafting by radical transfer [624], and grafting initiated by functional groups in backbone polymers [625,626]. Macromonomers of (a) have been synthesized by means of anionic polymerization techniques and have been copolymerized with styrene [627,628]. Only a few examples are known in which polymers from (a) and (b) were used as backbone [629]. Star shaped block copolymers with four arms were prepared by coupling living styrene/(a) block copolymers with 1,2,4,5-tetrakis-bromomethyl-benzene [626]. 

Similar micelle-like macromolecular co-assemblies were reported [50] for an anionic star-shaped bis-hydrophilic heteroarm copolymer PMAA-PEO coupled with a cationic bis-hydrophilic diblock copolymer, PDMAEMAQ-Wock-PEO (PDMAEMAQ-b-PEO), in alkaline media. In this case, however, the PEO blocks of both polymeric counterparts form a hydrophilic corona of each of the complex species formed. 

Star polymers consist of several linear polymer chains connected at one point. Prior to the development of CRP, star molecules prepared by anionic polymerization had heen examined. However, due to the scope of ionic polymerization, the composition and functionality of the materials were limited. The compact structure and globular shape of stars provide them with low solution viscosity and the core-shell architecture facilitates entry into several applications spanning a range from thermoplastic elastomers (TPEs) to dmg carriers. Based on the chemical compositions of the arm species, star polymers can be classified into two categories homoarm star polymers and miktoarm (or heteroarm) star copolymers... 

Anionic and cationic polymerizations are often associated. Feldthusen et al. [5f] prepared copolymers containing linear and star-shaped blocks a living polyisobutylene chain was prepared by cationic polymerization, its ends were converted into 2,2-diphenylvinyl groups, then metallated and used as initiators of the ferf-butyl methacrylate anionic polymerization. 

Feldthusen J, Bela I and Mueller A H E (1998) Synthesis of linear and star-shaped block copolymers of isobutylene and methacrylates by combination of living cationic and anionic polymerizations, Macromolecules 31 578-585. 

On the other hand, reports on microphase separation of star copolymers have apparently not been published to date. We have recently synthesized A B starlike (comb-shaped) copolymers by anionic polymerization of binary vinylbenzyl-terminated PS and PI macromonomers and investigated the mi-crophase-separated structures [75]. These A B star copolymers formed a clear microphase-separated structure. In this type of A B star copolymer, there are two entropic effects at work that oppose one another, according to de la Cruz and Sanchez s theory [3]. The first is the entropy of the melt. The entropy of the star copolymer is smaller than that of the corresponding diblock copolymer melt because of the additional constraints on the A-B junction point. However, for noncritical compositions, there will be a lowering of the transition entropy... 

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