Controlled radical mechanisms, block copolymers

The living radical polymerization of some derivatives of St was carried out. The polymerizations of 4-bromostyrene [254], 4-chloromethylstyrene [255, 256], and other derivatives [257] proceed by a living radical polymerization mechanism to give polymers with well-controlled structures and block copolymers with poly(St). The random copolymerization of St with other vinyl... 

Iniferter polymerizations were also combined with anionic polymerization. The representative example involves the synthesis of PCL-l7-(PMMA-co-PSt)-l7-PCL. ° A polymeric thermal iniferter, PCL-substituted tetraphenylethane, was prepared by anionic polymerization of CL in the presence of aluminum triisopropoxide and benzopinacol. The benzopinacolate groups incorporated into the polymer chain initiated the polymerization of St and MMA via a controlled radical mechanism at 95 °C to yield the desired block copolymers (Scheme 47)... 

The counter radical method has been studied with various monomers more or less successfully. However, the synthesis of only few block or grafted copolymers is effectively described. This is a strong indication that a true control of the polymerization is still not achieved with all monomers although progress is constant. Nevertheless, it is clear that the possibility of reversibly controlling the termination step offers a tool of choice for the synthesis of well-defined and pure block copolymers and many studies are still necessary to understand properly the precise mechanism of macroradical end capping in order to control the reversible character and possible secondary reactions. 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

Novel rathenium complexes with carborane ligands were employed as efficient catalysts for controlled polymer synthesis via Atom Transfer Radical Polymerization (ATRP) mechanism. The ability of carborane ligands to stabihze high oxidation states of transition metals allows the proposed catalysts to be more active than their cyclopentadienyl counterparts. The proposed catalysts do not reqnire additives such as aluminium alkoxides. It was shown that introdnction of amine additives into the polymerization mixture leads to a dramatic increase of polymerization rate leaving polymerization controlled. The living nature of polymerization was proved via post-polymerization and synthesis of block copolymers. 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

Several methods can be used to synthesize block copolymers. Using living polymerization, monomer A is homopolymerized to form a block of A then monomer B is added and reacts with the active chain end of segment A to form a block of B. With careful control of the reaction conditions, this technique can produce a variety of well-defined block copolymers. This ionic technique is discussed in more detail in a later section. Mechanicochemical degradation provides a very useful and simple way to produce polymeric free radicals. When a rubber is mechanically sheared (Ceresa, 1965), as during mastication, a reduction in molecular weight occurs as a result of the physical pulling apart of macromolecules. This chain rupture forms radicals of A and B, which then recombine to form a block copolymer. This is not a preferred method because it usually leads to a mixture of poorly defined block copolymers. 

In a well-controlled radical system, the monomer conversion is first order, molar mass increases linearly with monomer conversion, and the molar mass distribution MJM is below 1.5. In addition, chain end functionalization and subsequent monomer addition allow the preparation of well-controlled polymer architectures, for example, block copolymers and star polymers by a radical mechanism, which had been up to now reserved for ionic chain growth polymerization techniques. 

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