Controlled radical mechanisms, block

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 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... 

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. 

A special case of control over the structure of copolymers may include the first stereoblock copolymers made by CRP. By applying either RAFT or ATRP to polymerization of acrylamides in the presence of rare-earth triflates such as Y(OTf)3 and Yb(OTf)3, it was possible to enhance isotacticity of A(A/-dmiethylacrylamide (DMA) from 50% meso to 90% meso dyads (170). At the same time control of molecular weights and polydispersity was preserved. Similar results were obtained for RAFT of A(-ispropylacrylamide (171). The ATRP and RAFT of DMA was applied to the first one-pot stereoblock synthesis by radical mechanism. RAFT or ATRP of DMA were started without Lewis acid to produce the first atactic block. Subsequently, the complexing agent was added at the desired conversion to continue the chain growth with the preferential isotactic placement (170). 

Living polymerization was discovered in anionic system by Szwarc (see p. 476) in 1950, which, as we shall see in Chapter 8, offers many bene ts including the ability to control molecular weight and polydispersity and to prepare block copolymers and other polymers of complex architecture. Many attempts have then been made to develop a living polymerization process with free-radical mechanism so that it could combine the virtues of living polymerization with versatility and convenience of free-radical polymerization. Considering the enormous importance and application potential of living/controlled radical polymerization techniques, these will be considered in detail in another chapter (Chapter 11) with a state-of-the art discussion on the subject. 

In most reports, the peptide-polymer-conjugates are prepared by using a polymeric macroinitiator for the polymerization of the polypeptide however, the sequence can also be reversed. Polypeptides can be prepared and used as macroinitiators for a polymerization. Particularly suited for this approach are controlled polymerization techniques because they usually allow good end-group control and adjustment of the molecular weight and the molecular weight distribution of the polymer block. There are different mechanisms for a controlled radical polymerization that can be used for this purpose stable free-radical polymerization (SFRP), ATRP, and reversible addition fragmentation chain transfer (RAFT) polymerization. 

Since the number of monomers, and thus the resulting polymer structures, are limited by any of the specific living polymerization techniques, appropriate combination of different polymerization mechanisms can lead to a variety of new and useful polymeric materials. Therefore combinations of controlled radical polymerizations and other polymerizations applied to synthesize block copolymers have been developed. Generally, polymers with active sites, such as carbon-halogen or nitroxide or dithioester terminal groups, are synthesized by other living polymerizations, and the product is further used to initiate the controlled radical polymerization. In many cases, this method is essentially a variant of the macroinitiator method discussed above. However, in some cases, these kinds of macromolecules do not act as initiators, and may act as transfer agents. For example, an AB-type amphiphilic block copolymer, CLB-2 was prepared by RAFT polymerization of 2-(N-dimethylamino)ethyl methacrylate... 

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