Synthesis homopolymer formation

Polymeric initiators based on azo or triazene modified poly(organosiloxane)s can be used to synthesize graft copolymers with silicone backbone and thermoplastic side chains. The azo functionality has some advantages such as lower thermal stability, known reaction mechanism, no homopolymer formation and cleaner graft products. However, the synthesis of the triazene polymeric initiators requires fewer synthetic steps. The graft products microseparate but form stable films. 

Obviously, one could also use the benzoin site of the azo-benzoin initiator for a photopolymerization in the first step and activate the azo sites in a subsequent, thermal reaction [213]. However, regarding homopolymer formation, the method depicted in reaction (47) has to be given priority because benzoin groups are thermally stable, whereas azo groups may be photolyt-ically ruptured. Benzoin-azo initiators are, furthermore, suitable for the synthesis of block copolymers with one of two monomers being cationically polymerizable [214,215]. 

Grafting provides a convenient means for modifying the properties of numerous polymers. It is often required that a polymer possess a number of properties. Such diverse properties may not be easily achieved by the synthesis of homopolymers alone but can be achieved through the formation of copolymers or even terpoly-mers. The formation of graft copolymer with sufficiently long polymeric sequences of diverse chemical composition opens the way to afford speciality polymeric materials. 

Selective functionalization of the 7-hydroxyl group as the monomethoxytrityl (MMT) ether and phosphoramidite formation at 0-5 under standard conditions delivered 234 (58% over two steps). This material could be taken through to the solid-phase ON synthesis. This synthesis utilizing 234 and 235 as starting materials delivered homopolymers, 0T15 and oA15, each of which was 15 nucleoside bases long. 

Most of the biochemical studies on polysaccharide synthesis to date have been concerned with the formation of homopolymers even when it is known that the synthesis of the homopolymer chain occurs in vivo as part of a heteropolysaccharide (4-6). Cytochemical investigations have made no such distinctions and the polymers located by these studies have nearly always been sites at which heteropolymers were present and where deposition in the wall occurred. The bulk of the polysaccharides that occur in the wall, with the exception of cellulose and callose, are heteropolymers. Generally the polysaccharides of the hemicelluloses and pectins are composed of poly-... 

In 1965, Milkovich (. ) reported that divinylbenzene could be utilized for the formation of star-branched macromolecules. Later, Rempp and coworkers (2, 3, 4) successfully applied this method for the synthesis of star-branched polystyrenes. Moreover, Fetters and coworkers (54 ) used this procedure for the synthesis of multi-arm star-branched polyisoprene homopolymers and poly-... 

Rogovin, Z. A., T ung Sun, A. D. Virnik, and N. M. Khvostenko CIV. Synthesis of new cellulose and other polysaccharide derivatives. 19. Synthesis of cellulose and carbon-chain graft copolymer withour simultaneous formation of homopolymers. Vysokomolekul. Soedin. 4, 571 (1962). 

Given the morphological complexity of AB diblock and ABA triblock copolymers, it might be expected that the phase behaviour of ABC triblocks would be even more rich, and indeed this has been confirmed by recent experiments from a number of groups. From a practical viewpoint, ABC triblocks can also act as compatibilizers in blends of A and C homopolymers (Auschra and Stadler 1993). In addition to the composition of the copolymer, an important driving force for structure formation in these polymers is the relative strength of incompatibilities between the components, and this has been explored by synthesis of chemically distinct materials. 

As already stated in Sect. 4.3, the synthesis of block copolymers via radical reactions is generally accompanied by the formation of homopolymer. Thus, if the block copolymers are to be characterized thoroughly, the homopolymer and block copolymers must be separated. Since such separations are system-specific, they will be described in connection with the chosen examples. 

A typical procedure for the synthesis of a propylene-MMA diblock copolymer is as follows. A living polypropylene (Mn = 16,000, Mw/Mn = 1.2) was prepared at —78 °C in a toluene solution of the V(acac)3/A1(C2H5)2C1 catalyst, followed by the addition of MMA. The block copolymerization with MMA was carried out for 5 h at 25 °C, resulting in the formation of an almost monodisperse block copolymer (K I0 = 18,000, Mw/Mn = 1.2). The block copolymer was treated with acetic acid in which the homopolymer of MMA would be soluble. No soluble polymer was detected. In addition, the insoluble polymer was treated with boiling acetone. Again, no soluble polymer was found. These results indicate the formation of a diblock copoly-... 

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