Multiblock copolymers synthesis

Somewhat limited work has been reported over the last decade. There are several reports on the synthesis and physical and structural characterization of styrene-dimethylsiloxane 141 144) and methylmethacrylate-dimethylsiloxane145> diblock, triblock and multiblock copolymers. Several reports are also available on the thermal223), solution 224,2251 and surface196 2261 characterization of various styrene-dimethyl-siloxane block copolymers synthesized by anionic techniques. 

The A-B type iniferters are more useful than the B-B type for the more efficient synthesis of polymers with controlled structure The functionality of the iniferters can be controlled by changing the number of the A-B bond introduced into an iniferter molecule, for example, B-A-B as the bifunctional iniferter. Detailed classification and application of the iniferters having DC groups are summarized in Table 1. In Eqs. (9)—(11), 6 and 7 serve as the monofunctional iniferters, 9 and 10 as the monofunctional polymeric iniferters, and 8 and 11 as the bifunctional iniferters. Tetrafunctional and polyfunctional iniferters and gel-iniferters are used for the synthesis of star polymers, graft copolymers, and multiblock copolymers, respectively (see Sect. 5). When a polymer implying DC moieties in the main chain is used, a multifunctional polymeric iniferter can be prepared (Eqs. 15 and 16), which is further applied to the synthesis of multiblock copolymers. 

The synthesis of some multiblock copolymers was attempted by successive polymerization using this iniferter technique. However, pure tri- or tetrablock copolymers free from homopolymers were not isolated by solvent extraction because no suitable solvent was found for the separation. In 1963, Merrifield reported a brilliant solid-phase peptide synthesis using a reagent attached to the polymer support. If a similar idea can be applied to the iniferter technique, pure block copolymer could be synthesized by radical polymerization. The DC group attached to a polystyrene gel (PSG) through a hydrolyzable ester spacer was prepared and used as a PSG photoiniferter (Eq. 53) [186] ... 

Figure 1. Schematic of self-assembly assisted polypolymerization (SAAP) of triblock copolymers in a selective solvent for the synthesis of long multiblock copolymers with a controllable chain sequence and block length. [3 5]...

Other authors have been interested in the synthesis of multiblock copolymers starting from ozonization of PVC which essentially leads to a-co acid oligomers. Michel et al. [113,115, 126] synthesized the compounds shown in Scheme 40 using ozonization of PVC in solvents. 

Numerous authors took advantage of the reactivity of double bonds towards ozone to prepare a-(0 functional oligomers usable in the synthesis of multiblock copolymers by copolycondensation, or in the synthesis of precursors of surfactants or ionomer resins. Results in this field of investigation are numerous, mainly in terms of industrial applications. 

The cyclic tin alkoxides have the additional advantage of offering a convenient synthetic pathway for the synthesis of macromers, triblock, and multiblock copolymers [81,82]. Macromers from l-LA [83],e-CL [84], and l,5-dioxepan-2-one (DXO) [85] have been synthesized as well as triblock poly(L-LA-b-DXO-b-L-LA) [86] and multiblock copoly(ether-ester) from poly(THF) and e-CL [87]. The polymerization proceeds by ring expansion and the cyclic structure is preserved until the polymerization is quenched by precipitation. 

The ability of these macromonomers to participate in free-radical copolymerization was not tested. They were used28b) for the synthesis of tri- and multiblock copolymers by means of hydrosilylation, i.e. an addition reaction between vinylsilyl groups and the —Si — H functions at the chain end of poly(dimethylsiloxane)mole-cules. 

It is seen that free radical micromolecular or macromolecular initiators have been successfully employed for the synthesis of di-, tri- or multiblock copolymers. However, once again, the structure of these block copolymers depends upon the termination step of the polymerization, and especially on the recombination or disproportionation of macroradicals produced. Besides, such a method also generates homopolymers. Separation and purification of these different structures are usually very difficult or even impossible. Moreover, the copolymers obtained usually exhibit a broad polydispersity, a defect inherent in the classical radical process. 

According to a similar process, macromolecular bis(silyl benzopinacolate) derivatives have been of particular interest in the synthesis of multiblock copolymers containing siloxane sequences proposed by Crivello et al. [211], These authors used a high molecular weight PDMS macroinitiator 35. 

Olefin metathesis is one of the few fundamentally new organic reactions discovered over the last few decades that has revolutionized organic and polymer chemistry. Olefin metathesis provides a convenient and rehable way to synthesize imsaturated molecules that are often hard to prepare by any other method. A munber of reviews [8-17] and books [18-20] have been published in this area, aU of which focus on the ever-increasing use of olefin metathesis in organic synthesis and polymer chemistry. Particularly in the latter research area, ROMP has become a powerful and popular method to synthesize polymers with narrow molecular weight distributions. Due to the hving nature of polymerizations initiated by state-of-the-art initiators, well-defined diblock, triblock or multiblock copolymers are available today. 

The type of block copolymer produced in this S3m-thesis is dependent on the termination reactions of both monomers. If both monomers terminate by disproportionation diblock copolymers result. If one reacts by combination and one by disproportionation triblock copol3nners are formed and if both monomers terminate by combination multiblock copolymers result. This process is imavoidably accompanied by the formation of homopolymers. So the central question of this synthesis is How high can the fraction of block co-pol3raiers be ... 

With monofunctional initiators (e.g., n-butyllithium), AB, ABA, and multiblock copolymers can be formed. For example, the synthesis of an AB block copolymer can be shown schematically as... 

Bachari, A., Belorgy, G., Helary, G., and Sauvet, G., Synthesis and Characterization of Multiblock Copolymers poly[poly(L-lactide)-block-poly-dimethylsiloxane]. 1995, Macromol. Chem. Phys., 196 411... 

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