Block copolymers step-growth polymerization

Block copolymer—These copolymers are built of chemically dissimilar terminally connected segments. Block copolymers are generally prepared by sequential anionic addition or ring opening or step growth polymerization. 

We make copolymers by incorporating two or more different monomers into a single polymer. We can make copolymers via either chain growth or step growth polymerization methods. Copolymers are characterized based on the ordering of their monomers in the final chain. Figure 2.16 illustrates several of the more common classes of copolymer random, alternating, block, and branched block. 

The type of copolymer formed during step growth polymerization depends on the reactivity of the functional groups and the time of introduction of the comonomer. A random copolymer forms when equal concentrations of equally reactive monomers polymerize. The composition of the copolymer, then, will be the same as the composition of the reactants prior to polymerization. When the reactivities of the monomers-differ, the more highly reactive monomer reacts first, creating a block consisting predominandy of one monomer in the chain the lower reactivity monomer is added later. This assumes that there is no chain transfer and no monofunctional monomer present. If either of these conditions were to exist,... 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

The resulting macromolecules are still reactive toward additional diazoalkanes. The above step-growth polymerization reactions can also yield block copolymers ... 

Chain-growth catalyst-transfer polycondensation (CTP) is a rapidly developing polymerization method, as it allows, in many cases, the above-mentioned limitations of step-growth polymerizations to be overcome. CTP provides a straightforward access to well-defined conjugated homopolymers (e.g., polythiophenes (1), polyfluorenes (2), polyphenylenes (3), etc.), alternate donor-acceptor copolymer e.g., 4) and all-conjugated block polymers (e.g., 5), Chart 20.1. 

Synthesis. Block copolymers are synthesized by a variety of methods most important are sequential (41-46) and step-growth polymerization (47). In sequential polymerization, a polymer (A) is first synthesized in such a way that it contains at least one group per molecule that can initiate polymerization of another monomer B. 

The polyetherimide/polysiloxane block copolymers are synthesized by step-growth polymerization (47). Many other synthetic methods for preparing block copol5miers have been described (14,16-20,22-25) but are currently not believed to be commercially important. 

A benefit of the relatively stable end groups of polymers prepared by controlled/ living polymerizations, is that they can be isolated and stored as macroinitiators with relative ease. Such is not the case for polymers prepared by ionic polymerizations the active anion or cation will be quenched by advantageous moisture. This also allows one to modify polymers prepared by other methods so that they can become macroinitiators for controlled/ living radical polymerization. Such mechanism transformation can be used to prepare a wide array of novel polymers block copolymers of combinations of radically prepared polymers with those synthesized by step-growth polymerizations [160,276], ROMP [159,277], cationic [161,278] and anionic polymerizations [255,279] have been prepared (Table 3). 

More specific topics, such as block copolymer synthesis by changing the polymerization mechanism [18], by step-growth polymerization [19], via macroinitiators [20], living free-radical polymerization [21, 22] or ionic polymerization [23] were reviewed later on, as well as the synthesis of selected block copolymer types, for example hydrophilic-hydrophilic copolymers [24], copolymers based on PEO [10,16]. 

Montero de Espinosa M, Meier MAR (2011) Synthesis of star- and block-copolymers using ADMET head-to-tail selectivity during step-growth polymerization. Chem Commun 47 (6) 1908-1910... 

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