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.1 If it is possible to minimize the contribution ofchain transfer and termination during the polymerization, it is possible toachieve a level of control over the resulting polymer and achieve a predeter-mined number average molecular weight and a narrow molecular weight dis-tribution (polydispersity).If such an ideal scenario can be created, thenumber of polymer chains that are produced is equal to the number of initi-ator groups; the polymerization will proceed until all of the monomer hasbeen consumed and the polymer chain ends will remain active so that furtheraddition of monomer will lead to continued polymerization.This type ofpolymerization was termed a  living polymerization by Szwarc2 in 1956 andrepresents one of the ultimate goals of synthetic polymer chemists.Flory3determined that in the absence of termination, the number of propagatingpolymer chains must remain constant and that the rate of polymerization foreach growing chain must be equal.In this situation, the number averagedegree of polymerization (DPn) and hence the molecular weight of the poly-mer can be predicted by simple consideration of the monomer to initiatorratio (see eqns (1) and (2), respectively).[monomer]DPn (1)[initiator]Number average molecular weight (polymer)DPn molecular weight (monomer) (2)Several key criteria are used to elucidate the  living nature of apolymerization.4 For a polymerization to be considered  living , the rate ofinitiation must exceed the rate of propagation.Therefore, all the propagatingpolymer chains are formed simultaneously and grow at the same rate.If thisW.Hayes and S.Rannardsituation did not occur, the first chains formed would be longer than thoseinitiated later and the molecular weight distribution of the propagating chainswould broaden.In addition, an ideal  living or  immortal polymerization mustnot exhibit any termination of the propagating polymer chains over the lifetimeof the reaction.Consequently,  living polymerizations are characterized byvery narrow molecular weight distributions (Mw/Mn 1.2).The ability of thepropagating species to undergo polymerization until full monomer conversionFunctional group Functional groupsXPolymer X Polymer XEnd-functionalized polymers Bifunctional/telechelic polymersA B A B AAB block copolymers ABA block copolymersPolymer APolymer APolymer BPolymer BGraft/brush polymers Ladder-type polymersStar polymersFig.3.1 The variety of polymer and copolymer architectures that can be synthesized by living polymerization techniques.1003: Controlled/ living polymerization methodsand then to continue polymerizing upon addition of another monomerfeed is another key feature of  living polymerizations (i.e.the active chainends are stable enough to enable synthesis of block copolymers via sequentialmonomer addition).If all of these criteria have been satisfied, polymers andblock copolymers exhibiting Poisson-type molecular weight distributions willbe produced and a linear relationship exists between the number averagemolecular weight (Mn) of the resultant polymer and monomer conversion.As a consequence of the development of  living polymerization methodo-logies, synthetic polymer chemists are now able to construct, in a precise man-ner, a wide variety of polymer architectures5 (Figure 3.1) that were previouslyinaccessible using uncontrolled chain-growth methods such as free-radicalpolymerizations that employed initiators such as azobisisobutyronitrile(AIBN) or benzoyl peroxide.6 For example, in light of recent developments in living anionic polymerization techniques, ABA block copolymers such asstyrene butadiene styrene for use in thermoplastic elastomers can be pro-duced in a reliable fashion on an industrial scale.5 The implications of  livingpolymerization methods upon materials science are enormous, as syntheticpolymer chemists now have a range of powerful synthetic tools available tothem to facilitate the construction of well-defined polymers almost to order.Inthis respect, meaningful structure property relationships (analogous to thoseemployed in drug discovery by the pharmaceutical industry) can be carried outfor the first time, enabling detailed development of high technology polymericmaterials.2.Covalent  living polymerization: group transferpolymerizationSeveral  living polymerization techniques have been shown to initiate andpropagate by the reaction of an active covalent end group with monomers.One such method is the Group Transfer Polymerization (GTP) [ Pobierz całość w formacie PDF ]