Criteria for Living Polymerization

Following on from the above, various methods have been described to test and/or rank the livingness of polymerization processes." Ul7 20 All of these tests have limitations.. The following list paraphrases a set of criteria for living polymerization set out by Quirk and Lee11 who also critically assessed their applicability primarily in the context of living anionic polymerization. 

This general equation reduces to the Poisson distribution when 1 = 1. The magnitude of I will define the narrowness of the MWD. At sufficiently high MW, 1 = 25 still yields DP ,/DP j < 1.1. The MWD criteria for living polymerizations... 

Quirk RP, Lee B. Experimental criteria for living polymerizations. Polym Int 1992 27 359-367. 

Quirk, R.P and Lee, B. (1992) Experimental Criteria for Living Polymerization. Polymer International,... 

Doi, Ueki and Keii47,48) have found that the soluble catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 polymerizes propylene in toluene at —78 °C to give a syndiotactic living polypropylene having a narrow molecular weight distribution (Mw/Mn = 1.05-1.20). This low-temperature polymerization of propylene was shown to satisfy all criteria for the living polymerization 47). 

In the following sections the available literature on Nd-catalyzed polymerizations of dienes is discussed regarding the compliance with each of the criteria put forward for a strictly living polymerization. 

Styrenic block copolymers derive their useful properties from their ability to form distinct styrene (hard phase) and diene (rubber phase) domains, with well defined morphologies. To achieve this requires an unusual degree of control over the polymerization. The polymerization must yield discrete blocks of a uniform and controlled size, and the interface between the blocks must be sharp. This is best achieved by so-called living polymerization. For a polymerization to be classified as truly living, it is generally accepted that it must meet several criteria [3] ... 

Several key criteria are used to elucidate the living nature of a polymerization.For a polymerization to be considered living , the rate of initiation must exceed the rate of propagation. Therefore, all the propagating polymer chains are formed simultaneously and grow at the same rate. If this... 

Not long ago criteria for the generation of living cationic polymerizations have been developed [443-445]. These criteria include an almost instantaneous and quantitative initiation and the absence of chain transfer and termination reactions. These requirements were satisfied when iodine was used to polymerize NVK in dichloromethane solution containing tetrabutylammonium iodide at low temperatures [446,447]. Polymer molecular weights increased in a linear fashion with monomer consumption during the reaction and upon the addition of successive aliquots of monomer. 

More extensive discussion, however, is given in Reference 11, which also describes major criteria for botb living and controlled polymerizations. In addition to the lUPAC definitions, the AGS definitions are quoted bdow. In prindple, there is no difference between the viewpoints of these two groups of experts. 

Chain-transfer side reactions (see Scheme 1.1) can also cause substantial increases in D-values. Macrocychzation is particularly poor in this respect, leading to a complete equihbrium and an especially broad molar mass distribution (see e.g. Figure 1-4). On the other hand, a reversible polymerization devoid of macro-cyclization, but accompanied by the segmental exchange reactions, can fulfill the criteria of the living process [95-99]. However, in this case the D-value also increases with conversion, reaching at equilibrium a value which is predicted by Equation 1.24 and characteristic of the most probable molar mass distribution. Figure 1.7 illustrates the dependence of M /M determined for LA polymerization initiated with Sn(ll) alkoxide [98]. 

Quirk and Lee concluded there is no single criterion which is satisfactory for determination of whether a given polymerization is living or not. " Most of the radical polymerizations discussed in this chapter meet one or more of these criteria. 

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