Polymerization, degree living,

Thus, many effective methods for PolyP determination and characterization have now been developed. These allow PolyP assays not only in extracts, but also in cell homogenates or even in living cells. The most appropriate information about the PolyP content, polymerization degree and metabolism may be obtained by combining different methods, including the extraction of PolyPs from cells. 

The molecular mass of the generated macromolecules is most easily predicted for living polymerizations with rapid initiation (see Chap. 5, Sect. 8.1). Somewhat more complicated is the case of systems that, although are not living, at least polymerize without transfer. The number average polymerization degree, P, of the product is a simple function of the mean kinetic chain length, v... 

All of the observations above indicate the presence of living systems however, attempts to extend these well-defined systems above a limit of M = 100,000 have been unsuccessful, except at very low temperatures (< -70° C) [270]. Thus, polymers with predetermined polymerization degrees, low polydispersities, and with desired end groups can be obtained only for a sufficiently low molecular weight range. This indicates that contribution of transfer increases with temperature and with chain length [cf. Eq. (2) in Section II.C]. In the presence of transfer and termination polydispersities increase with the chain length and with conversion. 

Cationic polymerization of THF fulfills all the requirements of living polymerization. With several initiators, the initiation is relatively fast and quantitative, and propagation proceeds without transfer or termination. The dependence DP = ([M]0 - [M)e)/[I)0 holds up to high polymerization degrees. The only limitation is that, due to the reversibility of propagation, the molecular weight distribution is broadened and reaches the value of MjMn = 2 in equilibrium. Polymers with narrower MWD were obtained by terminating the polymerization at lower conversion [56]. 

In contrast, transfer reactions are not detected by following the monomer conversion if the rate of reinitiation is comparable to that of propagation. In this case, transfer is detected by a nonlinear dependence of the polymer molecular weight or degree of polymerization as a function of monomer conversion, polymer yield or the ratio of monomer to initiator concentrations, [M]o/[IJo (Fig- 2). However, a linear dependence of molecular weight on conversion is often used erroneously to demonstrate that a polymerization is living [9-11]. That is, termination does not affect the number of chains in the system, and therefore does not affect Afn or )/ if initiation is fast and monomer conversion goes to completion (i. e. if all of the chains are not terminated). 

The molecular weight distribution of living chains is exponential p(N) = conste (Floty distribution, see eqn [42]). The number-average and the weight-average polymerization degrees are... 

Metallocene catalysts usually require a large excess of MAO or MMAO to achieve high polymerization activity. The effect of the amount of dMMAO was investigated by the living polymerization of propene with 4-dMMAO, because the turnover-frequency (TOP) can be precisely evaluated from the number-average polymerization degree (Pn) of produced polymer and fp as PJt. The relation between Pn/fp and Al/Ti ratio is displayed in Fig. 7. The PJt value increased exponentially... 

The formation of polymer can be considered as a quasi-living polymerization. After the polymerization is complete, it can be reinitiated with the addition of more monomer to the unquenched polymer. However, the degree of polymerization cannot be predicted by the monomer/initiator molar ratio, the polydispersity is 1.5-2.0, and water, or even carboxylic acids, act as inhibitors and do not terminate the polymerization [10]. 

This relation was verified experimentally7 49 and it was shown that the degree of polymerization in a system containing "living polymers is independent of concentrations of initiator or monomer and of temperature. Furthermore, if all the growing centers were formed in a time much shorter than the time of polymerization, a Poisson molecular weight distribution would be obtained. Indeed, by using this technique samples of polystyrene were obtained for which MjMn = 1.04. 

It is not necessary that living radical polymerizations be slow. However, it follows from the above discussion that, for a high fraction ofliving chains, either the final degree of polymerization must be significantly lower than that in an otherwise similar conventional process or that conditions must be chosen such that the rate of polymerization is substantially lower. 

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