Rate of living polymerization

RATE OF LIVING POLYMERIZATIONS AND MOLECULAR MASS DISTRIBUTION CURVES OF PRODUCTS... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

The rate of living cationic polymerization is expressed as the rate of propagation of ion pairs... 

We have followed the propagation rate of the polymerization of isoprene initiated by cyclohexane solutions of the complexes oli-goisoprenyllithium/TMEDA and oligoisoprenyllithium/PMDT in two very different ranges of living ends concentration. In each case we have determined the order of the reaction with respect to the living ends concentration. 

Preirradiation of the solution of gold salt prior to the addition of monomer enhances the rate of dark polymerization, although the rate is much slower in comparison to polymerization under continuous irradiation. Formation of some long-living active species by preirradiation is obvious. It is not possible, however, to say whether the photoirradiation is effective in producing active species alone or whether it sensitizes further reactions of active species. Certainly, further investigation is required to derive conclusions on mechanism. 

The alkylaluminum component combined with V(acac)3 has an influence on the kinetic behavior of the propylene polymerization at —78 °C47 82 83). In the polymerization with V(acac)3 and dialkylaluminum monohalide like Al(i-C4H9)2C1, Al(n-C3H7)2C1, A1(C2HS)2C1 or Al(C2H5)2Br, M of polypropylene increased proportionally to the polymerization time, and the polydispersity (M Mj was as narrow as 1.15 0.05 (see Fig. 9). This is the case of living polymerization. As can be seen from Fig. 9, the rate of increase of Mn, i.e. the rate of propagation of living chains as expressed by lvln/(42 t), is influenced by the kind of aluminum component and decreases in the series... 

Most epoxide polymerizations have the characteristics of living polymerizations, that is, the ability to polymerize successive monomer charges forming block copolymers. The expressions for the rate and degree of polymerizations are essentially those used in living chain polymerizations (see Chapter 8). The polymerization rate is given by... 

Under certain conditions, polymerizations of cationic cyclic ethers show the characteristics of living polymerizations in that the propagating species are long-lived and narrow MWDs are obtained. The rate and degree of polymerizations are then given by expressions previously described [Eqs. (10.15) and (10.16)]. Living polymerizations occur when initiation is fast relative to propagation and there is an absence of termination processes. Such conditions are found for polymerizations initiated with acylium (I) and... 

The rate of assisted lactam polymerization is dependent on the concentration of base and JV-acyllactam, which determine the concentrations of activated monomer and propagating chains, respectively. The degree of polymerization increases with conversion and with increasing concentration of monomer or decreasing A -acyllactam concentration. These characteristics are qualitatively similar to those of living polymerizations, but lactam polymerizations seldom are living [15,16]. 

Many authors have dealt with such problems and have tried to develop analytical methods for the diagnosis of living polymerizations [1,19,20,21,24, 29,30,45,51,55-57]. For example, the most recent effort is due to Penczek et al. [21] who combined the M vs Wp (molecular wieights vs weight of polymer produced) and — ln(l — C) vs t (rate of polymerization vs time) plots and expressed conversion (C) as a function of the degree of polymerization (DPn) ... 

The kinetics of these polymerizations is complex. Both complexed ion pairs and free ions are involved in the propagation reactions and the free ion rate constants depend on monomer concentration. The relative reactivity of complexed ion pairs and free ions is temperature dependent. Above the inversion temperature of —35°C, free ions are more reactive than ion pairs, but below this temperature the ion pairs are more reactive. At 30 °C in DMF, the observed (average) propagation rate constant is 0.13 l/(mol s) [146], The anionic polymerization of a,a-dialkyl-P-propiolactones such as pivalolactone (a,a,-dimethyl-P-propiolactone) initiated with carboxylate anions exhibits the main characteristics of living polymerizations. 

---