Initial stages, living polymerization

The homopolymerization of MMA with the soluble catalyst was found to exhibit the characteristic of living polymerization at the initial stage of polymerization ( 5 h) giving poly(MMA) with a narrow molecular weight distribution (Mw/IVln = 1.2, Mn = 2400), at 25 °C. To elucidate the mechanism of the MMA polymerization, the copolymerization of MMA with styrene was carried out. The observed reactivity ratios (rs = 0.5, rMMA = 0.4) indicated that the living polymerization of MMA occurred via a radical intermediate. 

When the polymerization of St was carried out with 51 under conditions identical to those in Fig. 3, i.e., [7]/4=[8]/2=51=2X 10-3 mol/1, the formation of benzene-insoluble polymers was observed from the initial stage of the polymerization. Although 7 and 8 induced living radical mono and diradical polymerization similar to that previously mentioned, benzene-insoluble polymers were formed in the polymerization with 51, and the molecular weight of the soluble polymers separated decreased with the reaction time. This suggests that a part of the propagating polymer radicals underwent ordinary bimolecular termination by recombination, leading to the formation of the cross-linked polymer, which was prevented by the addition of 13. 

The strategy involved initiation of St polymerization by the living PIB arms of octa-arm stars. Scheme 7 illustrates the key steps and structures. The overall methodology of living IB polymerization by the two-stage procedure has been followed (see Sect. 4.1). Briefly, stars with predetermined arm molecular weights were prepared by the use of 1, in conjunction with BClj-TiC coinitiators used in two stages, then St was added to obtain the star blocks. 

The ESR method combined with a flow system should be very powerful for studying short lived transient radicals during vinyl polymerization in solutions. As will be made clear later, however, the conditions for the reaction occurring in the flow cell are quite different from the conventional solution polymerization studied during a steady-state process. Nevertheless, the hyperfine structure of the ESR spectra observed by the flow technique, can provide straightforward information on the structure, concentration, reactivity, and even the steric conformation of the transient radicals involved, particularly at the initial stage of the polymerization. 

The polymerization of 3,3-bis(chloromethyl)oxetane (BCMO) initiated with living polytetrahydrofuran (polyTHF) was studied by Saegusa 117). In this system, due to the high equilibrium concentration of THF, some monomer remains in equilibrium with the polymer after the first stage is completed (i.e. at the polyTHF THF equilibrium). After addition of the second monomer, the remaining THF may participate in copolymerization with added BCMO (cf. Sect. 15.2.2.1., copolymerization above T0), leading to random copolymer. Only after the complete consumption of THF the second monomer may form the required homoblock. Thus, the two homoblocks are separated by a third random BCMO/THF copolymer block ... 

The anionic polymerization of 1,1-dimethyl and 1,1-diethylsilacyclobutane with butyl lithium in THF/hexane (1 1) at -48 °C proceeds with a living nature. Mn increases in proportion as the yield of polymer increases. Addition of new monomer to the reaction restarts the polymerization at the same rate as in the initial stage. 

There are indications of a living chain-ffowth mechanism in boron trifluoride-diethyl ether initiated polymerizations of propylene sulfide at conversions of 5-20%. In these early stages of polymerization the molecular weight corresponds to that calculated for typical living polymers. This is believed to take place through formations of stable sulfonium ions ... 

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