Cross-propagation, living polymerization

In a living system, if Mi is much more reactive than M2 and polymerization is allowed to proceed to completion, the end-product is a tapered block copolymer, in which only the middle section contains units of both monomers, e.g. with anti-1-methylnorbomene (Mi)/syw-7-methylnorbomene (M2), see Section VIII.C.4 also with norbornene (Mi)/cyclooctatetraene (M2), catalysed by 8W (R = Me)360. In the extreme case the cross-propagation reactions may be so slight that the product is indistinguishable from a perfect block copolymer, e.g. with bicyclo[3.2.0]hept-2-ene (Mi)/norbornene (M2) catalysed by 18109,597, or with awft -7-methylnorbornene (Mi) syn-1 -methylnorbomene (M2), catalysed by 7 (R = Me)128. The successive polymerization of the two monomers can be readily followed by NMR. 

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. 

From these experimental and modeling studies, the mechanism of the living free radical polymerizations initiated by a combination of TED and DMPA have been elucidated. The TED produces DTC radicals that preferentially cross-terminate with the propagating carbon radicals. By this cross-termination reaction, the carbon radical concentration is kept low (as was shown in figure 6) and the rate of polymerization is decreased, as is the autoacceleration effect. This suppression of the autoacceleration peak in HEM A polymerizations and, interestingly, in DEGDMA polymerization has been observed to increase as the TED concentrations are increased. This behavior has been predicted successfully by the model as well. 

The irreversible decay of the dormant alkoxamine chains stops the monomer conversion rather abruptly at the time t = H( nk. For methyl methacrylate polymerizations this stop has been observed, and it has been demonstrated that it is caused by a considerable fraction of cross-disproportionation between the nitroxide and the propagating radicals.51,97112 Unfortunately, the factors governing disproportion-ation-to-combination ratios in radical—radical reactions are not well understood up to now, but stereo-electronic effects are certainly very important.112 Hence, one cannot yet predict a nitroxide structure that will allow living methacrylate polymerizations up to large conversions. 

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