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Two types of network formation affected by physical factors are discussed: (1) the effect of strong intermolecular interactions in the formation of polyurethane networks, and (2) the effect of strong intermolecular repulsion operative during anionic polymerization of oligoester acrylates. The intermolecular interactions influence also the formation of supermolecular structures and the reaction kinetics. During recent years much attention has been given to the possibility of the occurrence of some kind of local ordering in polymerizing systems. This ordering is considered to be a result of intermolecular interaction1 6, Many experimental structural and kinetic data can be explained if we take into account the existence of such regions7. These data allow us to think that for the most part the reactions of polymer molecule formation, especially in melt, are not those of isolated molecules. Many reactions, even in solution. are reactions between molecules which are found at an interface with some kind of supermolecular structures. composed perhaps of molecular aggregates8. According to theory92 and some experimental data, the reactions of three-dimensional polymerization or polycondensation are microheterogeneous' 315 It seemed16 that at a definite stage of reaction in the reaction system there arise some particles of the colloid size (microgel). The chemical interaction between these three-dimensional particles leads to gel formation. Experimental data are in accordance with this mechanism1 . There are several possible reasons for the arising of microheterogeneity in the oligomer and polymer systems. First, the strong intermolecular interaction which can lead to the formation of molecular aggregates even in the initial oligomer system before the beginning of the reaction. A typical example of such a system is that in which the reaction of polyurethane formation proceeds. Secondly, an incompatibility of components in the course of the reaction. This case is also typical of polyurethane systems. Thirdly, the difference in the reaction ability of functional groups which take part in three-dimensional polymerization and polycondensation1 8,

Structural interpretation of the mechanism of polymer network formation