Step polyaddition

Structure and Elasticity of Loose Step Polyaddition Networks... 

Step reactions (polycondensation, step polyadditions, etc) are relatively slow, so that their rates are controlled by the equilibrium given by ki/k i. This ratio is not much affected by viscosity since the concentration of the activated complex is given by the residence time of the two groups in the "reaction volume". 

The rate of polymeric chain generation used to help in the classification of polymerization processes, and to this day chain polymerizations and step polyadditions are distinguished. This classification was introduced long ago and is mainly based on the observation that the molar mass of the product is sometines roughly independent of time (conversion) whereas in other cases it increases considerably. Strictly speaking, every polymerization is a chain polymerization and, at the same time, each is a polyaddition. 

The borderline between chain and step polyadditions has never been strictly defined. In fact it is mobile, and if the molar mass change with time is accepted as a criterion, this borderline is expected to change with the development of experimental methods. Polymerizations proceeding on the multiple bonds of monomers or by opening of their rings need not be classified in this way ... 

As a process characterizing name, the term step polyaddition is preserved for cases where a monomer with multiple bonds is copolymerizing with saturated monomers. The formation of polyurethanes is an example of such a process... 

Step polyaddition (urethane formation) Polyurethanes with large M can be prepared using a method based on the Wnrtz alcohol test. In the presence of a basic catalyst such as a diamine, ionic addition takes place, for example, between 1,4-butanediol and 1,6-hexanediisocyanate ... 

In the first stage of our study, three aromatic diamines were used as chain extenders a non-symmetrical chain extender, 2,4-toluene diamine (TDA) two symmetric aromatic diamines 4,4 -methylene dianiline (MDA) and 4,4 -diamino-dibenzyl (DAB). TDA was used in order to improve the mechanical properties and thermal stability. The materials were synthesized by a two steps polyaddition reaction as... 

Segmented polyurethane (PU) elastomers can be prepared either by the one-shot or by the two-step (prepolymer) method, where it has been shown that the two-step polyaddition theoretically results in a narrower distribution of hard blocks than the single-step process of the same stoichiome-... 

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