Formation of high-molecular-weight product

These reactions often lead to cyclic acetals that interfere with the formation of high molecular weight products. Useful polyacetals, however, can be formed fix)m pentaeiythritol and acetals of dialdehydes ... 

Silacarbocycles with sp -hybridized endocyclic carbon atoms showed much higher reactivity, including ROP, than silacycloalkanes. This is associated not only with the participation of multiple bonds in some reactions (as in the case of silacycloalkenes) but also with increased strain and manifestation of properties typical for alkenylsi-lanes. For example, at the allyl location of the sp - hybridized system in a cycle (with respect to the Si-atom), the endocyclic bond Si-CAn (or Si-CBenzyi) becomes weaker. As a result, not only 4-membered but also 5-membered silacarbocycles can undergo ROP with formation of high-molecular-weight products. 

The addition polymerization of diisocyanates with macroglycols to produce urethane polymers was pioneered in 1937 (1). The rapid formation of high molecular weight urethane polymers from Hquid monomers, which occurs even at ambient temperature, is a unique feature of the polyaddition process, yielding products that range from cross-linked networks to linear fibers and elastomers. The enormous versatility of the polyaddition process allowed the manufacture of a myriad of products for a wide variety of appHcations. 

Redox polymerizations are usually carried out in aqueous solution, suspension, or emulsion rarely in organic solvents. Their special importance lies in the fact that they proceed at relatively low temperatures with high rates and with the formation of high molecular weight polymers. Furthermore, transfer and branching reactions are relatively unimportant. The first large-scale commercial application of redox polymerization was the production of synthetic rubber from butadiene and styrene (SBR1500) at temperatures below 5 °C (see Example 3-44). 

This experimental procedure must be followed carefully to avoid partial decomposition of ethyl a-(hydroxymethyl)acrylate. The reaction is stopped rapidly after the addition of the carbonate solution (5 min) to prevent formation of high molecular weight by-products which result from transesterification and Michael addition, both of which occur in the basic medium. However, about 25% of the product is lost. Addition of diethyl ether during cooling minimizes side reactions. 

Note Top reaction sequence represents hydrolysis of isocyanate functions to corresponding amines. The remainder of the reaction scheme represents some of the potential side reactions of hexamethylene diisocyanate and the hydrolysis products of hexamethylene diisocyanate. Extended repetition ofthese reactions results in the formation of high molecular weight polyurea compounds. 

As in purely thermal degradation, thermal oxidation of rubber is accompanied by formation of low-molecular-weight products in yields too high to be accounted for by random attack on the... 

In this section an attempt is made to construct a general scheme into which as many as possible of the conclusions reached about macrozwitterion polymerization can be fitted. It has already been pointed out that chemical reactions between electrophilic and nucleophilic molecules which yield charged products or intermediates have been studied by organic chemists for many years. For a polymerization to occur, a reaction pathway for consecutive addition of at least one molecule to the bipolar species must be available. Such reactions are properly the domain of the polymer chemist. Formation of high molecular weight polymer requires that charges be separated until there is no longer any inductive or electrostatic interaction between them. Various authors have realized that this cannot be accomplished with the enthalpy, released when monomer bonds are broken. 

Photooxidation of adrenochrome melanin under oxygen at high pressure led to its degradation and formation of low molecular weight products (316). 

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