Depolymerized high-molecular-weight

It is however much more difficult to prepare the cyclic monomer. The general procedures for preparing cyclic disulfides involves depolymerizing high molecular weight polymers [115-118], treatment of organic thiosulfate with cupric salts followed by steam distillation [119], reaction of the dibromide with an alcoholic solution of sodium disulfide [120] and oxidation of dithiols [121,122]. Polymeric species are also formed in the oxidative reaction [123]. Hence precautions must be taken to prevent polymerization during the synthesis. 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

Scheme 3.1 Preparation of cyclic terephthalate esters via acid chlorides or depolymerization, and polymerization to high-molecular-weight polymer...

Many, if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Exercise 29-2 Suppose a bottle of 1,3-cyclopentadiene were held at a temperature at which polymerization is rapid, but depolymerization is insignificant. Would the polymerization result in conversion of all of the 1,3-cyclopentadiene into essentially one gigantic molecule Why or why not How would you carry on the polymerization so as to favor formation of polymer molecules with high molecular weights ... 

Table V shows yields and structural parameters of SP-300 fractions. These fractions were black solid materials like the feed coal, as compared to dark-brown viscous liquids of the HVL-P fractions. The overall average molecular weight of SP-300 was more than three times that of HVL-P. SP-300 fractions have much larger R, RN and //cl than HVL-P fractions. The //cl values disclosed that there were, on the average, 2.8 aromatic clusters per molecule in SP-300, as compared to 1.1 aromatic clusters per molecule in HVL-P. Thus SP-300 fractions are high molecular weight materials,in which the degree of depolymerization is relatively low. This in turn suggests that SP-300 has retained more of the original structures of the parent coal than has HVL-P.

The polymerization of ketones was studied for theoretical reasons. Acetone has an unusually low ceiling temperature and yileds high-molecular-weight polymers only at low temperatures (ca. 70 K) its polymer depolymerizes at temperatures of about 100 K. 

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