Depolymerization, copolymerization

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. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Several important assumptions are involved in the derivation of the Mayo-Lewis equation and care must be taken when it is applied to ionic copolymerization systems. In ring-opening polymerizations, depolymerization and equilibration of the heterochain copolymers may become important in some cases. In such cases, the copolymer composition is no longer determined by die four propagation reactions. 

One critical issue is the evaluation alternative. In the case of methanolysis, the alternatives are to make DMT and EG by depolymerization or secure materials from traditional petrochemical sources. For hydrolysis, the alternatives are TPA and EG by depolymerization or from traditional sources. For both technologies, the amount of copolymerizing isophthalate and/or 1,4-cyclohexane dimethanol is likely to be too little to justify the cost of recovery. For the various forms of glycolysis and the methanolysis/BHET hybrid, the alternative is the BHET and BHET-like materials made by the combination of a terephthalate and isophthalate plus EG and various glycols. Market prices exist for TPA and EG. BHET is not an item of commerce, and so the value must be imputed from the market price for TPA (the modern terephthaloyl) and EG, plus a conversion cost. 

The above theory can be extended to deal with other more complex cases. For example, the two ends of a biopolymer need not behave identically, and, as noted earher, MTs are helical polymers of asymmetric protomer units. Thus, two sets of on- and off-constants might be necessary. In other cases, such as in the polymerization of tubulin in the presence of tubulin-colchicine complex (Sternlicht et one may need to consider copolymerization. The kinetics of microtubule depolymerization are the reverse of elongation, and are gener-... 

Aldehyde Copolymer Self Developing Electron-beam Resists. The ceiling temperature for the copolymerization of aliphatic aldehydes is usually below 0°C and the copolymers are easily depolymerized into monomeric aldehydes above 150°C under vacuum. This depolymerization into monomers also occurs on electron-beam or X-ray exposure as evidenced by combined gas-liquid partition chromatography-mass spectrometry. As a result, the copolymers of aldehydes behaved as self-developing positive resists and almost complete development was accomplished without any solvent treatment. Electron-beam exposure characteristics of the aliphatic aldehyde copolymers studied here are... 

Equation 5 is valid only for irreversible reactions. For a copolymerization reaction that contains a polymerization-depolymerization equilibrium, Equation 5 is oversimplified. Depolymerization (Equation 2) is not considered in Equation 5. The following derivation of a generally valid equation for a copolymerization will include this retroreaction. 

In the following the reversibility of the polymerization of a-methyl-styrene has been taken into account. The copolymerization curves are calculated via Equation 33 together with Equation 34. The equilibrium constants necessary for the calculation are taken from Table II. The depolymerization of methyl methacrylate (I) can be neglected in the temperature range investigated since the equilibrium constants for this monomer (fC2) are extremely small compared with the value for -methyl-styrene—e.g., (21) at 100°C, Ki = 22.9 mole/liter, K2 — 0.12 mole/liter at 80 °C, Ki = 12.9 mole/liter, fC2 = = 0 049 mole/liter. 

One can describe the copolymerization of a-methylstyrene and methyl methacrylate with Equations 5 and 33. Equation 5 reflects only a mathematical approach, whereas Equation 33 takes into account the polymerization-depolymerization equilibrium investigated in the homopolymerization of a-methylstyrene. 

In Figure 6 Equation 37 combined with Equation 38 was used to evaluate the copolymerization behavior. It is now assumed that the sequence with two monomer units Mi cannot depolymerize, but that longer sequences are subject to the polymerization-depolymerization equilibrium. 

It is not possible to predict which mechanism is involved in a certain copolymerization. In the system a-methylstyrene-methyl methacrylate depolymerization of sequences of two monomer units seemed to occur as well as depolymerization of a-methylstyrene from longer sequences. In the system a-methylstyrene -acrylonitrile the sequence of two monomer units of a-methylstyrene is stable and does not depolymerize. The reversibility of the polymerizations of a-methylstyrene and methyl methacrylate can be explained by sterically induced strain in the polymer chain (13). In the copolymer a-methylstyrene-methyl methacrylate this strain involves the whole polymer chain whereas in the a-methylstyrene— acrylonitrile system the strain is broken by the acrylonitrile sequences and is built up again in the a-methylstyrene. This explains the difference in the depolymerization tendencies of sequences of two units of a-methylstyrene and longer sequences in this system. 

Tn the cationic polymerization and copolymerization of trioxane in the - melt or in solution, an induction period usually exists, during which no solid polymer is formed and the reaction medium remains clear. Nevertheless, reactions are known to occur during this period. By using BF3 or an ether ate as catalyst, in homopolymerization, Kern and Jaacks (I) reported the formation of formaldehyde via depolymerization of polyoxymethylene cations. 

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