Rates depolymerization reactions

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

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. 

P n is a reactive molecule of the polymer with n monomer units in the chain, and it is unimportant whether the polymerization mechanism is radical or ionic. The rate constant of the propagation step is kp. Under certain conditions, monomer units can be split off the reactive polymer molecule. Then it is necessary to consider also the depolymerization reaction (with the rate constant kd). 

Finally, the solvent isotope effect observed in the crystallization of zeolite A provides further insight into the mechanism. The deuteroxide anion (0D") is a stronger base than OH and should promote the depolymerization equilibria.(32) Also, D20 is more structured than H20 and should promote nucleation.(33) The isotope effect leading to a slower rate of reaction will occur in the condensation polymerization reaction leading to crystal growth, which involves elimination of both H20 and OH". (2) It is a combination of all these factors that lead to the observed nucleation rate. 

The behavior of the reaction rate as a function of temperature dispels any notion that the reaction is simple. Figure 3 shows that there is a maximum in the first-order rate constant-temperature curve at approximately 80 °C. At such a low temperature, the rate-temperature maximum cannot be explained by depolymerization, nor can it be explained by deactivation of the catalyst as a result of more rapid polymer accumulation on the catalyst at higher temperatures since the maximum is obtained for initial rates measured as a function of temperature. Theoretical considerations predict that a maximum in the rate-temperature curve may be expected from the Langmuir-Hinshelwood model for polymerization on solid surfaces but not from the Rideal model (5). The rate of reaction for the Langmuir-Hinshelwood model is given by ... 

Methyl methacrylate (MMA) has the formula CH2 = ClCHa) (COOCH3). The vapor pressure of MMA due to its vaporization is estimated by [3] In P(Atm) = 11.79 — 4410/T(K), assuming a constant heat of vaporization. For the depolymerization reaction, the temperature which gives 1 atm of MMA (the product) is 164 C [26]. This temperature establishes the pressure equilibrium constant and therefore the Gibbs fiiee energy for the depolymerization reaction. Answer The depolymerization of PMMA rate constant given by [26] rate constant = 3.87 x 10 exp(—175 kJ/mole/RgT) gm/cm sec. 

The etch resistance of poly (butene-1 sulfone) in fluorocarbon-based plasmas can be enhanced by prior treatment of the surface in an oxygen plasma. This pretreatment inhibits or retards the depolymerization reaction that characterizes normal etching in fluorocarbon plasmas, thereby permitting formation of a surface-modified layer which exhibits a substantially reduced etch rate. Pretreating PBS in an oxygen plasma enables it to be used subsequently in selective reactive-ion etch processes involving fluorocarbon plasmas to delineate submicron, anisotropically etched patterns. 

The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects the change of the pH and of the relative amount of TPAOH to silico-aluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of OH" availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. 

As the concentration of polymer amide groups increases during the polymerization, the equilibrium (25) decreases the concentration of lactam anions and increases the concentration of polymer amide anions (P ). Hence, an increasir fraction of polymer is formed in the sequence of reactions (30) and (31) whereas the contribution of reaction (24) to the chain growth decreases. Similarly, the contribution of the bimolecular depolymerization reaction (29) will be increasing as compared with the monomolecular depolymerization in reaction (24). The rate of the monomolecular depolymerization is proportional to the concentration of amide anions in the vicinity of the acyllactam, viz. 

With increasing ring strain and degree of polymerization (n), the bimolecular depolymerization will prevail, while the monomolecular reaction is important only at the very beginning of polymerization. For cis-lactams reaction (31) can be assumed to proceed faster than reaction (30), so that the rate determining reactions can be summarized in the equation... 

The main depolymerization reaction consists of a carboxyl group catalysed elimination of lactam from the amine end group, and the rate of lactam formation follows the equation [217]... 

However, catalysts such as the potassium slloxanolate catalyst are not transient. Non-transient catalysts must be neutralized or removed by some other method In order to give a thermally stable polymer. If the catalyst Is not removed. It will cause depolymerization at high temperatures. For example, a silicone gum prepared by reacting D4 with O.OlZ KOH has been reported to lose over 99% of Its weight at 2S0 C In 24 hours (11). Non-transient catalysts can often be used at much higher temperatures than the transient catalysts, leading, of course, to faster rates of reaction. 

At pH values of 5-6 pectin solutions are stable only at room temperature. As the temperature 1s raised, pectin chains cleave by a beta-elimination reaction (25-39) (Figure 1), a reaction which is stimulated by organic anions (40). Deesterification of pectin proceeds simultaneously with the beta-elimination depolymerization reaction, which occurs only at monosaccharide units that are esterlfied. At pH values above 6, deesterlfIcatlon eind depolymerization are rapid reactions even at room temperature, the rate of each reaction increasing with Increasing pH. 

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