Equilibrium conditions depolymerization

If polymerization is carried out under conditions of high nucleation, polymer chains are deposited rapidly and indiscriminately and the active sites necessary for growth are occluded. If growth is carried out near the equilibrium concentration, depolymerization becomes important and again the active sites become occluded. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

We can consider three situations (a) if [M]oo > [M] , there will be a net increase in polymerization (b) if [M] < [M]oo, there will be net depolymerization and (c) when [M] = [M] , the monomer and polymer are in equilibrium. The last case does not imply a static condition rather, monomer addition and loss can constantly occur, but the total polymer weight concentration will always remain unchanged. In head-to-tah polymerizations, hke those of actin and tubulin assembly in vitro, each of the two polymer ends can interact with monomer the critical concentration [M]o then equals (koff +... 

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. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

This reaction also plays a role in the degradation of polysulfides. A back-biting mechanism as shown in equation 6 results in formation of the cyclic disulfide (5). Steam distillation of polysulfides results in continuous gradual collection of (5). There is an equilibrium between the linear polysulfide polymer and the cyclic disulfide. Although the linear polymer is favored and only small amounts of the cyclic compound are normally present, conditions such as steam distillation, which remove (5), drive the equilibrium process toward depolymerization. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

Endo, Suzuki, Sanda, and Takata [57] showed (Fig. 14b) that dithiol-linked bifunctional spiro orthoesters 48 can generate cross-linked polymers 49 in the bulk under acidic conditions (2 mol% CF3COOH). The increase of the temperature shifts the equilibrium in favor of the monomer. Conversely, the cross-linked polymers can be depolymerized from CH2C12 suspensions to form the initial monomers, also under acidic conditions (5 mol% CF3COOH). The yield of these processes is not quantitative. 

High concentrations favor oligomerization, whereas at a lower concentration, an intramolecular reaction or depolymerization may compete with polymerization.4 In fact, many cases are known where a monomer and its oligomers exist in equilibrium under various conditions. 

It is not practical to conduct free-radical polymerizations under conditions where there is an equilibrium between polymerization and depolymerization processes. The polymer synthesis is effectively irreversible in normal radical polymerizations. The course of the reaction is then determined kinetically, and the molecular weight distribution cannot be predicted statistically as was done for equilibrium step-growth polymerizations described in Chapters. 

Thus the values shown in Table 1.8 are for standard conditions and represent just one of a series of ceiling temperatures for various monomer concentrations above which polymer formation is not favoured. Thus, in a bulk polymerization reaction the ceiling temperature may change with conversion in such a way that complete conversion is not achieved. For example, if methyl methacrylate is polymerized at 110°C the value of [M]c calculated from the above equation is 0.139M and this will be the monomer concentration in equilibrium with the polymer. The polymer, when removed from the monomer, will have the expected ceiling temperature as given in Table 1.8 and will depolymerize only if there is a source of free radicals to initiate the depolymerization (Section 1.4.1)... 

A silanol terminated PDMS heated under isothermal conditions at different temperatures first shows an increase in molecular weight (MW) as a function of time151 (Figure 9). This process is the condensation of silanol groups on the ends the PDMS chains which leads to an increase of MW. However, the increase in MW is limited probably due to either a lowering in concentration of silanol groups or to the equilibrium nature of the silanol condensation reaction. When the temperature is further increased, a second process is observed which is the depolymerization. A mixture of cyclic oligomers... 

Polymer yields are optimized by avoiding the use of diluents. If solution polymerization is desired, then in order to avoid the formation of increased amounts of equilibrium cyclics it is necessary to employ more reactive cyclic reactants and selective polymerization conditions, e.g., strained cyclics such as (Me2SiO)j or Me2SiCH2CH2SiMe20 can be selectively transformed to linear polymers using conditions that are so mild that little of the resulting polymers are depolymerized to other cyclics. ... 

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