Molecular weight distribution depolymerization

The primary polymerization product ia these processes has a relatively wide molecular weight distribution, and a separate step is often used to narrow the polydispersity. Such a narrowkig step may consist of high vacuum stripping to remove volatile polymer chains, often followed by a solvent fractionation step (35,36), sometimes a solvent fractionation step alone (37,38), or a fractional precipitation from organic solvent (32). The molecular weight distribution can also be narrowed by depolymerization at elevated temperatures ia the presence of a depolymerization catalyst (217—220). 

In intestinal contents and faeces from germfree rats quite high amounts of galacturonan were found, especially in the case of the pectin with the highest DE (Table 3). The isolated pectins were depolymerized to a small extent. The molecular weight distribution of pectins from intestinal contents and faeces remained relatively unchanged (Figures 1 and 2). 

Polymerization occurs very quickly and the process is controlled via kinetic effects rather than thermodynamic ones. The net result is that the molecular weight distribution of the product does not match the thermodynamically stable one. If the chains were not capped with monofunctional phenols, the polymer chains would depolymerize, allowing the monomers to rearrange themselves at elevated temperature to approach the thermodynamically stable... 

Depolymerization is in functional correlation with the molecular weight distribution and with the type of terminal groups which are formed in chain initiation and chain termination. 

Boyd, R. H. Theoretical depolymerization kinetics in polymers having an initial most probable molecular weight distribution. Journal of Chemical Physics 1959 31 321. 

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. 

The Flory distribution is a random distribution useful in several modes of polymerization. This distribution results from addition polymerization reactions when the only significant processes that interrupt macromolecular growth are either or both of chain transfer (to any species but the polymer) or termination by disproportionation. Likewise, this molecular weight distribution describes linear condensation polymerization when equal reactivity is assumed for all ends only when the reaction involves an equilibrium between polymerization and depolymerization. The model describes the distribution with one parameter which is the number average molecular weight. The distribution equation is ... 

A method for the gel permeation chromatographic analysis of the molecular weight distribution of wood pulp holocellulose as the carbanilate derivative has been applied to red maple Acer rubrum) and loblolly pine Pinus taeda). Either the chlorine-ethanolamine or acid-chlorite method could be used to prepare the holocellulose and the derivative was obtained by heating at 80 with phenylisocyanate in pyridine. Higher temperatures caused depolymerization. 

This technique has been used to estimate the molecular weight distribution in coal but the (number average) molecular weights (Mn) of the material soluble in pyridine after depolymerization can be affected by the presence of colloidal material. Whilst the reported value may be in the region of 400, removal of the colloidal material (by, say, centrifugation) may increase this value to ca. 1000. 

In contrast to chain scission, the kinetics of depolymerization is very specifically dependent on the kind of initiation reaction used to produce the polymer, the molecular-weight distribution, etc. The kinetics thus has to be calculated for each specific case. Kinetics can depend, for example, very much on the type of end groups. To obtain a 50% degradation to the monomer within 45 min, a temperature of 283°C is needed in the case of poly(methyl methacrylate) prepared using benzoyl peroxide. With a thermally polymerized product, on the other hand, the temperature must be raised to 325°C. 

This trend can be developed further by chemical modification. Fig. 4 shows the changes in molecular weight distribution which can be obtained by controlled oxidative depolymerization of the latex rubber. These materials are particularly interesting as they have intrinsic... 

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