Molar mass distribution polycondensation

Due to the fact that different end groups can be formed during the polycondensation, the reaction products may exhibit a functionality-type distribution in addition to the molar mass distribution. Although SEC is suitable to analyze the molar mass distribution, it does not yield information on different end groups. For the determination of the functionality-type distribution, other types of liquid chromatography must be used. 

The chemical constraint reduces the number of possible reactions considerably, and consequently it leads to a much narrower molar mass distribution. Furthermore, the extent of reaction a of the A-group can cover all values from zero to unity, but the extent of reaction P of the equally reactive 5-groups cannot become larger than P=a/(f-l). One important consequence of this strict constraint is that gelation can never occur [1,13]. A much higher branching density than by random polycondensation can be achieved. For this reason one nowadays speaks of hyperbranching. 

In a polymerization process the chain length distribution or molar mass distribution (MMD) is influenced by a large number of factors and conditions the kinetics of the reaction plays a very important role. The calculation of the resulting MMD is thus very complicated. For one of the simplest cases, a step reaction with polycondensation, a first-order approach is given here. As an example we take a hydroxy acid HO-R-COOH, which, upon condensation, forms the chain -[-O-R-CO-]n. 

Very early reports on these systems described them as polycondensates, consisting of broad molar-mass distributions with randomly branched topologies. The methods of synthesis included Friedel-Crafts coupling of benzyl alcohols [108] and the polymerization of 2,5,6-tribromophenol involving aryl ether formation [109], In addition, hyperbranched natural carbohydrate polymers, such as amylopectin, dextrin, and glycogen have been extensively studied [73-75]. 

Due to their statistical build-up, hb polymers exhibit very broad molar mass distributions, which become even broader with the DP. This effect can be ascribed to the greater probability of larger molecules reacting with the monomer due to the higher number of reactive groups. Compared to linear polycondensates, which in theory approach a polydispersity (M, /M ) of 2, the molar mass distribution of statistically hb samples will depend directly on the DP, as M /Mn DP/2 [125]. 

In thecfe novo synthesis the nucleoside triphosphates are polycondensed to polynucleotides under the influence of certain enzymes and in the absence of templates, primers and Mg " ions. One-strand homopolymers are produced by this method, for example, poly(dA) or poly(dG). The molar mass distribution corresponds to that of the most probable Schulz-Flory distribution. 

Recently, typical step-reaction polymerizations, as in polyesters, polyethers, and polyamides, have been forced into chain-reaction mechanisms by designing complex chain ends that react fast with the monomer only. Under the proper conditions, the step reaction can be suppressed almost completely. Such chain-growth polycondensation may even yield living polymers with narrow molar-mass distribution. A link to the initial literature is given in the General References for this section. 

Gupta et al. simulated polymerization of caprolactam including all important reactions reversible ROP, polycondensation, polyaddition, and cyclization reactions as well as the reaction with monoftinctional acids and found that reactions constituting the ring-chain equilibria influence the molar mass distribution of product. 

Dendrimers (Newkome et al., 1996) and hyperbranched polymers, HBP, look like functional microgels in their compactness but they differ in two aspects they do not contain cyclic structures and, more importantly, they are much smaller, in the range of a few nanometers in size. They are prepared stepwise in successive generations (dendrimers) or they are obtained by the polyaddition/polycondensation of ABf monomers, where only the A + B reaction is possible (HBP Voit, 2000). Both molecules have tree-like structures, but a large distribution of molar masses exists in the case of HBP. 

A series of close-to-spherical styrene/DVB resins of varying particle size and pore diameter were employed as supports for non-covalent adsorptive attachment of CALB by hydrophobic interaction. The effect of matrix particle and pore size on CALB i) adsorption isotherms, ii) fraction of active sites, iii) distribution within supports, and iv) catalytic activity for s-CL ring-opening polymerizations and adipic acid/l,8-octanediol polycondensations is reported. Important differences in the above for CALB immobilized on methyl methacrylate and styrene/DVB resins were found. The lessons learned herein provide a basis to others that seek to design optimal immobilized enzyme catalysts for low molar mass and polymerization reactions. 

Statistical DB values were found for aU samples, with a marginal increase in the DB in line with the molar mass. A larger increase in the DB at higher DP-values was identified after the fractionation of an aliphatic-aromatic polyester [155] this finding was in agreement with previous kinetics studies on the one-pot melt polycondensation of this type of ahphatic-aromatic polyester performed by Schmaljohann et al. [30] Consequently, in these samples the coexistence of two different distributions was proven, namely molar mass and DB. It should be taken into account that, in contrast to the one-pot synthesis of the hb poly(ether amide)... 

The distribution of molar masses of the polycondensates formed throughout polymerization can be calculated using statistical methods. Since all unreacted functional groups are assumed to be equally reactive and behave independently of one another, the probability that a reaction occurs to link any -X to any -Y is equal to the extent of reaction (p)... 

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