Polymerization average chain lengths

The changes in the average chain length of a solution of semi-flexible selfassembling chains confined between two hard repulsive walls as the width of the sht T> is varied, have been studied [61] using two different Monte Carlo models for fast equihbration of the system, that of a shthering snake and of the independent monomer states. A polydisperse system of chain molecules in conditions of equilibrium polymerization, confined in a gap which is either closed (with fixed total density) or open and in contact with an external reservoir, has been considered. 

Solution The number and weight average chain lengths for a self-condensing polymerization are obtained from Equation (13.18) and (13.19) by setting Sab= E ... 

FIGURE 13.5 Representative profiles of viscosity versus conversion for free-radical and condensation polymerizations. The endpoint is 99.9% conversion with a number average chain length of 1000. 

This enzyme [EC 2.4.2.30] (also referred to as NAD+ ADP-ribosyltransferase, poly(ADP) polymerase, poly-(adenosine diphosphate ribose) polymerase, and ADP-ribosyltransferase (polymerizing)) catalyzes the reaction of NAD+ with [ADP-D-ribosyl] to produce nicotinamide and [ADP-D-ribosyl]( + i). The ADP-d-ribosyl group of NAD+ is transferred to an acceptor carboxyl group on a histone or on the enzyme itself, and further ADP-ribosyl groups are transferred to the 2 -position of the terminal adenosine moiety, building up a polymer with an average chain length of twenty to thirty units. 

Average chain length for reacted species remains approximately constant throughout the polymerization... 

Chain transfer can also occur with initiator, impurity, solvent, or other additive present in the polymerization system. While the average chain length is equal to Rp divided by the sum of all termination reactions, it is customary to control all termination steps except the one that is being studied. Chain transfer to all other molecules, except solvent or some special additive, is typically negligible. 

The broad single peak distribution found with the mutual sample is not unexpected. It is known that with the film thickness and monomer-solvent mixture used the graft polymerization is diffusion controlled. The average chain length reached will be dependent on the distance of the growth site from the film surfaces resulting in the broad spectrum of chain lengths actually found. 

Twice-recrystallized Pro-NCA was heated to 130-135 °C in vacuo for 2h. The transparent solid product was dissolved in the minimal amount of glacial AcOH and precipitated by the addition of five volumes of E O with quantitative yield. The average chain length was n= 67-133. A polyPro preparation with an average degree of polymerization of n=67 gave [a]D20 —372 (c 2.0, glacial AcOH), [a]D20 —483 (c 1.5, formic acid), and [a]D20 -353 (c 2.0, H20). 

So how is this polymerization initiated A small amount of water is added to hydrolyse some of the caprolactam to 6-aminohexanoic acid. The amino group can then attack another molecule of caprolactam and so on. The amount of water added influences the average chain length of the polymer. 

The traditional method of determining the monomer transfer constant is the Mayo method [294,295], where the inverse of the number average chain length Pn is extrapolated to zero polymerization rate. To obtain reliable values, one needs to measure rather large P values to high precision that can then be extrapolated to zero polymerization rate. In addition, linear extrapolation is not guaranteed if bimolecular termination reactions are chain-length-dependent [296]. 

Fig. 21 Calculated development of the weight-average chain length during the model emulsion polymerization...

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