Addition polymerization average chain lengths

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. 

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). 

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]. 

The quantity Fi, the instantaneous copolymer composition, is analogous to x , the instantaneous number-average chain length in free-radical addition polymerization. Like x , it depends on the conditions in the reactor at a particular instant. It, too, is really an average, since not all the copolymer formed at a particular instant has exactly the same composition. However, the instantaneous distribution of compositions is normally much narrower than the instantaneous distribution of chain lengths, and because the fact that it is an average is not normally of great practical importance and cannot be controlled anyhow, the overbar is left off of Fj. 

Silicone oils. These are formed when the crude hydrolysis product mentioned above is made to undergo further polymerization using acid or alkali as catalyst. In this equilibration process the hydroxy-ended oligomers condense with elimination of water, and the cyclic oligomers open and also produce long chain polymers. The average chain length is determined by the addition of exactly controlled amounts of hexamethyl-disiloxane ... 

The elastically effective network chains should obey Gaussian statistics. They should therefore be long enough, and their average degree of polymerization should be known. In addition the distribution of chain-lengths is expected to be rather narrow. 

In addition to the conversion of the functional groups, the degree of polymerization, and the number average molecular weight we are interested in the distribution of chain lengths, n, (i.e. molecular weights M ),... 

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