In the preparation of novolac resins, three possible dimers with slightly different molecular dimensions may be produced in the polymerisation reaction. Gel permeation studies were carried out on four novolac resins prepared using different acid catalysts, and the results indicated that the nature of the catalyst influences the type and quantity of dimer produced. The weight and number average molecular weights were also calculated from the chromatograms obtained.
A new light‐scattering instrument is described, in which the incident light beam is rotated by a mirror system, and the photomultiplier cell (E.M.I. type 5311) which measures the scattered light remains stationary. A study of the osmotic and light‐scattering molecular weights (Mw) of a series of polyvinyl acetate fractions shows that the process of fractionation separates the several molecular species present according to both size and shape. The average dimensions of the polymer chain (D) can be deduced from the dissymmetry measurements, assuming the random chain model. These results show that the most insoluble fraction has the highest molecular weight, but is also the least extended in solution. The lower‐molecular‐weight samples are relatively longer. The ratio Mw/D2 is a measure of the degree of branching of the polymer chain, and the minimum value measured for ten samples is 0.5. This is believed to be the value characteristic of the linear molecule. A photopolymer prepared at −25°C. and low conversion, that is, under conditions favoring the formation of linear polyvinyl acetate molecules, also had this same minimum value of Mw/D2 = 0.5.
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