Dilatometrically determined glass transition temperatures and thermal expansion coefficients of 47 different polymers, most of them acrylics, are reported. The experimental values are discussed in terms of the effect of side‐chain flexibility, bulkiness, and polarity on the glass transition temperature and are compared with the isofree‐volume theory of the glass transition temperature, as recently set forth by Simha and Boyer. Possible reasons for the poor agreement with this theoretical treatment are discussed. The effect of equilibrium time at each temperature during a dilatometric run was determined for one sample, polystyrene, which was measured both in the usual way and with long equilibrium times (over a week) at each temperature, yielding glass temperatures of 97 and 89°C. for the usual and for the long equilibrium times, respectively.
METHACRYLATE) BY THE GEL MELTINGPOINT Sir:Most of the quantitative methods for determining the tacticity of stereoregular poly-(methyl methacrylate) depend on nearest-neighbor interactions and, therefore, except in special cases,2 measure the fraction of monomer placements which may be in one tactic configuration or another. This is the case with two of the more useful methods thus far described, high-resolution n.m.r.2 and infrared spectroscopy. 1,3,4 Since complete steric control of polymerization probably never is attained with methyl methacrylate, it is also of importance to learn the average length of the stereoregular sequences, and the distribution of these sequence lengths. We report here a new method, specific to poly-(methyl methacrylate), which we believe gives a relative measure of average syndiotactic sequence length.This method is based on the observation that when a solution of isotactic poly-(methyl methacrylate) in a solvent such as dimethylformamide is mixed with a solution of syndiotactic poly-(methyl methacrylate) in the same solvent, a rigid continuous gel is formed rapidly. If this gel is heated, it will melt sharply to a clear solution. This gel melting point is (1) reproducible to j = O . 5 O , (2) only slightly affected by changes in the original polymer concentration from about 1 to 9% by wt. or by changes in the ratio of isotactic to syndiotactic polymer from about 1:4 to 4:l by wt., (3) independent of the moje of preparation of the isotactic polymer if it has Mv > ca. 100,000, (4) only slightly affected by the molecula; weight of the syndiotactic polymer if it has Mv > ca. 50-100,000, ( 5 ) markedly dependent on the temperature of polymerization of the syndiotactic polymer. Both n.m.r. and infrared spectroscopy have established that the fraction of syndiotactic placements increases with decwasing temperature of polymerization. SYNDIOTACTIC POLY-(METHYL (1) T. G . I'ox, B.Standard conditions therefore can be established so that the gel melting point becomes entirely a function of some element of tacticity of the syndiotactic poly-(methyl methacrylate).A single is0 tac tic poly-(me thy1 met hacry late) sample was used as the isotactic component of all gels. Gels were prepared with equal weights of various syndiotactic polymers in dimethylformamide a t a total concentration of 5% by wt. The isotactic polymer4 was prepared by initiation with phenylmagnesium bromide in toluene at 3 O , and had M, = 130,000; the syndiotactic polymers6 were prepared by free radical initiation at various constant temperatures from -50" to +140°. The gel melting points were measured by determining the temperature a t which the gel, contained in the tip of a 1-ml. pipet inserted into a snug-fitting testtube, flowed readily from the tip. Some results are given : Temp. of polymerization, OC. @V X 1CF Gel melting point, O C . -50 8 . 0 92.7 -30 19.0 87.5 0 11.5 7 6 . 5 60 25.1 60.0 140 5 . 7 4 3 . 5Gornick'j has applied the Flory theory of copolymer crystallization7 to partially t'actic systems, and concludes that ...
1. Isopropyl alcohol ``induces'' the chromic acid oxidation of manganous salts to MnO2. The limiting induction factor (ratio of moles of MnO2 to moles of acetone formed) is one-half. 2. In the presence of manganous ion the rate at which chromic acid is reduced by isopropyl alcohol is decreased; the maximum decrease in rate observed under a wide variety of experimental conditions is 50 percent. 3. These facts, together with the previously determined reaction kinetics, demand the formation of an intermediate chromium compound in which chromium has the valence four or five. 4. Within chemically reasonable limits, all the reaction mechanisms involving such transitory intermediates have been examined. In the presence of manganous salts the only possible mechanism is HCrO4−+CH3CHOHCH3→Cr4+CH3COCH3,Cr4+Mn++→Cr++++Mn+++,2Mn++++2H2O→Mn+++MnO2+4H+.In the absence of manganous salts several mechanisms (all of which are listed) are possible.
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