Jiří Podešva

Hydrogenation of low-molar-mass, OH-telechelic polybutadienes. I. Methods based on diimide

Low-molar-mass, OH-telechelic polybutadienes were hydrogenated (1) by diimide alone and (2) by using a novel method, consisting of the following two steps: up to some 95% degree of conversion by gaseous hydrogen with conventional Ziegler–Natta catalysts, and, only then, up to almost full saturation by diimide. The two-step method, which has been found to be equally efficient, enables one to decrease substantially the necessary feed of p-toluenesulfonylhydrazide, by the thermal decomposition of which diimide is generated. The crude saturated products, which could not be purified by a conventional (re)precipitation technique due to their low molar mass, contained a relatively large amount of a side-product, bis(p-tolyl)disulfide (TDS). It was found that free TDS can be converted quantitatively by reduction cleavage into p-tolyl mercaptan (TM) without changing the structure of the polymeric product, and TM can then be removed from the mixture by alkaline extraction. Alternatively, the crude product can be freed from TDS by chromatography. With the two-step hydrogenation method, only a small amount of the fragments and/or precursors of TDS add to the 5% residual CC double bonds of the partially hydrogenated polybutadiene chains. After any of the two purification procedures, the fully saturated products usually contained less than 1 wt % of such undesirable substituents only, which is comparable with the reported single-step diimide hydrogenation of the initial, fully unsaturated polybutadiene in the presence of a proton scavenger (tri-n-propylamine).

Hydrogenation of low-molar-mass, OH-telechelic polybutadienes. II. Nuclear magnetic resonance and infrared spectrometric determination of hydroxyl end groups before and after hydrogenation by diimide

1H nuclear magnetic resonance (1H-NMR), 13C-NMR, and infrared spectroscopies were used to determine concentrations (cOH, in mmol/g) of the secondary hydroxyl end groups in the low-molar-mass, OH-telechelic polybutadienes, and their hydrogenated analogs. Mean OH-functionality (fOH ≤ 2), that is, an average number of OH groups per one polymer chain, was calculated from cOH for each sample and each method, and the results were compared with those obtained by a conventional acetic anhydride titration method. It has been found that with molar masses of the samples studied (2310 to 3410 g/mol), the differences between individual spectrometric methods are usually not higher than approximately 10%, which corresponds to an expected relative experimental error. Certain differences in fOH between individual methods are discussed. No systematic change of fOH after virtually total hydrogenation of the olefinic double bonds of the polymeric substrate by diimide was observed.

Effect of cyclodextrins on pH-induced conformational transition of poly(methacrylic acid)

A pH-induced conformational transition of atactic poly(2-methylprop-2-enoic acid) (poly(methacrylic acid), PMMA) from the contracted to expanded conformation was investigated by viscometry, potentiometric titration, and anthracene solubilisation in the presence of low-molecular-mass non-ionogenic co-solutes-glucose, α-cyclodextrin (αCD), and γ-cyclodextrin (γCD), respectively. No effect of glucose and αCD on the conformational transition was observed with either of the methods used. On the other hand, the characteristic features of the conformational transition were absent in the presence of γCD. The different effects of the co-solutes indicate that the interaction between PMAA and γCD corresponds to the partial inclusion of the PMAA chain into the γCD cavity. The viscometry and anthracene solubilisation imply that γCD promotes the expanded conformation of PMAA at low pH. The potentiometric titration does not support this conclusion. Even though there is no break on the Henderson-Hasselbalch plot, a characteristic of the conformational transition, the potentiometric behaviour corresponds to that of the contracted PMMA conformation. Thus the results suggest the hierarchical picture of the PMAA conformation at low pH in which the local arrangement of the PMAA chain is a prerequisite for clustering on a larger scale.

1H NMR Analysis of Long-Chain-Branched Strong Polyelectrolytes Obtained by Vinyl/Divinyl Monomer Copolymerization in Aqueous Medium

Statistical long-chain branching occurring during the free-radical polymerization of the system sodium 4-vinylbenzenesulfonate (SSNa) and N, N′-methylene-bis(acrylamide) (MBAA) in aqueous medium was studied by ¹H NMR spectroscopy and SEC. The former method yielded the degree of conversion and the content of the three monomer units in the chain, i.e., SSNa, MBAA with pendant vinyl groups, and MBAA as branching points. It was found that the number of individual monomer units per a total of 1000 monomer units does not perceivably change with the degree of conversion and only slightly increases with increasing ratio MBAA/SSNa in the feed.