Crosslinked styrene/divinylbenzene network systems
Differences in properties of pure metaand pure para-divinylbenzene crosslinked polystyrene and their sulphonated products have been correlated with probable network structural differences. The para-divinylbenzene copolymerizes more slowly than does the meta isomer to give a crosslinked copolymer which swells less, sulphonates less rapidly, and, when sulphonated, gives an ion exchanger which has lower selectivity. It is concluded, on the basis of copolymerization data, that the para crosslinked network is tighter and less uniform than that of the meta crosslinked network. Techniques have been developed for the synthesis, separation and purification by gas chromatography, polymerization, and copolymerization of 99.5_99.9 per cent pure metaand para-divinylbenzenes. Kinetics of the polymerization and copolymerization have been determined by radiocarbon techniques and computer analysis of high-conversion data. The preparation and sulphonation of the bead copolymers of styrene with the pure metaand para-divinylbenzenes have been developed in detail and the exchange isotherms of the sulphonated beads have been evaluated. INTRODUCTION Most ion exchange resins are prepared from styrene/divinylbenzene crosslinked copolymers in bead form by processes that are not usually specified. These processes use for divinylbenzene the commercially available mixture of the metaand para-divinylbenzene isomers. This mixture also contains varying amounts of other ingredients—chiefly metaand para-ethyistyrenes. This ambiguously defined process leads to a network structure that is difficult to obtain reproducibly, and problems are thus added to the usual complications involved in studies of crosslinked polymer systems. In our studies we have obtained data on the properties of pure metaand pure para-divinylbenzenes2'3' 8,142330; on the kinetics of their poiymerization'2'26 and styrene copolymerization4'9' 152O283233; on the characteristics of the copolymers'8' 19,25, 27 and their suiphonation" 13, 16, 18,21,24; and some data on their suiphonated products" 10, 31, Some related data have been obtained for ort.ho-divinylbenzene, 1,2,4and 1,3,5trivinylbenzenes, and a series of divinyl compounds of the type CH2 CHC6H4—X—C6H4CHCH21'6"7'20'22 57 RICHARD H. WILEY The questions for which we have sought answers are: (1) what kind of data establish a difference between the meta and the para crosslinked systems and (2) what are the structural implications available from such differentiations? It is the extent to which we can provide answers to these questions that we wish to consider in this report. D1V1NYLBENZENE MONOMERS The commercially available divinylbenzene is a complex mixture of 8—12 components. It is made by dehydrogenation of diethylbenzenes, in turn obtained by Friedel—Crafts alkylation of benzene with ethylene. The ethylbenzenes are a mixture of ortho, para and meta isomers. The ortho isomer is converted to naphthalene in the dehydrogenation and there is little to be gained in separating the ethylbenzenes prior to dehydrogenation, as the dehydrogenation catalyst re-equilibrates the isomeric mixture. One dehydrogenation process has been operated using an iodine catalyst which does not equilibrate the isomeric mixture, and during the time of its development relatively pure metaand para-divinylbenzenes were available. Also, a process23 for the separation of the divinyl isomers based on a Werner complex formation was at one time operated on a developmental basis. We have reported analytical gas chromatographic data for commercial divinylbenzene samples. Typical data are given in Figure 1. A complete listing of the components as identified by comparisons with retention times of known mixtures is given in Table 1. It is essential, for the purposes of establishing possible differences in the network systems obtained on copolymerization, that the pure meta and para isomers of divinylbenzene be separated and purified. The methods we have developed for this purpose are based on preparative gas chromatography (° _____________________________________