Monosized microspheres of hypercrosslinked polystyrene. Synthesis and adsorption properties

Abstract

Hypercrosslinked polystyrene is considered to be a promising new stationary phase for high performance liquid chromatography. A key problem in this direction consists of preparing hypercrosslinked polystyrene in the form of monosized microbeads of 3 to 5 μ in diameter. This paper describes a new developed approach to resolving the problem, the physico-chemical properties of resulting polymers and their adsorption properties towards hydrophilic pharmaceuticals. Monosized microbeads of styrene copolymers with 0.5 and 4% divinylbenzene have been obtained by precipitation polymerization in17% solution in the mixture of n-octane/polydiethylsiloxane oil. The yield of 3 μ spheres is independent of the synthesis conditions (temperature, time, type and quantity of radical initiator, monomer concentration, inert atmosphere, etc.) and amounts to 50%. The completing of bead formation at that low styrene conversion is conditioned upon the low distribution coefficient of the residual monomer between the phase of precipitated beads and dispersion medium (5:95) as well as an extremely low rate of styrene polymerization in diluted (9%) siliconhydrocarbon solution.
The monosized copolymer beads thus obtained were crosslinked with 1.5 or 2.5 mole monochlorodimethyl ether via Friedel-Crafts reaction till maximum possible crosslinking degrees of 300 and 500%, respectively (it means that theoretically each phenyl rung of polystyrene chains gets connected with adjacent ones by three or five methylene groups). As judging by gas chromatography technique, in all cases after accomplishing reaction, the liquid phase contains no more than 0.5% unreacted ether. The networks based on styrene-0.5% DVB copolymer contain only 1.7-4% residual chlorine and so the real crosslinking degree of these products is pretty close to nominal one. The real crosslinking degree of polymers prepared by bridging styrene-4% DVB copolymer appears to be far from the expected degree since the final hypercrosslinked polymers contain 7.5-10% residual chlorine. All microspherical (MS) hypercrosslinked samples are porous materials, apparent inner surface area of MS-300/0.5 and MS-500/0.5 reaches correspondingly 1200 and 560 m2/g while MS-300/4 and MS-500/4 exhibit 200 m2/g only. Pore volume of the polymers calculated from bulk weight of initial beads and final products is 0.7 and 0.1 cm3/g for the polymers based on the low crosslinked copolymer and only 0.1 and 0.05 cm3/g for the samples on the basis of styrene-4% DVB. Nevertheless, all hydrophobic hypercrosslinked polymers take up hydrophilic pharmaceuticals from aqueous solutions, especially high loading capacity being demonstrated by MS-300/0.5 sample. Indeed, it adsorbs as large amount of pentoxiphylline as 500 mg/g, 265 mg/g N-acetyl-L-cysteine and 118 mg/g ascorbic acid. Taking into account the enormous density of this network crosslinking, we may explain its high loading capacity only if we reasonably hypothesize a certain heterogeneity of ultimately crosslinked polymers, namely, that ultimately crosslinked domains alternate with cavities facilitating the diffusion of sorbate molecules into the interior of microbeads.