Styrene macroporous copolymers

A macroporous polystyrene-divinylbenzene copolymer, produced by copolymerizing a mixture of styrene and divinylbenzene, is dissolved in an organic liquid such as t-amyl alcohol or isooctane, which is a solvent for monomers. This solvent is unable to substantially swell the resulting copolymer. Macroporous cation-exchange beads are also produced from these macroporous copolymers (25,26). 

A research group in Lehigh University has extensively studied the synthesis and characterization of uniform macroporous styrene-divinylbenzene copolymer particles [125,126]. In their studies, uniform porous polymer particles were prepared via seeded emulsion polymerization in which linear polymer (polystyrene seed) or a mixture of linear polymer and solvent were used as inert diluents [125]. The average pore diameter was on the order of 1000 A with pore volumes up to... 

The growing popularity of reversed phase chromatography in particular has prompted polymer manufacturers to investigate the use of polymeric media for this mode of operation. Macroporous copolymers of styrene and divinylbenzene have similar properties to silica based stationary phases bonded with alkyl chains. However, the absence of leachables and stability at high pH can offer advantages under certain circumstances. High quality, mechanically stable macroporous polymeries are now manufactured at much larger scales than the... 

Tatsuzawa et aq 36,37,45,59 separat.ed cold drugs and neuroleptics by using a styrene-divinyl benzene-methyl methacrylate copolymer as stationary phase. The best results were obtained with methanol - ammonia (99 1) as mobile phase. The effect of the pH and of the composition of the mobile phase on the separation were discussed. Aramaki et al.70 analyzed a series of alkaloids on a macroporous styrene-divinylbenzene copolymer with alkaline acetonitrile - water mixtures as mobile phase (Fig. 7.10). The columns showed excellent stability, and also under the strong basic conditions used for the analysis of the alkaloids. 

Column, Hitachi Gel 3010 (macroporous styrene-divinylbenzene copolymer), 10 pm (22ox4.6 mm ID), mobile phase, SI acetonitrile - water (3 7) containing 0.02 M tetrabutylammonium hydroxide, S2 acetonitrile - water (6 4) containing 0.02 M tetrabutylammonium hydroxide, S3 acetonitrile - water (6 4) containing 0.02 M ammonia, flow rate 1 ml/min, detection UV 254 nm. 

Figure 2. Fluorescence emission of solvent equilibrated macroporous styrene-divinylbenzene copolymers prepared by suspension polymerization.

The porogen is usually an inert solvent, or mixtures of inert solvent and polymers. The meso- and macropores in the polymer network are the voids once occupied by the porogen. Individual recipes for the preparation of macroporous polymer beads may seem complex in terms of the number of components involved and the required control of the experimental conditions. The technology, however, for their preparation has been developed to such a degree that excellent control over their properties (e.g. particle size, shape, porosity and chemistry) is routinely achieved. The vast majority of current macroporous polymers are based on styrene-divinylbenzene copolymers. Other suitable monomers include acrylates, methacrylates, hydroxyalkylacrylates, vinylpyridines and vinyl acetate. A wide range of products are available for HPLC in particle sizes from 5-20 p,m, pore diameters from about 2-400 nm, and surface areas from about 50-500 m /g [141,144,146-148]. 

Macroporous poly(styrene-divlnylbenzene) copolymer, FRF-1, columns were used as the stationary phase in the reverse-phase HFLC of the synthetic 3-lactam antihiotlc aztreonam [2S-[2a,33(Z)]]-3-[[(2-Amino-4-thiazolyl)[(1-carhoxy-l-methylethoxy)-imino]acetyl]amino]-2-methyl-4-oxo-l-azetidlnesulfonlc acid and related compounds. Aztreonam was separated better from its precursors and therefore could be assayed more accurately. In most cases, the elution order of compounds tested on a FKF-1 column followed that in conventional reverse-phase, suggesting a similar separation mechanism. For various separations Investigated, FRP-1 was found to be more suitable for our applications than the silica-based reversed-phase columns. 

According to the lUPAC recommendations [238], the term micropores should be apphed to voids smaller than 20 A in diameter, mesopores are those with diameters between 20 and 500 A, and macropores have diameters larger than 500 A. The micropores and thin mesopores mosdy contribute to the value of the inner surface area, whereas the wide mesopores and, in particular, macropores make up most of the total pore volume of porous materials. It should be noted that the overwhelming majority of porous styrene—DVB copolymers are typically meso-porous materials however, the initially introduced name macroporous polymers became generally accepted and we will not deviate from this traditional scientific jargon. 

If the polymer does not swell with certain solvents (such as methanol or octane in combination with macroporous styrene—DVB copolymers), the pore volume may be estimated from the uptake of the solvent by simply weighing the dry and wet polymer samples [248]. 

For a large number of macroporous styrene—DVB copolymers, sulfo-nated cation exchangers KU-23, and weak basic anion exchangers AN-221, the specific volumes and the true and apparent densities were measured independently and the values of and F were correlated in accordance with Eq. [3.3]. The coefficients a and b of the above equation, obtained after statistical treatment of all the results [250], are given in Table 3.1. 

Pore size and pore size distribution. Particularly two methods have been used to determine the pore size in adsorbing materials of both organic and inorganic nature, namely, the gas adsorption technique and the mercury intrusion porosimetry. On the basis of information provided by these methods, a number of serious conclusions have been drawn on the porous structure of macroporous styrene-DVB copolymers. This necessitates a more critical analysis of the possible errors in the interpretation of the results of measuring adsorption isotherms and mercury intrusion. 

Figure 3.3 Dependence of inner surface area on the volume fraction of diluent for the macroporous copolymers of styrene with fech-DVB prepared in the presence of (3-7) f)-heptane and (1,2) isooctane, the concentration of DVB, % (1,4) 20, (2,5) 30, (3) 8, (6) 40, (7) 60. After [271],...

Figure 33 Pore size distribution for (1) dry conventional styrene-6% tech-DVB copolymer and macroporous copolymers prepared in the presence of (2) n-heptane, (3) isooctane, and (4-7) buthyl alcohol the concentration of DVB % (4) 2, (1-3,6) 6, (7) 20. After [282].

Figure 3.6 Dependence of inner surface area upon (a) the concentration of DVB and (b) the degree of dilution with n-heptane for (1, 2) macroporous styrene-DVB copolymers and (3, 4) sulfonates based on the copolymers with (1, 3) p-DVB and (2, 4) fech-DVB (a) the volume fraction of n-heptane 0.57, (b) the concentration of DVB 20%. After [298],...

Figure 3.9b shows the same boundaries confining the macroporous domain II for real styrene-DVB copolymers prepared in the presence of 2-ethyl-1-hexanoic acid, benzyl alcohol, heptane, or pentanol [314]. Judging from the shape of the plots, 2-ethyl-1-hexanoic acid and benzyl alcohol generate macroporous copolymers with cauliflower texture and high surface area in a wider range of crosslinking densities and dilutions compared with heptane or pentanol diluents. 

The third approach to the formation of macroporous structure of styrene— DVB copolymers consists of adding a linear polymer, largely polystyrene, to the monomer mixture. 

Figure 7.18 Kinetics of swelling in toluene of macroporous copolymers (1) styrene-15% DVB copolymer prepared in the presence of 60wt% n-hexane, (2) polydivinylbenzene prepared in the presence of 60wt% toluene, (3) polydivinylbenzene prepared in the presence of 60 wt% n-hexane. (After [159]).

---