Solvent-swollen macroporous polymer

Macroporous and isoporous polystyrene supports have been used for onium ion catalysts in attempts to overcome intraparticle diffusional limitations on catalyst activity. A macroporous polymer may be defined as one which retains significant porosity in the dry state68-71 . The terms macroporous and macroreticular are synonomous in this review. Macroreticular is the term used by the Rohm and Haas Company to describe macroporous ion exchange resins and adsorbents 108). The terms microporous and gel have been used for cross-linked polymers which have no macropores. Both terms can be confusing. The micropores are the solvent-filled spaces between polymer chains in a swollen network. They have dimensions of one or a few molecular diameters. When swollen by solvent a macroporous polymer has both solvent-filled macropores and micropores created by the solvent within the network. A gel is defined as a solvent-swollen polymer network. It is a macroscopic solid, since it does not flow, and a microscopic liquid, since the solvent molecules and polymer chains are mobile within the network. Thus a solvent-swollen macroporous polymer is also microporous and is a gel. Non-macroporous is a better term for the polymers usually called microporous or gels. A sample of 200/400 mesh spherical non-macroporous polystyrene beads has a surface area of about 0.1 m2/g. Macroporous polystyrenes can have surface areas up to 1000 m2/g. 

Macroporous68 71 > polymers present an additional diffusion step in catalysis. They have permanent pores created during synthesis in addition to the micropores, which are the spaces between chains in the polymer network filled by solvent in solvent-swollen form. Transport of a reactant from the surface of a macroporous catalyst particle to an active site may proceed first through the liquid-filled macropores to the internal surface of the catalyst particle and then through the polymer matrix. A mathematical treatment of catalysis kinetics of macroporous ion exchange resins is available 72). 

The most common supports are insoluble particles of polystyrenes (PS), cross-linked with divinylbenzene (DVB), and silica gel (SG). Soluble polymers have been used as supports and separated by precipitation or by ultrafiltration. Insoluble PS beads are prepared by suspension polymerization. They can be either microporous or macroporous (synonymous with macroretic-ular). " Typical average particle diameters are 50 gm for peptide synthesis and 500 jum for ion exchange. PS beads are used in the form of solvent-swollen gels. The micropores are created by solvent, and removal of solvent collapses the pores. A macroporous polymer retains pores in the dry state and may have as much as 700 m g" of internal surface. The macropores are created during polymerization by a solvent from which the polymer precipitates as it is formed. A macroporous polymer is usually, but not necessarily, highly cross-linked. In a good solvent, the polymer phase of a macroporous PS also becomes a microporous, solvent-swollen gel. 

The fact that adding a better solvent to the mixture results in a shift of the distribution to smaller pore sizes has been explained by the mechanism of pore formation, postulated for macroporous resins in the late 1960s [101-103]. The addition of a poor solvent causes the phase separation to occur early, whereas the precipitated polymer nuclei are swollen with monomers, which present a better solvating agent than the porogen. Due to the high monomer concentration within the globuli. 

In a general sense, the swollen polymer films can be considered as a polymer, polyelectrolyte gel [3]. Various microscopic techniques have revealed a pronounced heterogeneity of the surface layer [151-159]. In this respect, one has to distinguish between macropores (whose diameter exceeds 10 nm considerably) and nanopores (which represent solvent molecules and ions between the polymer chains). Inside the macropores, the thermodynamic and transport properties of ions and solvent molecules practically do not differ from that of the contacting bulk solution. The space-charge regions (electric double layers) are formed at the interface between the polymer and solution phases whose thickness is much lower than the characteristic sizes of macroelements (fibrils, grains, and pores). The... 

Typical crosslinked polymers do not possess porosity other than when they are swollen in solvents. After solvation, the polymer chains are separated from each other, and they form pores that are places (spaces) filled with solvent. This limited swelling takes place for low crosslinked polymers or macroporous gels (products easily deformable under pressure) and generally precludes their use in bead form (in liquid chromatography, for example). 

Reactions with insoluble polymeric reagents are conducted as stirred mixtures in the same manner as reactions with soluble reagents. A microporous polymer usually is swollen for efficient use. Macroporous PS and SG with lipophilic surfaces may require wetting with a water miscible organic solvent before use in water. After reaction the polymer usually is filtered with a glass frit. The frit can... 

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