Macroreticular network

Under certain reaction conditions so-called macroreticular or macro-porous networks are produced. In such cross-linked networks, the cross-linked chains of the polymeric substance are not completely randomly distributed over the whole of the volume occupied by the substance. They give a structure that is more or less porous (Figure 2-7). With equal branch densities, macroreticular networks are much more permeable to solute and solvent molecules, which leads to their use as ion-exchange and gel-... 

J mol ). This is additional evidence in favor of rate limitation by inner diffusion. However, the same reaction in the presence of Dowex-50, which has a more open three-dimensional network, gave an activation energy of 44800 J mol , and closely similar values were obtained for the hydrolysis of ethyl acetate [29] and dimethyl seb-acate [30]. The activation energy for the hydrolysis of ethyl acetate on a macroreticular sulphonated cationic exchanger [93] is 3566 J mol . For the hydrolysis of ethyl formate in a binary system, the isocomposition activation energy (Ec) [28,92] tends to decrease as the solvent content increases, while for solutions of the same dielectric constant, the iso-dielectric activation energy (Ed) increases as the dielectric constant of the solvent increases (Table 6). 

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. 

Most of the carriers used in organic synthesis are lightly crosshnked gel-type resins. In contrast to macroreticular (macroporous) resins which are characterized by a permanent porosity, the gelatinous resins have to swell in appropriate solvents in order to build up the polymeric network and to make the reactive sites located on the polymeric strands accessible to the reactants. Good swelling properties are therefore essential for these gel-type resins. The functional groups are introduced either by copolymerization with functionalized monomers or by a posteriori functionalization of the polymeric component. Thus the reactive sites are statistically distributed on the polymer chains and more than 99% are positioned inside the polymeric beads and not on the surface. 

During copolymerization of styrene with divinylbenzene in the presence of a solvent, the polymer precipitates as it forms. At high conversion the polymer consists of submicroscopic fused polymer particles and solvent filled pores 122.231. Removal of the solvent leads either to collapse of the network or to permanent pores. Polymers with permanent porosity are called macropoFous or macroreticular. The more highly cross-linked e network, and the poorer the solvent used as diluent during polymerization, the more likely the product is to be macroporous. 

Millar, J. A., Smith, D. G., Marr, W. E., and Kresmarm, T. R. E. Solvent modified polymer networks. Part 1. The preparation and characterization of expanded-networks and macro-porous styrene-DVB copolymers and their sulfonates. J Cherw Soc, pp. 218-225 (1963). Kun KA, Kunin R. Macroreticular resins III. Formation of macroreticular styrene-divinyl-benzene copolymers. JPolym Sci, PartAl, Polym Chem, 6,2689-2701 (1968). 

J. H. Barrett and D. H. Clemens, Method of Decolorizing Sugar Solutions with Hybrid Ion Exchange Resins, U.S. Pat. 3,966,489 (1976). IPN ion exchange resin. Macroreticular polymer network I. High crosslink levels. 

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