Polymeric gels with macroporous structure

Hi. The monomer polymerization route. Compared with the resin-functionalization route, the homo- and copolymerization of organotin-containing monomers permits one to influence the polymer resin structure to a greater extent. In principle, it is possible to prepare gel-type, macroporous, microporous or nonporous polymers. The pore structure, tin loading, solubility and other factors which influence the reactivity of the polymer-supported organotin reagents can be controlled by appropriate... 

During the preparation of macroporous materials by crosslinking copolymerization in the presence of precipitants, phase separation takes place within a relatively short period of time. This fast phase separation naturally results in the formation of microdroplets of the rejected porogen. Since the conversion of comonomers at that moment is very low, the rapidly growing polymeric network fixes in the gel the emerging liquid droplets. The nonequilibrium microsyneresis thus transforms into the stable form of phase separation within a heterogeneous system. Thus, the fast arrival at the unstable local polymer-solvent relationship, as compared with the slow rates of solvent macrosyneresis and of network relaxation, leads to the formation of gel-included microdroplets and, finally, to a permanent macroporous structure of copolymers. 

Macroporous Polymers Prepared by Bulk Polymerization. With toluene as diluent (DVB-50-B-T) the highly crosslinked networks closely parallel the pure solvent correlation line indicating the probe is highly solvated even in poor polymer solvents (Figure 3). This result indicates a gel phase with a lilgh degree of permanent micropore structure. There is essentially no difference between this material and that prepared by suspension polymerization (DVB-50-S-T). 

A lot of research has also been devoted to the infiltration of macroporous templates by reactive components. Porous polymeric material was synthesized by either infiltration of a monomer-initiator mixture with subsequent polymerization [29], or by infiltration of a prepolymer solution, which can be UV-cured afterwards [27]. A quite common route to fabricate metal oxide networks is to infiltrate the precursor structure with its corresponding sol-gel solution, which eventually hydrolyzes and solidifies in the desired porous shape. This technique has been shown for a great variety of materials (compare Table 2 in [10], Table 1 in [37], and Table 1 in [38]), such as silica [52], titania [30,50], zirconia [30] or aliunina [30], just to mention a few. Another pathway to metal oxide structures was introduced by Park et al., who precipitated acetate salt solutions of the desired material in the free voids. After addition of oxalic acid the porous metal oxide was formed during the combustion of the latex template [73]. 

As far back as in the late 1940s to the early 1950s, it was noticed that if the suspension copolymerization of mono- and divinyl monomers is carried out in the presence of an inert solvent, it wiU yield beads that are opaque in appearance and much more resistant to osmotic shock compared with the known gel-type copolymers. It soon became evident that the improved physical properties of the new resins are caused by their special internal structure, which, in its turn, results from the phase separation of the initially homogeneous comonomer—diluent solution. This finding opened the door to a new generation of polymeric adsorbents, the so-called macroporous resins that exhibit stable porosity in both the dry and the solvated states. 

To increase the surface-to-volume ratio in the preconcentration channel without the need for particles, the channel can be filled with a polymeric rod. These are formed by in situ polymerization, during which the polymer material also reacts with the wall of the channel. As a result, no frits are needed to hold the material in place. Columns filled with a polymeric rod, so-called monolithic columns, were originally developed for conventional liquid chromatography. They are made by sol-gel technology, " which enables the formation of a highly porous material containing macropores and mesopores in its structure. The use of a monolithic phase circumvents the problems encountered when packing a column with particles. 

These three different approaches are distinguished by the type of pore formers that are introduced in each case leaving particulates, molecules or functional groups in the former, self-organized entities (mainly micelle and lamellar structure formers) in the second approach, and a continuous polymeric phase in the latter approach. The three different approaches also yield, respectively, very different gel morphologies microporous or macroporous material mesoporous materials and hierarchical pore structures with macro- or mesoporosity as well as nanoscale pores within the same material domain. 

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