Counterion-Based Polymerizable PorogensDirect Preparation of Nanoporous Polymer Matrices with Control over Pore Size and Carboxylic Acid Content

We recently introduced the idea of a polymerizable porogen (PolyPo), wherein a pore-generating PEG segment was covalently linked to a styrenic unit via a cleavable urethane linkage. Copolymerization of the PolyPo with divinyl benzene (DVB) led to cross-linked matrices wherein the PEG segment microphase-separated to generate a bicontinuous morphology, via a spinodal-type decomposition; hydrolytic removal of the PEG segment left behind a 3D network of the amine-functionalized nanoporous matrix, where the pore size was controlled by the PEG length. In the present study, the pore-generating segment is electrostatically bound to the polymerizable unit; in other words, they form the ion pair, which makes the removal of the porogenic segment simple, and in turn would leave behind the complementary functional group on the pore walls. To achieve this, commercially available Jeffamines (both mono- and difunctional) were quaternized and used as the counterion for 4-vinyl benzoate, thereby generating an ionic PolyPo. These ionic PolyPos were copolymerized with DVB to generate transparent cross-linked monoliths, which upon extraction with methanolic HCl yielded porous cross-linked matrices with COOH groups decorating the pore walls. The concentration of the COOH groups was controlled by varying the wt fraction of the PolyPo in the polymerization mixture; here, the pore size remained almost constant, since the size of the pore-generating segment was fixed. However, varying the size of the porogenic Jeffamine segment permitted control over the pore size, and, as expected, the pore size scaled with molecular weight as Mn^0.5. To increase the density of functional groups using PolyPos, one must increase their relative mole fraction; this unfortunately also leads to a decrease in cross-link density, which makes the matrix susceptible to pore collapse and consequently reduces the surface area. To avoid this, a new cross-linkable PolyPo was designed based on 3,5-divinylbenzoate with a Jeffammonium counterion; using this cross-linkable PolyPo, the functional group density can be increased without compromising on the matrix rigidity. Finally, using an additional comonomer, namely, 4-chloromethyl styrene, along with the ionic PolyPo and DVB, porous cross-linked matrices with hierarchical porosity were generated; this was carried out utilizing the hyper-cross-linking strategy via Friedel–Crafts alkylation, which further cross-links the PS matrix and generates the micropores, in addition to the mesopores formed via polymerization-induced microphase separation.