Macroporous polymer monoliths

Peters EC, Svec F, Frechet JMJ. Rigid macroporous polymer monoliths. Adv Mater 1999 11 1169-1181. 

The synthesis of macroporous polymer monoliths using SCCO2 as the porogenic solvent. 

Synthesis of Macroporous Polymer Monoliths using scCOz as the Porogenic Solvent... 

J. A. Tripp, F. Svec, J. M. J. Frechet, Grafted macroporous polymer monolithic disks a new format of scavengers for solution-phase combinatorial chemistry. J. Comb. Chem. 2001, 3, 216-223. 

Cooper AI, Wood CD, Holmes AB (2000]. Synthesis of well-defined macroporous polymer monoliths by sol-gel polymerization in supercritical CO2. IndEng Chem Res, 39,4741-4744. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Svec, E (2004a). Preparation and HPLC applications of rigid macroporous organic polymer monoliths. J. Sep. Sci. 27, 747-766. 

The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The effect of the temperature can be readily explained in terms of the nucleation rates, and the shift in pore size distribution induced by changes in the polymerization temperature can be accounted for by the difference in the number of nuclei that result from these changes [61,62]. For example, while the sharp maximum of the pore size distribution profile for monoliths prepared at a temperature of 70 °C is close to 1000 nm, a very broad pore size distribution curve spanning from 10 to 1000 nm with no distinct maximum is typical for monolith prepared from the same mixture at 130°C [63]. 

N-isopropylacrylamide 1 is added to the polymerization mixture to increase hydro-phobicity of the monolith required for the separations in reversed phase mode. Vinylsulfonic acid 12 provides the chargeable functionalities that afford electroosmo-tic flow. Since the gelation occurs rapidly already at the room temperature, the filling of the channel must proceed immediately after the complete polymerization mixture is prepared. The methacryloyl moieties attached to the wall copolymerize with the monomers in the liquid mixture. Therefore, the continuous bed fills the channel volume completely and does not shrink even after all solvents are removed. Fig. 6.8 also shows scanning electron micrograph of the dry monolithic structure that exhibits features typical of macroporous polymers [34],... 

Fig. 16.3. Scanning electron micrographs of cross-sections of a MIP-filled capillary column. The super-porous morphology of the polymer monolith can be seen. Micrometre-sized globular units of macroporous MIP surrounded by interconnecting super-pores (left). A superpore of about 7 pm in width (above, right). Covalent attachments of the MIP to the capillary wall (below, right). Reprinted from [39] Copyright (1997), with permission from American Chemical Society.

Digestion can also be achieved using a trypsin IMER, where trypsin is immobilized to a solid support, e.g, macroporous silica [38], on POROS material (Porozyme IMER) [39-40], a PVDF membrane in a microreactor [41], or silica-based [42] or porous polymer monoliths [43-45]. 

Frechet and coworkers have reported the development of a functionalized polymer monolith for use in parallel solution phase synthesis in continuous flow applications [10]. In this report, the authors outline the preparation of an azalac-tone-functionalized monolith for scavenging nucleophiles. This method involves the preparation of a macroporous polyfchloromethylstyrene co-divinylbenzene) monolith via the polymerization of the relevant mixture of monomer, initiator and porogen. These are allowed to react with a free radical initiator (4-cyanovaleric acid), followed by reaction with the monomer of choice, to synthesize the functionalized monolith. The authors have thus prepared monoliths functionalized with VAZ to provide an azalactone-functionalized monolith. These monoliths were then demonstrated to completely remove amines after flowing a solution of amine in THF through the monolith for 30 min. They have also reported the reaction of these monoliths with alcohols as well. A small demonstration library of ureas was prepared and after 8 min of residence time up to 76% of the alkyl amines were found to be scavenged (Scheme 8.6). 

Mini-columns for analyte separation/concentration can also behave as reactors, resembling the packed bed reactor. In this context, organic polymer monoliths, largely used in the medical and biological fields [73], should be highlighted. Monolithic mini-columns consist of continuous beds with macropores and mesopores which are characterised by low back-pressure effects. These columns offer several other advantages [74], as emphasised in Chapter 8. In the context of flow analysis, monolithic mini-columns were implemented in a sequential injection analyser in 2003 [75] and the potential and limitations of the approach, called Sequential Injection Chromatography, were recently reviewed [76]. 

Finally, w/c and c/w PFPE based emulsions have been used for the synthesis of porous materials, which are the skeletal replica of the emulsions after removal of the internal phase. W/c microemulsions allowed for macroporous polyacrylate monoliths to be produced (80-82). Conversely, c/w emulsions may be used for the preparation of well-defined porous hydrophilic polymers (83). 

Figure 3. Effect of monomer concentration on morphology of macroporous crosslinhed polymer monoliths. Scale bar in electron micrographs = 10 pm.

Figure 4. Effect of CO2 pressure on on morphology of macroporous crosslinked polymer monolith, (a) BET surface area (continuous line = total surface area, hed line = micropore surface area) (b) Percent micropore volume (c) Median pore diameter (d) Intrusion volume (macropore volume).

In the early 1990s, yet another category was developed. These rigid macroporous organic polymer monoliths were formed by a very simple in situ molding process in which a liquid mixture of monomers and solvents was polymerized under carefully controlled conditions and immediately used within a closed tube or similar container. Many review artieles describing various aspects of these materials have been published during the years since their inception. ... 

The early attempts at fabricating molecularly imprinted capillary monoliths adapted the procedure set forth by Frechet and Svec [4] for the in situ preparation of non-MIP macroporous polymer rods for FC separation. In this procedure, porogenic solvents cyclohexanol and dodecanol (80 20 v/v) were used with a methacrylate-based polymer system to produce porous monoliths. When this system was applied to the fabrication of molecularly imprinted monoliths for CEC, the polymers obtained were sufficiently porous but resulted in poor enantiomeric separations [36]. It is thought that the polar-protic nature of the porogens used may have inhibited the formation of well-defined imprints. Polar-protic solvents such as these are often poor porogens for the noncovalent imprinting approach because they interfere... 

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