Monomers and cross-linkers

A broad range of functional monomers and cross-linkers has been used for the preparation of MIPs. The choice of the functional monomers depends on the nature and functionalities of the print molecule. The most widely used monomer is methacrylic acid, which has been shown to interact through ionic interactions and hydrogen bonds with amines, amides, carbamates and carboxylic acids [13-15]. The monomers and the print molecules self-assemble upon mixing and the strength of the complex is of importance for the selectivity of the polymer. For this reason, a considerable amount of research effort has focused on finding optimal monomers for various classes of print molecules and functionalities. For example, for some print molecules, polymers prepared with vinylpyridines [16,17], 2-(trifluoromethy-l)acrylic acid [18] or acrylamide [19] resulted in higher selectivities and affinities than polymers made from methacrylic acid. Mixtures of functional monomers 

2-acryiamido-2-methyl- acryiamide methyi methacryiate 1-vinyiimidazole 1 -propanesulfonic acid 

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Porous organic polymers (Figure 10) are potential electrochromatographic stationary phases for the analysis of pharmaceuticals. The polymer network is generally formed inside the capillary by a stepwise chain polymerization reaction. Polymerization reaction mixtures usually consist of a combination of monomers and cross-linker, initiator, and a porogenic mixture of solvents. 

Organic monoliths are based on copolymerization of a monofunctional and a bifunctional (uncommonly trifunctional) organic precursor in the presence of a suitable initiator and a porogenic solvent. During the last 15 years, a vast number of different monomers and cross-linkers have been introduced and copolymerized using different polymerization techniques and initiators. A general survey of the tremendous amount of scientific contributions can be gained from numerous reviews [25-32],... 

Methacrylate monoliths have been fabricated by free radical polymerization of a number of different methacrylate monomers and cross-linkers [107,141-163], whose combination allowed the creation of monolithic columns with different chemical properties (RP [149-154], HIC [158], and HILIC [163]) and functionalities (lEX [141-153,161,162], IMAC [143], and bioreactors [159,160]). Unlike the fabrication of styrene monoliths, the copolymerization of methacrylate building blocks can be accomplished by thermal [141-148], photochemical [149-151,155,156], as well as chemical [154] initiation. In addition to HPLC, monolithic methacrylate supports have been subjected to numerous CEC applications [146-148,151]. Acrylate monoliths have been prepared by free radical polymerization of various acrylate monomers and cross-linkers [164-172]. Comparable to monolithic methacrylate supports, chemical [170], photochemical [164,169], as well as thermal [165-168,171,172] initiation techniques have been employed for fabrication. The application of acrylate polymer columns, however, is more focused on CEC than HPLC. 

Kubo et al. [122] have covered the sensing area of a gold electrode lattice with a polyimide layer which supports an atrazine selective MIP prepared with MAA and EDMA and functional monomer and cross linker, respectively. The detection limit was 50 nM (11 ppb) with a working range up to 15 pM atrazine. Other herbicides... 

An MIP-QCM chemosensor for determination of carbamate pesticides, such as carbaryl, has been devised [130]. The chemosensor featured a thin film of PVC, containing carbaryl-imprinted polymer microspheres, which was deposited on top of the gold-sputtered quartz crystal transducer. The microspheres were prepared by thermo-induced co-polymerization in ACN of MAA and EGDMA, used as the functional monomer and cross-linker, respectively, in the presence of carbaryl and AIBN serving as the template and initiator, respectively. The chemosensor performance was evaluated for determination of carbaryl exhibiting the linear concentration range of 10-1000 ng mL-1 in the Britton-Robinson buffer of pH = 8.0. This chemosensor was highly stable. It selectively discriminated carbaryl from its structural counterparts, such as carbofuran and aldicarb, with LOD of 1.25 ng mL-1 carbaryl. 

Molecular imprinting allows the generation of specific three-dimensional cavities in polymer matrices by using a template molecule around which functional monomers and cross-linker are self-assembled in a pre-polymerisation state. Following polymerisation and template removal, the polymer matrix is left with the free three-dimensional cavities capable of rebinding the molecule, or others structurally very similar, used for the imprinting. 

The choice of the co-monomers and cross-linker used together with the template will have a very important role in determining the physical properties of the final materials and, ultimately, the rebinding and catalytic properties of the polymers. 

Polyacrylamide gels are formed as the result of polymerization of acrylamide (monomer) and AT,AT-methylene-bis-acrylamide (cross-linker) (Fig. 4-5). The acrylamide monomer and cross-linker are stable by themselves or mixed in solution, but polymerize readily in the presence of a free-radical gen-... 

Chemical gels are covalently cross-linked polymer networks, featuring very high viscosity and well-defined pore structure. Polyacrylamide is the most widely used chemical gel material, usually cross-linked with N,N-methylene-bisacryl-amide (BIS). The pore size of the gel is determined by the relative concentration of monomer and cross-linker used during polymerization (%T, total monomer concentration and %C, cross-linker concentration as a percent of the total monomer and cross-linker concentration [34]). Highly cross-linked ( 5%C) poly-... 

Clinical applications of thermosensitive hydrogels based on NIPAAm and its derivatives have limitations [121], The monomers and cross-linkers used in the synthesis of the hydrogels are still not known to be biocompatible and biodegradable. The observation that acrylamide-based polymers activate platelets upon contact with blood, together with the unclear metabolism of poly(NIPAAm), requires extensive toxicity studies before clinical applications can merge. 

The resulting cross-linked polyacrylamide possesses characteristics that are determined by the quantities of monomer and cross-linker used. The [monomer]/[cross-linker] ratio determines the pore size in which enzyme is entrapped. The total [monomer] + [cross-linker] quantities used will determine the so-called mechanical properties of the gel its stability and rigidity. 

The pore size of a polyacrylamide gel controls the mobility and resolution of components because of the sieving effect of the pores on macromolecular species.6 The pore size may be controlled by varying the total concentrations of monomer and cross-linker, and by varying their ratio. Gel compositions are dehned by two parameters, their %T and %C values, that represent the total and crosslinker contents, respectively. These parameters are defined by Eqs. 9.5 and 9.6. 

In another study, Kempe and Kempe159 employed multivariate data analysis (Modde 6.0 software, Umetrics, Umea, Sweden) for the optimization of monomer and cross-linker ratios in design of polymer specific for propranolol. 

Most functional monomers and cross-linkers contain one or more vinyl functionalities. Polymerization of this type of compound for the preparation of MIPs is traditionally performed as a free-radical polymerization, initiated via either ther-molytic or photolytic homolysis of an initiator. One of the most commonly used free radical initiators for this purpose is 2,2 -azobis (isobutyronitrile) (AIBN). Other examples of free-radical polymerization initiators are phenyl-azo-triphenyl-methane, tert-butyl peroxide (TBP), acetyl peroxide, benzoyl peroxide (BPO), lauroyl peroxide, tert-butyl hydroperoxide and tert-butyl perbenzoate. 

The cross-linking density cannot be estimated from the stoichiometric relation between monomer and cross-linker, e.g., NlPAAm and MBAAm, since the possibility of cychzation during NlPAAm polymerization is high (see Sect. 2, in chapter General properties of hydrogels ). A more realistic number of net chains can be determined by compression or stress-strain measurements, preferred on a gel swollen in organic solvent. 

Li, W. Gao, H. Matyjaszewski, K. Influence of initiation efficiency and polydispersity of primary chains on gelation during atom transfer radical copolymerization of monomer and cross-linker. 

Gao, H. Miasnikova, A. Matyjaszewski, K. Effect of cross-linker reactivity on experimental gel points during ATRP of monomer and cross-linker. Macromolecules 2008, 41 (21), 7843-7849. 

The aliphatic monomers and cross-linkers such as 30 were also used to examine the properties of the resulting particulates for lysosomotropic delivery [22]. Particulates derived from 30 and related aliphatic monomers displayed good biocompatibility and fast hydrolysis rates at acidic pH values. This aliphatic cross-linker undergoes degradation to generate acetone as the carbonyl-related degradation... 

Can existing functional monomers and cross-linkers be used in step-wise assembly processes or if not, can the more specialized reagents and chemistries be employed on sufficient scales to be commercially useful ... 

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