Reversible functional monomers

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

Experiments supporting the actual participation of amine acid salts In the crossUnklng of cyclic hemlamidals were described In the companion paper (1). Amine add salt catalysts are milder than mineral adds, causing less degradation of the substrate the resulting secondary amine appears to be a less reversible crosslink than the ether formed with mineral adds and amine crosslink centers have the potential to Involve up to three functional monomer units 1n the crosslink, leading to a tighter network (Scheme 2). 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

There are two general classes of imprinted polymers covalent and noncovalent MlPs. These two categories refer to the types of interactions between the functional monomer and the template in the prepolymerization complex. There are also hybrid MlPs that utilize a combination of covalent and noncovalent interactions in the preparation and rebinding events (Klein et al. 1999). Covalent MlPs utilize reversible covalent interactions to bind the template to the functional monomers. In contrast, noncovalent MlPs rely on weaker noncovalent functional monomer-template interactions. Each type has specific advantages and disadvantages with respect to sensing applications that will be addressed in subsequent sections. 

A very important issue in the synthesis of MIPs is the study of the pre-organized complexes formed between the functional monomer(s) and the template. Preferably, this complex should be sufficiently stable to withstand polymerization conditions on the one hand and satisfactorily labile to allow both facile release of a template after polymerization and fully reversible rebinding of an analyte on the other. 

The target molecule may be covalently or noncovalently linked to a functional monomer. Usually, with an increase in binding sites between target molecule and functional monomer, the specific molecular recognition will be increased, but the reversibility of sensor will be decreased. Under certain conditions, we are only interested in specific molecular recognition. For example, we prepared a MIP as an alternative to an antibody in an immunoassay. In this application, the reversibility of MIP is not important. However in other situations, we have to achieve a balance between the specific recognition and reversibility for sensor design. 

The preparation of the materials starts by positioning the functional monomers around a template molecule. The monomers interact with sites on the template via interactions that can be reversible covalent or non-covalent (hydrogen, ionic, Van der Waals, n-n, etc.). They are then polymerised and cross-... 

Mediated Radical Polymerization " (NMRP) or Reversible Addition Fragmentation Transfer polymerization (RAFT). Various coil blocks have been grown from these macro-initiators polystyrene, polyaciylates and polymethacrylates derivatives, including functional monomers, and... 

Two main approaches are used to produce MIPs the noncovalent [48] and the covalent [49] approach. In the covalent approach (Figure 5.12a), the functional monomer is covalently bonded to the template molecule before polymerization. When polymerization is complete, the covalent bonds between the template molecule and the polymer are cleaved and the template molecule is extracted. The resulting imprint is then able to recognize and rebind the imprinted analyte via reversible covalent bonds. However, this technique suffers from lack of generality owing to the difficulties of finding suitable monomers. 

The covalent approach or the pre-organized approach implies the formation of a template-functional monomer complex through reversible covalent bonds prior to polymerization. After synthesis and removal of the template, in the subsequent rebinding step, the initial covalent linkage is reconstituted between the polymer and template. Therefore, only a low number of non-selective binding sites are expected to be formed because of the well-defined stoichiometry taking place between the functional monomer and template. Unfortunately, this approach is only applicable to a limited number of template molecules. 

With the development of controlled radical polymerization techniques like nitroxide-mediated radical polymerization (NMRP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization (see Section 3.2), the field of linear glycopolymers has significantly flourished, especially as control of molar mass and monomer sequence has become available, even for functionalized monomers. This enables incorporation of new and more complex glycomonomers as well as allows controlled dispersity, end group functionality, and monomer sequences in block, star-shaped, and graft copolymers, and eventually... 

Sellergren [58] has used the same type of binding partners in a reverse manner, preparing pentamidine imprinted polymers for solid-phase extraction with methacrylic acid as functional monomer. Addition of an excess of acid to an isopropa-nolic, template containing polymerization mixture led to formation of a precipitate which was dissolved by adding water. 

The covalent approach does not suffer from any of the problems caused by using an excess of functional monomer, since the template is covalently bound to an appropriate stoichiometric amount of functional monomer in the polymerization mixture. The result is that all functional groups in the resultant imprinted polymer are present only in the imprint sites and in the precise spatial arrangement for rebinding. This would appear to represent an ideal situation for the creation of an imprint, however, the range of template functionality for which efficient reversible complex formation is... 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

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