Crosslinking single-step

The use of homobifunctional reagents in two-step protocols still creates many of the problems associated with single-step procedures, because the first protein can crosslink and... 

Figure 1.24 A two-step protocol using a homobifunctional crosslinking agent offers more control than single-step methods, but still may result in oligomer formation.

Bz s-imidoesters like DMS may be used to couple proteins to PE-containing liposomes by crosslinking with the amines on both molecules (Figure 22.24). However, single-step crosslinking procedures using homobifunctional reagents are particularly subject to uncontrollable polymerization of protein in solution. Polymerization is possible because the procedure is done with the liposomes, protein, and crosslinker all in solution at the same time. 

In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of the chiral monomer 0-9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 1 or 0-9-(tert-butylcarbamoyl)-ll-[2-(methacryloyloxy) ethylthio]-10,ll-dihydroquinine 2 (see Figure 1.34a), the comonomer 2-hydroxyethylmethacrylate (HEMA), the crosslinker ethylenedimethacrylate (EDMA) in presence of the binary porogenic solvent mixture cyclohexanol and 1-dodecanol, directly in a single step within fused-silica capillaries. Initiation of the polymerization by either thermal treatment or UV irradiation yielded microglobular polymer morphologies, such as those well known from their corresponding nonchiral... 

Homogeneous ideal networks, also called closed networks, result from a single-step polymerization mechanism of a stoichiometric mixture of monomers, reacted to full conversion. Many amine-crosslinked epoxies of Tg < 200°C and polyurethanes obtained using a single isocyanate monomer and a single polyol belong to this family. 

Bank et al. (1997) developed a sensitive single-step HPLC procedure, by which hydrolysates from a wide variety of tissue sources can be analysed without prior sample treatment down to a detection limit of 0.4 pM for the PYR/dPYR crosslinks. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

It is therefore very difficult to use the lathe cut method to satisfy this need. An appropriate response is to adopt the direct method using complete precision polymerization of an aqueous solution with a single step process in a mold. It is also necessary to develop an appropriate monomer, prepolymer, or photopolymerizable materials. At this point, optimization of design concept as shown in Fig. 3 becomes necessary. The raw material must be water-soluble monomers or polymers that can be aqueous-solution polymerized. During polymerization, 3D crosslinking must take place. Furthermore, the lens must be molded while it contains water and therefore dimensional accuracy will be guaranteed. 

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

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