Cross-linking of pre-formed polymer

There are numerous ways of attaching polymers to a solid surface. Examples are photo cross-linking of pre-formed polymer chains, in-situ atom transfer radical polymerization (ATRP), electron beam irradiation,and plasma polymeriza-tion.f Electrochemical techniques are particularly suited for conducting substrates. For example, Palacin et al. have grafted vinylic monomers from anhydrous solutions.f This technique is mainly based on an anionic polymerization and leads to a covalent link between the polymer and the metal. Schuhmann et al. 

Cross-linking of a pre-formed polymer or formation of a polymer network in the... 

Oxidative phenolic cross-linking of polymer molecules at pre-existing reactive sites is a mechanism common to many biological systems. The two types of cross-links which form are shown in Fig. 14. The symbol R represents the connection between the phenolic group and the polymer which carries it. 

A further group of cross-linked polymers are polymers formed by two or more networks. An interpenetrating network (IPN or full IPN) is an intimate combination of two polymers, both in network form [971UP1]. The networks are not connected to each other by covalent bonds, but they cannot be separated unless chemical bonds are broken. A mixture of two or more pre-formed polymer networks is not an IPN. ... 

Most investigations of polymer-supported onium ion phase transfer catalysts have used cross-linked polystyrenes. Not all of them have the same structure, even when they have the same formal degree of cross-linking with divinylbenzene. (The effect of percent cross-linking is considered in a later section). Two principal methods have been used to functionalize polystyrene for phase transfer catalysts, chloromethylation of pre-formed beads and copolymerization of chloromethylstyrene monomer with styrene and divinylbenzene. The chloromethylation route employs chloromethyl methyl ether (a cancer suspect agent), and a Lewis acid, usually stannic chloride.Substitution proceeds >90% para and is accompanied by some intrapolymer alkylation, which puts additional cross-links into the polymer... 

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Ringsdorf showed a nice example of confinement of supramolecular polymers.192 In cyclohexane, hexa-cinnamoyl azacrown 37 self-assembles to form supramolecular columnar polymers. The periphery of photopolymerizable groups forms cross-links via photocycloaddition and allows the pre-assembled supramolecular polymer to be transferred into a rodlike covalent polymer (Figure 21). Performing the reaction on molecularly dissolved molecules gives rise to a randomly cross-linked polymer with a lower DP. 

The study of unconjugated diolefin polymerizations is usually considered quite difficult because complex, three-dimensional networks are set up at very low conversion. Such cross-linked materials are generally intractable. The situation is quite different in the case of diallyl esters, especially in the case of diallyl phthalates. Both diallyl o-phthalate and diallyl m-phthalate may be converted to prepolymers, which are soluble in a variety of solvents. These two prepolymers are commercially availabe. When dissolved in their respective monomers and heated in the presence of typical initiators, such solutions are converted to cross-linked resins. Most of the shrinkage related to the conversion of monomers to pol)mers has taken place when the pre-polymer was formed originally. Therefore solutions containing relatively high levels of diallyl o-phthalate prepolymer shrink little on polymerization. 

The approach relies on the formation of a pre-polymerization complex between monomers carrying suitable functional groups and the template. A cross-linker is then added and the polymerization initiated. Then, the highly cross-linked polymer forms around the template-monomer complexes. The template is then removed from the polymer via extraction with a solvent, which disrupts the non-covalent interactions present in the pre-polymerization complex. Subsequent template re-binding takes place through the formation of the same non-covalent interactions. 

Polymer pyrolysis to form advanced ceramics allows the production of highly covalent refractory components (fibers, films, membranes, foams, joints, monolithic bodies, ceramic matrix composites) that are difficult to fabricate via the traditional powder processing route [1-4]. Yajima was the first to demonstrate the feasibility of producing high-strength SiC-based fibers from pyrolysis of polycarbosilane [5]. In this process, a thermoplastic pre-ceramic polymer is first shaped into the desired form, cross-linked into a pre-ceramic network and finally converted into a ceramic material by a pyrolysis process in a controlled atmosphere (Fig. 1). A common feature of the polymer route is the formation of intermediates called amorphous covalent ceramics (ACC) [6]. These are formed after removal of the organic components and before crystallization that occurs at higher temperatures. 

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