Chemical modification methods

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

An important chemical modification method is the chemical coupling method. This method improves the interfacial adhesion. The fiber surface is treated with a compound that forms a bridge of chemical bonds between fiber and matrix. 

Many of the chemical derivatization methods employed in these strategies involve the use of an activation step that produces a reactive intermediary. The activated species then can be used to couple a molecule containing a nucleophile, such as a primary amine or a thiol group. The following sections describe the chemical modification methods suitable for derivatizing individual nucleic acids as well as oligonucleotide polymers. 

Most of the chemical modification methods investigated to date have involved the chemical reaction of a reagent with the cell wall polymer hydroxyl groups. This can result in the formation of a single chemical bond with one OH group (Figure 2. Id), or cross-linking between two... 

Physical Modification Methods Chemical Modification Methods Radiation Modification Methods... 

Physical or chemical modification methods have been employed to increase the toughness of polymer materials. The chemical modifications include random copolymerization, block copolymerization, grafting, etc. the physical ones include blending, reinforcing, filling, interpenetrating networks etc. [24-26]. 

Before the advent of recombinant DNA technology in the 1970s, the ability to engineer novel enzymes was limited to chemical modification methods in which specific residues in an enzyme... 

The first report of a polymer-supported approach to this reaction appeared in 1987 [48]. Enantiopure amino alcohols such as ephedrine, prolinol, and 3-exo-amino-isoborneol were attached to Merrifield polymer. The use of polymer-supported 3-exo-aminoisoborneol 40 resulted in quite high enantioselectivity ( 95 % ee) in the ethylation of aldehydes with diethylzinc (Eq. 15), a result comparable with those obtained from the corresponding low-molecular-weight catalyst system (Eq. 16). A similar system was also reported in 1989, this time using ephedrine derivatives (41,42) and prolinol derivative (43) [49]. A methylene spacer was introduced between the polymer and the amino alcohol to improve activity [50]. Despite this the selectivity was always somewhat lower than that obtained from the low-molecular-weight catalyst (44). These chiral polymers were all prepared by the chemical modification method using Merrifield polymer. 

Polymer-supported TADDOL-Ti catalyst 79 prepared by chemical modification was poorly active in the Diels-Alder reaction of 3-crotonoyloxazolidinone with cyclo-pentadiene (Eq. 24) whereas polymeric TADDOL-Ti 81 prepared by copolymerization of TADDOL monomer 80 with styrene and divinylbenzene had high activity similar to that of the soluble catalyst. In the presence of 0.2 equiv. 81 (R = H, Aryl = 2-naphthyl) the Diels-Alder adduct was obtained in 92 % yield with an endolexo ratio of 87 13. The enantioseleetivity of the endo product was 56 % ee. The stability and recyclability of the catalyst were tested in a batch system. The degree of conversion, the endolexo selectivity, and the enantioseleetivity hardly changed even after nine runs. Similar polymer-supported Ti-TADDOLate 82 was prepared by the chemical modification method [99]. Although this polymer efficiently catalyzed the same reaction to give the (2R,2S) adduct as a main product, asymmetric induction was less than that obtained by use of a with similar homogeneous species. 

Prior to the advent of recombinant DNA technology, the ability to design proteins was limited to chemical modification methods in which specific residues in a protein are modified at the protein level by chemical agents. Different strategies such as atom replacement and segment reassembly have been used to alter enzyme substrate specificity, activity, cofactor requirement, and stability.f These methods can introduce a diverse... 

Crystal polystyrene is a misnomer because the polymer is an amorphous material. The term "crystal" refers to the high optical clarity of the molded polystyrene homopolymer. It is one of the top five commodity polymer resins in the world. Because most chemical modification methods are expensive, they are impractical for low scale production. For low volume production, polymer blending is the most effective way to modify polystyrene. 

In addition to the effect of the PEI moiety, we have also compared the effect of two chemical modification methods (A-methylene phosphonation and quatemization) on the transfectability of the CP copolymer. A-methylene phosphonation is a chemical approach to increase the water solubility of chitosan via the introduction of the phosphonic acid function (Heras et al. 2001) whereas quatemization is an old process that comprises methyl iodide-mediated N, N, A-trimethylation for the fabrication of chitosan soluble in water at the physiological pH (Thanou et al. 2002). In weighing... 

An important chemical modification method is the chemical coupling method, which improves the interfacial adhesion. The fibre surface is treated with a compound, that forms a bridge of chemical bonds between fibre and matrix. The increase in the mechanical properties of the fibreboards due to chemical modification is an indication of improved interaction and stress transfer between the components. Some authors have reported that softening and increased thermo-plasticity of wood fibre surface facilitates contact and dispersion of the fibre with thermoplastics [61, 62]. 

The diverse potential applications of PHA in a number of fields demanded the production of smart polymers with minimal toxic impurities. Chemical modification methods are sometimes aggressive, and lead to reduced polymer molecular weight, unwanted side reaction(s) and toxic impurities. In some instances, a mild surface modification process is required without which the polymer may fail in its intended application(s). For example, neat polymer without the proper modification may cause delamination of adhesive bonds, poor cellular attachment, permanent staining of a fabric, or may influence proteinaceous membrane fouling etcJ These and many other reasons necessitate the application of physical methods (Table 7.1) in polymer modifications, as explained in the subsequent sections. 

TABLE 18.1 Chemical Modification Methods for Natural Fibers... 

Phosphorus-containing polystyrenes have been used for a variety of purposes and can be prepared either by copolymerization or by chemical modification methods. The latter are limited in number and often lead to unacceptable... 

The highly crystalline and hydrophobic nature of PLLA has interfered with modulation of their degradation rate and mechanical properties. It has also been difficult to impart flmctionality to these polymers by application of the ordinary chemical modification methods. JCimura and coworkers have developed a cyclic diester monomer, 3-( )-[(benzyloxycarbonyl)methyl]-l,4-dioxane-2,5-dione (BMD) and 3-(s)-[(dodecyloxycarbonyl)methyl]-l,4-dioxane-2,5-dione (DMD) which consist of both glycolate and a-malate components. The aliphatic polyesters synthesized by their pol)nnerization and copolymerization are functionalized with pendant ester groups that are derived from the a-malate units [21]. 

Physical treatments also clean the impurities of fiber surface, etch and roughen the fiber surface, and improve mechanical interlocking, chemical bonding, and finally better adhesion between the fiber and the matrix. Although physical treatments are ecofriendly, they only affect the external cell walls and do not change the chemical composition of the fibers. So, they are used as a preparation stage for chemical treatments [10], Chemical modification methods and their impact on mechanical properties of the resulting composites are scrutinized later in this chapter. 

This chapter describes recent progress in studies aimed at understanding effects of various chemical modification methods on the state of dispersion of clay particles in a high-temperature PI matrix and on the physical properties of polymer/clay nanocomposite films. Three different modification methods and their effects on the Pl/clay nanocomposite... 

Rubber vulcanization crosslinking was an early chemical modification method. Block and graft methods are also widely used in polymer modification. One of the successful examples of a block copolymer is a thermoplastic elastomer. It is a new material that can be processed like plastic and has elasticity like rubber. Among graft copolymers, the most widely used one is the acrylonitrile butadiene and styrene copolymer... 

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