Hydrolytic polycondensation

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

The formation of a sol-gel functional silica gel takes place by hydrolytic polycondensation of suitable precursors in the presence of a dopant in solution. An example is given in the following (unbalanced) equation ... 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

The overall hydrolytic polycondensation reaction (unbalanced) can be written as... 

When polymerizing A2B monomers there is a possibihty of losing the unique focal point due to intramolecular cyclization. The loss of the focal point in a hyperbranched polyester based on 4,4-(4 -hydroxyphenyl)pentanoic (Fig. 7) acid was closely examined by Hawker et al. [45]. The study showed no significant occurrence of intramolecular cyclization. One disadvantage of polycondensation polymers is that they are sensitive to hydrolysis, that is depolymerization, which might restrict their use. Some hyperbranched polymers are synthesized via substitution reactions which provide less hydrolytically unstable polymers. 

Hydrolytic polycondensation of trifunctional monomers of the type XSiYj leads to cross-linked three-dimensional as well as network and cis-syndiotactic (ladder-type) polymers, (XSiOj 5) With increasing amount of solvent, however, the cor-... 

The rate of the hydrolytic polycondensation of alkyltrichlorosilane markedly depends on the length and branching of the alkyl group at the silicon atom and decreases with X varying in the following order ... 

Phenyltrichlorosilane undergoes hydrolytic polycondensation more readily than ethyltrichlorosilane in spite of the steric difference of phenyl and ethyl groups. However, oligophenylsilsesquioxanes are not formed as the end products of this... 

Most regularities of the hydrolytic polycondensation of XSiYj monomers described in Section 3.1 are also characteristic of co-condensation reactions of the latter... 

The formation of oligosilsesquioxanes in the course of hydrolytic polycondensation of XSiYg monomers in diluted solvents, which follows a seemingly simple scheme... 

Galbis et al. described a variety of carbohydrate-based linear polyesters 61 of the poly(alkylene dicarboxylate) type that were obtained by polycondensation reactions of the alditols 2,3,4-tri-(9-methyl-L-arabinitol (9) and 2,3,4-tri-O-methyl-xylitol (10), and the aldaric acids 2,3,4-tri-(9-methyl-L-arabinaric acid (26) and 2,3,4-tri-(9-methyl-xylaric acid (27), butanediol, and adipic acid were also used as comonomers [28]. Copolyesters of the poly(aIkylene-c )-arylene dicarboxylate) type were obtained using bisphenols as comonomers (Scheme 1). Chemical polycondensation reactions were conducted in bulk or in solution. Enzymatic polycondensation reactions of adipic acid with the above-mentioned alditols were carried out successfully using Lipozyme and Novozyme 435. The hydrolytic degradations of some of these polyesters were also described. 

The ether elimination is also observed as the first step in thermal decomposition of these alkoxides. The same reaction appears also to be responsible for the observed difference in the hydrolytic behavior of molybdenum and tungsten alkoxides the hydrolysis of the W alkoxides leads as for the majority of other metal alkoxides to the formation of hydroxospecies, forming sols and gels on polycondensation, while in the case of Mo alkoxides the added water, being a stronger acid than alcohols, acts as a catalyst for the ether elimination reaction and causes the formation of individual isopolyanions (independently of the nature of the alkyl radical or quantity of water added) [1774] ... 

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