Condensation reaction polycondensation

Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

Early studies of the condensation reaction on the monomer level did not give the full picture of this process and only in the 1980s was polycondensation of siloxanols studied by using oligomeric model compounds (76,77). These studies revealed that in the presence of strong protic acids three processes must be considered linear condensation (eq. 14), cyclization (eq. 15), and disproportionation (eq. 16). 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

These are produced by the polycondensation of a phenol or a mixture of phenol with formaldehyde. This condensation reaction is catalysed by acids or alkalies. The nature of the product formed by this condensation reactions depends upon the type of catalyst and the mole ratio of the reactants. 

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

A majority of the hyperbranched polymers reported in the literature are synthesized via the one-pot condensation reactions of A B monomers. Such one-step polycondensations result in highly branched polymers even though they are not as idealized as the generation-wise constructed dendrimers. The often very tedious synthetic procedures for dendrimers not only result in expensive polymers but also limit their availability. Hyperbranched polymers, on the other hand, are often easy to synthesize on a large scale and often at a reasonable cost, which makes them very interesting for large-scale industrial applications. 

Since the time of Flory, only a few papers have appeared in the hterature in which the kinetics of A2B condensation reactions are treated. A purely theoretical paper was recently pubhshed by Moller et al. where Flory s theory of AjjB polycondensations was expanded to describe the distribution of molecules containing arbitrary numbers of branching units [43]. In another paper, Hult and Malmstrom studied the kinetics of a reacting system based on 2,2-bis(hy-droxymethyljpropionic acid [44]. 

This type of polymerisation generally involves a repetitive condensation reaction between two bl-functlonal monomers. These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers. 

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 rate expressions and values, mechanisms, and the activation energies for the condensation reactions forming polymers are similar to those of small molecule reactions. Reaction rate increases with temperature in accordance with the Arrhenius equation. Average DP also increases as the reaction temperature increases to the ceiling temperature where polymer degradation occurs. Long chains are only formed at the conclusion of classical polycondensation processes. 

The kinetics of polycondensation reactions might be expected to be similar to those found in condensation reactions of small molecules (evidence suggests that rate coefficients are independent of polymer size). Polyesterification reactions between dibasic carboxylic acids and glycols can be catalysed by strong acids. In the absence of added catalyst, it has been suggested that the acidic monomer should act as a catalyst, whereupon the rate of reaction should be given by... 

The International Union of Pure and Applied Chemistry [IUPAC, 1994] suggested the term polycondensation instead of step polymerization, but polycondensation is a narrower term than step polymerization since it implies that the reactions are limited to condensations—reactions in which small molecules such as water are expelled during polymerization. The term step polymerization encompasses not only condensations but also polymerizations in which no small molecules are expelled. An example of the latter is the reaction of diols and diisocyantes to yield polyurethanes (Eq. 1-6). The formation of polyurethanes follows the same reaction characteristics as the formation of polyesters, polyamides, and other polymerizations in which small molecules are expelled. 

Condensation polymerizations (polycondensations) are stepwise reactions between bifunctional or polyfunctional components, with elimination of small molecules such as water, alcohol, or hydrogen and the formation of macromo-lecular substances. For the preparation of linear condensation polymers from bifunctional compounds (the same considerations apply to polyfunctional compounds which then lead to branched, hyperbranched, or crosslinked condensation polymers) there are basically two possibilities. One either starts from a monomer which has two unlike groups suitable for polycondensation (AB type), or one starts from two different monomers, each possessing a pair of identical reactive groups that can react with each other (AABB type). An example of the AB type is the polycondensation of hydroxycarboxylic acids ... 

Fully aromatic polyamides are synthesized by interfacial polycondensation of diamines and dicarboxylic acid dichlorides or by solution condensation at low temperature. For the synthesis of poly(p-benzamide)s the low-temperature polycondensation of 4-aminobenzoyl chloride hydrochloride is applicable in a mixture of N-methylpyrrolidone and calcium chloride as solvent. The rate of the reaction and molecular weight are influenced by many factors, like the purity of monomers and solvents, the mode of monomer addition, temperature, stirring velocity, and chain terminators. Also, the type and amount of the neutralization agents which react with the hydrochloric acid from the condensation reaction, play an important role. Suitable are, e.g., calcium hydroxide or calcium oxide. 

Since the chemistry for fragment condensation is not known, we need some assumptions. One major assumption is now that the prebiotic fragment polycondensation was catalyzed by peptides originating from the NCA-condensation reaction itself. 

Polyethers and polyesters having methoxybenzalazine units with various alkylene groups (C4, C6 and Cg) in the main chain were synthesized from vanillin (7,8). The condensation reaction of 4,4 -alkylenedioxybis (3-methoxybenzaldehyde) [VI] with hydrazine monohydrate was applied to the synthesis of polyethers [VII] (Mn, 7.4 x 103 for C4, 7.3 x 103 for C6 and 4.1 x 103 for Cg derivatives), as shown in Scheme 3. Polyesters [IX] (77jnh, 0.35 dl/g for C4, 0.38 dl/g for C6 and 0.43 dl/g for Cg derivatives) were synthesized from 4,4 -dihydroxy-3,3 -dimethoxybenzalazine [VIII] and di-carboxylic acid chlorides by conventional low temperature solution polycondensation, as shown in Scheme 4. 

The synthesis of block copolycondensates by condensation reactions has also been described very often indeed the ineluctable presence of reactive end groups makes these molecules especially suitable for reactions with dibasic acids, diisocyanates, diacid chlorides, diamines, diols, etc. Using this method it was for example possible to synthesize polycondensates in which crystalline blocks alternate with amorphous blocks similarly it makes possible the synthesis of high molecular weight polymers from polycondensates of relatively low degree of polymerization. 

These polysiloxanes were also found to be synthesized by the condensation reaction between organosilanes and organoalkoxysilanes with release of alkane as an inert by-product. For this polycondensation, B(C6F5)3 was used as an effective catalyst [88]. This process involves cleavage of C-0... 

Depending on the reaction conditions, the solvent used and the length of the radical at the Si atom, the hydrolytic condensation of trifunctional halogen-containing compounds drastically changes the structure, composition and properties of polyorganosiloxanes formed as a result of hydrolytic condensation and polycondensation. This leads to the formation of branched (I), ladder (II) or cross-linked molecular structures in the polymer. 

Polycondensation. A condensation reaction which yields a polymeric substance as one of the products. See Condensation, Polyester. 

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