Polycondensations polyesterification

Polyesters have been obtained in organic medium by polyesterification of hydroxy acids,328,329 hydroxy esters,330 stoichiometric mixtures of diols and diacids,331-333 diols and diesters,334-339 and diols and cyclic anhydrides.340 Lipases have also been reported to catalyze ester-ester interchanges in solution or in die bulk at moderate temperature.341 Since lipases obviously catalyze the reverse reaction (i.e., hydrolysis or alcoholysis of polyester), lipase-catalyzed polyesterifications can be regarded as equilibrium polycondensations taking place in mild conditions (Scheme 2.35). 

AB-type polycondensations, 330 AB-type polymers, 135 Accelerated weathering tests, 245 Acceptor-catalytic polyesterification, 75 4-Acetoxy benzoic acid, poly esterification of, 74... 

While more complicated situations can occur, we will consider only the kinetics of simple polyesterification. The kinetics of most other common polycondensations follows an analogous pathway. 

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 effects described by Eqs. 4.1 to 4.4 apply to both stepwise addition and condensation polymerizations. In polycondensation, two further factors must be considered The condensation equilibrium and the exchange reactions. The condensation equilibrium limits the conversion and hence the average molecular weight. As in the case of esterification of monofunctional compounds, the corresponding polycondensations are to be treated as equilibrium reactions, governed by the law of mass action. For example, in the case of polyesterification, if... 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

The above acid-catalyzed polycondensations were carried out at more than 100 °C, whereas Takasu et al. reported room temperature polyesterification with scandium trifluoromethanesulfonate [Sc(OTf)3] or scandium trifluo-romethanesulfoimide [Sc(NTf2)3] [27,28]. Thus, the direct polycondensation of methylsuccinic acid and 1,4-butanediol proceeded in bulk under reduced pressure (0.3-30 mmHg) using 1.4 mol % of Sc(OTf)3 at 35 °C for 96 h to afford poly(butylene methylsuccinate) with Mn of 12400 (Scheme 5). When HfCl4-(THF)2 was used in this room temperature polymerization instead of Sc(OTf)3, only low molecular weight polyester (Mn = 1100) was afforded. The scandium catalysts did not promote transesterification ethanol selectively reacted with acetic acid even in the presence of equimolar methyl acetate. 

Furthermore, the chemoselective polyesterification of isophthalic acid with a bisphenol bearing amino group was investigated by direct polycondensation with DBOP and base (Scheme 25) [99]. Ueda and coworkers expected that phenolate was more nucleophilic than the aromatic amino group,... 

Regarding polyesters, Russell and coworkers reported the lipase (Novo-zyme-435)-catalyzed polycondensation of various triols and divinyl adipate for the synthesis of polyesters having pendant hydroxy groups (Scheme 28) [109]. Before this report, the divinyl adipate was reported to be available for enzymatic polyesterification of diols by Kobayashi and coworkers [110,111], in which the equilibrium of esterification is shifted in the forward direction by tautomerization of the eliminated vinyl alcohol to acetaldehyde. The Mw values of the resulting polyesters varied according to the triol used and ranged from 3000 to 14 000. Analysis by MALDI-TOF mass... 

In polyesterification and other polycondensation reactions, polymer molecules condense with one another by the same mechanism by which they add monomer units. There is no immediately apparent explanation, however, for the coupling of two polyarylene ether molecules of structure I. Three possible routes were first outlined by Finkbeiner (13), and each has received some support since. 

With the continual discovery of new reactions leading to the formation of polymers, and with the increased understanding of the mechanisms of those reactions, it is natural that certain definitions of descriptive terms will change with time. The term polycondensation is used here to denote those polymerization reactions which proceed by a propagation mechanism in which an active polymerization site disappears every time one monomer equivalent reacts. This may be illustrated for one propagation step in polyesterification by... 

Later work by Stevenson [72] supported this hypothesis. The preparation of PET catalysed by antimony trioxide was studied in thin films on metal surfaces that were carefully selected to avoid catalysis by surface effects or by dissolved metal as mentioned earlier, a large number of metals and their oxides, salts or other derivatives catalyse the polyesterification reaction. On inactive surfaces like silver or rhodium the catalysed polycondensation rate increased with decrease in film thickness. In the absence of added catalyst there was no tendency for the rate to increase with decreasing film thickness. Stevenson proposed that in thin films the catalyst-deactivating component was more readily lost, thereby increasing the reaction rate. 

Since the reactivity of two functional groups (or rate constant) is independent of the size of the molecule to which they are attached, it is possible to measure the polycondensation reaction rate simply by determining the concentration of functional groups as a function of time. For example, in a polyesterification reaction, the general reaction at any time t is as shown by Eq. (5.13) and, as the reaction proceeds, the functional groups -COOH and -OH disappear at the same rate. The rate of the polyesterification reaction can then be defined simply as... 

Figure 7 presents polycondensation using the example of polyesterification. As in the reaction illustrated above of ethanol and ethanoic (acetic) acid, producing acetic acid ethylester (ethyl acetate), only the functional groups are essentially important in describing esterification. Bifunctional monomers are required for polycondensation. In the example of polyesterification, diols such as ethandiol (glycol) and dicarboxylic acids such as terephthalic acid are used. 

PET and PBT can be eonveniently synthesized, both at laboratory and industrial scale, in the presenee of metal eatalysts in a two-step-polyesterification of the diols, ethylene glycol or 1,4-butanediol respectively, and dimethyl terephthalate (DMT) or terephthalic acid (TPA). The first step is basically a transesterification or esterification process depending on the use of DMT or TPA respectively, while the second step is the polycondensation of the resulting oligomers. The polycondensation step occurs at higher temperature and at reduced pressure to facilitate the distillation of the diols and consequently the polymerisation of the oligomers. 

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