Polycondensation by transesterification

Challa [68,69] studied the uncatalysed polymerization of bis 2-hydroxyethyl)terephthalate and its oligomers to PET. The equilibrium constant varied with degree of conversion as shown in Fig. 5, but only slightly with temperature (Fig. 6). 

It is difficult to suppress the reverse (glycolysis) reaction that occurs during the preparation of polyesters, though Griehl and Schnock [63] virtually eliminated it. Their results and later data of Challa [69], who took the reverse reaction into account, agree quite well. Griehl and Schnock proposed first-order kinetics however, if account is taken of the gradual increase in over-all rate coefficient, their data fit second-order kinetics better [69]. 

The second-order rate coefficients k and k for the polycondensation and glycolysis reaction of PET, showed roughly the same dependence on 

Challa [69] found that the monomer content of the polyesters was greater than that predicted by the Flory—Schulz distribution function, the so-called most probable distribution of molecular species. Challa proposed that the monomer molecule — in this case bis(2-hydroxy-ethyl)terephthalate, though the conclusion could be general for all polycondensations — lost more entropy on entering the transition state than did the longer molecules. 

Support for this proposal was given in another paper by Challa [70], where the redistribution reaction between end groups and chain bonds of polyesters was examined, viz. 

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Rate coefficients and activation energies for uncatalysed and catalysed polycondensations by transesterification... 

Most of the reported work on the kinetics of catalysed polycondensation by transesterification is quite recent and in the main deals with the preparation of PET, though related work on other esters from ethylene glycol has been reported. Reinisch et al. [34] found that the melt polycondensation leading to poly(ethylene sebacate) in 3—4 mm films under vacuum, with manganous acetate as catalyst, was second order from p = 0.70 to p = 0.97. Rate coefficients are listed in Table 5 for this reaction, for the formation of poly(ethylene sebacate-co-terephthalate) and for PET. 

The preparation of hydroxyesters or hydroxyacids from glycerol and their polycondensation by transesterification has been the subject of recent studies aimed at preparing biodegradable hyperbranched polycarbonates [29] and polyesters... 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

Step-growth condensation polymers, such as polyesters and polyamides, are formed by reversible reactions. In the case of PET, the commercial synthesis is essentially carried out by two reactions. The first is the formation of bishydroxyethyl terephthalate by esterification of a diacid with a glycol or by transesterification of a diester with a glycol. The second is the formation of the polymer by a polycondensation reaction. 

Lipase-catalyzed polycondensation and transesterification reactions are the subjects of intensive research activities but polyesters of low molecular weight are obtained by this technique [45-52]. 

Various polyesters derived from phosphorous or phosphoric acids were prepared. Efiicient polyphosphites were synthesised in the early 196(. Polyphosphite prepared from 152 and 4,4 -isopropylidenebis(cyclohexanol) was tested as a thermal stabilizer for PC [199] or as secondary AO for radiation sterilized EPM [200]. Built-in phosphites obtained by transesterification of triallcylphosphite with 4,4 -isopro-pylidenebisphenol or 4,4 -thiobisphenol possess antioxidant properties in polyolefins. Stabilizer containing phosphite moiety 153 was prepared from tris(2-hydroxy-ethyl)isocyanate, decyl alcohol and triphenylphosphite [201]. Various phosphites were derived firom polynuclear phenols or dihydric phenols. For example, a polycondensate prepared by reaction of phosphorus trichloride with 2,5-di-rert-butylhydroquinone was tested as heat and light stabilizer for PP [202], A linear polyester with a built-in phenolic moiety was synthesised from (2,6-di-tm-butyl-4-methylphenyl)bis(6-hydroxyhexyl)phosphite and dimethyl terephthalate [203]. 

Monomer 33 was made to undergo transesterification polymerization using Ti(OC4H9)4, while monomer 34 was appropriate for a Knoevenagel polycondensation. The transesterification polymerization resulted in the formation of an intractable material of unknown structure. Homopolymerization of 34 by the Knoevenagel technique afforded polymer 35 with a low molecular weight (M 6800). A major byproduct in this polymerization was a macrocyclic lactone, formed via an intramolecular Knoevenagel condensation (Scheme 10-14). 

It is convenient to consider the preparation of polyesters by transesterification as taking place in two stages (Sections 5.3.1 and 5.3.2) as shown by the last two reactions. The last, of course, also applies to the process of direct esterification when a two-fold or greater excess of volatile glycol is used. The glycolysis of DMT (Section 5.3.1) md the polycondensation stage (Section 5.3.2) are fundamentally the same, especially in regard to catalysis [56]. 

In 1973, Bonatz et al. [101] finally showed by cai efudy performed experiments diat die first reaction model proposed by Hoftyzer and van Krevelen [100] is correct. Thus, die polycondensation reaction (transesterification of bEG) takes place in die entire melt phase and the removal of EG is the rate-determining step for the overall polycondensation process. 

Low molecular mass linear and branched polyester resins are produced in a one-stage process at 125-240 C. The volatile condensation products are removed in vacuo (melt condensation process) or by passing a stream of inert gas through the resin melt (gas stream condensation process). Polycondensation in solution with azeotropic removal of water by solvent distillation (azeotropic process) is of lesser importance. High molecular mass copolyesters are produced in two stages as is used for poly(ethylene terephthalate). A precondensate is first obtained by transesterification of dimethyl terephthalate with an excess of diols. In the second stage, the molecular mass of the precondensate is adjusted to the desired value by polycondensation in special reactors with the maximum possible elimination of water and excess diols in vacuo at ca. 250 C. 

Polycarbonates are manufactured either by transesterification or by interfacial polycondensation. In the transesterification method, bisphenol A is transesterified in two stages with a slight excess of diphenyl carbonate ... 

Sugar-based fatty acid ester diols have also been prepared by transesterification of epoxidised oleates with methyl a-D-glucopyranoside and sucrose, followed by hydrolysis of the oxirane ring [57]. These fully bio-based monomers were polymerised with an aliphatic diisocyanate to produce PU whose structure could be oriented toward a linear architecture (when the sugar OH groups were not involved) or a network (if at least some of them participated in the polycondensation) by changing the solvent medium. 

Fig. 2.3 Scheme illustrating the polycondensation through transesterification catalyzed by titanium isopropoxide... 

Figure 5. 2 (1) Formation of bis-HBT and other hydroxybutyl-terminated terephthalate oligomers by transesterification of DMT with BD and (2) polycondensation of bis-HBT and hydroxybutyl-terminated oligomers resulting in PBT.

Polyethylene terephthalate (PET) (Table 5.3.13) is obtained either by polycondensation of ethylene glycol and terephthalic acid or by transesterification of terephthalic add dimethyl ester with ethylene glycol. The polymer has an unusually high melting point (260-270 °C) and very good mechanical properties. PET botdes and textile as well as industrial fiber applications are among the main applications of this polyester. 

The biocompatible dimerized fatty acid (DFA)-based poly(aliphatic-aromatic ester) elastomers (PED) have been synthesized and studied for biomedical applications by El Fray et al. [194-200]. The design of nanostructured elastomeric biomaterials (mimicking biological materials) has been realized by using renewable resources, i.e., DFA. They are prepared by transesterification and polycondensation from the melt (see Section 7). The exceptional properties of DFA, e.g., excellent resistance to oxidative and thermal degradation, allow the preparation of PEDs without the use of thermal (often irritating) stabilizers. This is a particularly important feature making these polymers environmentally friendly and additive-free. What is equally important, by the use of the same method and stabilizer-free conditions, it was possible to prepare specially modified PED copolymers with an increased surface hydrophobicity. 

PBT is produced by the transesterification of dimethyl terephthalate with 1,4-butanediol by means of a catalyzed melt polycondensation (19). PBT is also semicrystalline and is an extremely tough resin. Several commercial resins use a blend of PBT with another resin, such as PET, polycarbonate, or nylon. Typically, composites of PBT contain 20—30 vol % fiber glass. 

Polybibenzoates are a kind of thermotropic polyesters obtained by polycondensation of 4,4 -biphenyldicar-boxylic acid (p,p -bibenzoic acid) with a diol. These polyesters contain the biphenyl group, which is one of the simplest mesogens. They are synthesized by melt transesterification of the dimethyl or diethyl ester of p,p -bibenzoic acid and the corresponding diol, using a titanium compound as catalyst, according to the following scheme ... 

Alkyl esters often show low reactivity for lipase-catalyzed transesterifications with alcohols. Therefore, it is difficult to obtain high molecular weight polyesters by lipase-catalyzed polycondensation of dialkyl esters with glycols. The molecular weight greatly improved by polymerization under vacuum to remove the formed alcohols, leading to a shift of equilibrium toward the product polymer the polyester with molecular weight of 2 x 10" was obtained by the lipase MM-catalyzed polymerization of sebacic acid and 1,4-butanediol in diphenyl ether or veratrole under reduced pressure. ... 

Ester-thioester copolymers were enzymatically synthesized (Scheme 7). ° The lipase CA-catalyzed copolymerization of e-caprolactone with 11-mercaptoundecanoic acid or 3-mercaptopropionic acid under reduced pressure produced the polymer with molecular weight higher than 2 x 10". The thioester unit of the resulting polymer was lower than the feed ratio. The transesterification between poly(8-caprolactone) and 11-mercaptoundecanoic acid or 3-mercaptopropionic acid also took place by lipase CA catalyst. Recently, aliphatic polythioesters were synthesized by lipase CA-catalyzed polycondensation of diesters with 1,6-hexanedithiol. ... 

Unactivated esters, typically alkyl esters, often show low reactivity toward lipase catalyst for transesterifications. In the case of the lipase-catalyzed polycondensation of dialkyl esters with glycols, the polymer of high molecular weight was not obtained. The molecular weight improved when vacuum conditions were used Mw reached more than 2 x 104 in the combination of diethyl sebacate and 1,4-butanediol catalyzed by lipase MM [30]. 

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