Polycondensation functional polyesters

Candida antarctica lipase B was shown to catalyse efficiently the polycondensation of cw-9,10-epoxy-18-hydrox-yoctadecanoic acid, yielding epoxy-functionalized polyesters with Mw values ranging from 15000 to 20000. 

Functional polyesters were synthesized through the specific catalysis of lipase, and their properties and functions were evaluated. Enantio- and regioselec-tive polycondensations produced chiral and sugar-containing polyesters, respectively [20,23]. Using lipase catalyst reactive polyesters were conveniently obtained, some of which were crosslinked to biodegradable coatings. Recently, polyester-based biomaterials have been developed by lipase-catalyzed polymerizations. 

Another approach to increase the molecular weight of polyesters obtained in direct polycondensation is to use chain-extenders, for example through the reaction of oc,(jo-functionalized polyesters with diisocyanates or oxazo-lines " Using hexamethylene diisocyanate as a chain extender, an increase in the number average molecular weight from 33 000 to 72 000 g moU for copolymers with butylene succinate and butylene adipate units was reported. ... 

Figure 5.7 Degree of polycondensation as a function of retention time and temperature [12b]. From Weger, F., Solid-state postcondensation of polyesters and polyamides, presentation given at the FrankI and Thomas Polymer Seminar, 16 June, 1994, Greenville, SC, USA, and reproduced with permission of EMS Inventa-Fischer, GmbH Co. KG...

A different approach, presently appfied by the Perstorp company in the production of hyperbranched afiphatic polyesters from 2,2-bis(methylol)propionic acid [8], utihzes a B starter molecule with AB -groups condensed in consecutive steps see Fig. 1 in the middle of the diagram. In principle, the function of the B component can also be regarded as a chain stopper when all building blocks are polycondensed in one step. This also leads to a predictable and stable molecular weight at total conversion due to the excess of B-groups in the system. 

Another approach for the synthesis of networks relies on the polycondensation of multifunctionalized polyesters with the appropriate multifunctionalized agent, provided that one of the partner is at least tri-functionalized. Toward this end, several reaction have been reported, such as the Michael addition of amines onto acrylates [184], the coupling of ketones and oxyamines [185], the click copper(II)-catalyzed azide-alkyne cycloaddition [186], and esterification reactions [25, 159, 187]. Interestingly, if esterification reactions are used, the crosslinks are then degradable. 

Polycondensation of diols with dicarboxylic acids is often performed in the melt. However, it does not always lead to high-molecular-weight polyesters. Sometimes, the starting materials or the resulting polyester are thermally unstable at the high condensation temperatures. If the reactants and the polyester are well soluble, one can carry out the polycondensation in solution (see Example 4-2). The elimination of water from diols and dicarboxylic acids frequently occurs rather slowly. In such cases suitable functional derivatives of the diols and dicarboxylic acids (esters or anhydrides) can be used instead of the direct condensation, as described in Sect. 4.1.1.3. 

Tlie concept of functionality and its relationship to polymer formation was greatly expanded the theoretical consideration and mathematical treatment of polycondensation systems. Thus if a dibasic acid and a diol react to form a polyester, assuming there is no possibility of other side reactions to complicate die issue, only linear polymer molecules are formed. When the reactants arc present in stoichiometric amounts, the average degree of polymerization, x follows the equation ... 

Figure 14. Increase of the intrinsic viscosity during the polycondensation reaction of polyester as a function of the residence time. Comparison between free-falling-film reactor (13) and agitated thin-film machine...

The telomeric aliphatic polyesters were produced by polycondensation based on adipic acid and hexamethylene glycol in various stoichiometric amounts to generate polyesters of different end group functionality. The polyesters of different molar mass and corresponding reference samples were synthesized at the Center for Macromolecular Chemistry, Berlin, Germany. These types of polyesters are widely used as lacquers and precursors for the production of several important polyurethanes. 

The deformulation of a polyester was done with liquid-adsorption chromatography under critical conditions (LACCC) that was coupled to an SEC system. The dimension was separated by the end-group functionality to study byproducts formed during polycondensation. By 2D chromatography a number of species (e.g., cycles, ethers, alkyl terminated chains, and so on) could be identified. Some of them influenced the mechanical and thermal properties of the polyesters significantly. 

This polymerization process is a polycondensation in which the molecular weight builds up slowly as the small molecules of water are eliminated. Most step polymerization processes are polycondensations thus the terms step polymerization and condensation polymerization are often used synonymously. The stepwise reaction leads successively from monomers to dimers, trimers, and so on, until finally polymer molecules are formed. The polymers obtained are classified by taking into account the functional group of the repeating unit, for example, polyesters (— CO —O—), polyamides ( — CO— NH —), polyurethanes (—O — CO — NH —), polyethers ( — O —), and polycarbonates ( — O — CO —O —). 

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. 

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