Polyester step-growth polymerization reaction

Besides their use to trigger step-growth polymerization reactions, NHCs were mainly used as organic catalysts in chain-growth ROP of cyclic esters to produce linear as well as cyclic aliphatic polyesters, as summarized in Table 5. ... 

A number of polymers obtained in step-growth polymerization reactions (i.e., polyethers, polyesters, polyamides) are... 

In turn, the synthesis of saturated polyesters by the step-growth polymerization reaction under microwave irradiation was presented for polycondensation of 1,4-butanedioI and succinic acid in the presence of l,3-dichIoro-l,l,3,3-tetrabutyldistannoxane as a catalyst. The reaction mixtures were heated up to 200 °C in a microwave reactor, and, for comparison, polymerization under conventional conditions was carried out for 5 h in an oil bath preheated to 200 °C (Figure 31). ... 

The preceding discussions of the kinetics and molecular weight distributions in the step-growth polymerization of AB monomers are clearly exemplified by the esterification reactions of such monomers as glycolic acid or co-hydroxydecanoic acid. Therefore one method for polyester synthesis is the following ... 

The most common form of step growth polymerization is condensation polymerization. Condensation polymers are generally formed from simple reactions involving two different monomers. The monomers are difunctional, having a chemically reactive group on each end of their molecules. Examples of condensation polymerization are the formation of nylon 66, a polyamide, and of poly(ethylene terephthalate), a polyester. Because condensation poly-... 

Carothers pioneering studies were also based on aliphatic polyesters and then culminated in laying the foundations for condensation and step-growth polymerization and in establishing a relationship between molar mass and extent of reaction and the stoichiometric imbalance of functional groups. Fundamental studies relating structure to properties were carried out using these polymers. 

One important group of condensation polymers is the polyesters. The most important commercial polyester is formed from the reaction of terephthalic acid (a diacid) with ethylene glycol (a diol). This polymerization occurs in a stepwise fashion (hence the name step growth polymerization). First, one carboxylic acid group of a diacid molecule and one hydroxy group of a diol molecule combine to form an ester, with the loss of water. Then a second diol molecule reacts with the unreacted caiboxylic group on the other end of the diacid molecule, or a second diacid molecule reacts with the unreacted hydroxy group of the diol. Continuation of this process adds a new monomer unit at... 

In this experiment, both linear and crosslinked polyesters are synthesized by step-growth polymerization The reactions are carried out in the melt (or neat), meaning that no solvents are used in the preparation and the final product does not have to be purified. Preparations such as these are sometimes called green reactions. 

Condensation polymers result from formation of ester or amide linkages between difunctional molecules. Condensation polymerization usually proceeds by step-growth polymerization, in which any two monomer molecules may react to form a dimer, and dimers may condense to give tetramers, and so on. Each condensation is an individual step in the growth of the polymer, and there is no chain reaction. Many kinds of condensation polymers are known. We discuss the four most common types polyamides, polyesters, polycarbonates, and polyurethanes. 

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

In step-growth polymerizations with unfavorable values of K, it is therefore standard practice to operate at high temperatures and reduced pressures to remove the condensation products. This is typical of the manufacture of linear polyesters where the final stages of the polymerization are at pressures near I mm Hg and temperatures near 280 C. Alkyds (Section 5.4.2) are branched polyesters produced by esterification reactions of mixtures of polyhydric alcohols and acids with varying functionalities. They are used primarily in surface coatings. Alkyd syntheses are completed at temperatures near 240°C. It is not necessary to reduce the pressure to pull residual water out of the reaction mixture, because the final products are relatively low-molecular-weight fluids that are diluted with organic solvents before further use. In one process variation, a small amount of a solvent like xylene is added to the reactants to facilitate water removal by azeotropic... 

Polyesters are formed by step-growth polymerization using nucleophilic acyl substitution reactions, as we learned in Section 22.16B. For example, the reaction of terephthalic acid and ethylene glycol forms polyethylene terephthalate (PET), a polymer commonly used in plastic soda bottles. Polyethylene terephthalate is also sold as Dacron, a lightweight and durable material used in textile manufacturing. 

Step-growth polymers, such as polyamides and polyesters, are prepared by reactions between difunctional molecules. Polyamides (nylons) are formed by step-growth polymerization between a diacid and a diamine polyesters are formed from a diacid and a diol. 

Step-growth Polymerization The simplest scheme of this polymerization involves the reaction of a difunctional monomer AB, which contains both functional groups A and B in the molecule. For example, A can be an amine and B a carboxylic acid group. Another scheme involves the reaction between two difunctional monomers of the type AA and BB. In any case, each polymer linkage will have involved the reaction of the functional groups A and B coming from two molecules (monomers or chains). Some examples of polymers synthesized by this mechanism are polyurethane, polyamide, and polyester. 

All polymerization reactions can be categorized into two different types chain- and step-growth polymerization, which are incompatible in terms of monomer structure, experimental conditions, reaction rates, etc. In the past years, research concerning step-growth polymerization has been oriented to the preparation of new polyester materials by the combination of condensation and free radical techniques [28], as shown in Figure 3.10. 

Step-growth polymerizations generally involve either one or more types of monomers. In either ease, each monomer has at least two reactive (functional) groups. In cases where only one type of monomer is involved, which is known as A-B step-growth polymerization, the functional groups on the monomer are different and capable of intramolecular reactions. An example is the formation of an aliphatic polyester by the self-condensation of co-hydroxycaproic acid (Equation 2.24). 

Both polyesters and nylon 6,6 are prepared by step-growth polymerizations. The activation energies for such reactions are of the order of 84 kJ/mol. It is therefore usual to employ elevated temperatures to accelerate these reactions. The step-growth polymerizations shown are characterized by polymeiiza-tion-depolymerization equilibria, with equilibrium constants given by... 

A process used to convert monomer molecules into a polymer is called polymerization, and the two most important groups are step-growth and addition. A step-growth polymerization is used for monomers with functional groups such as —OH, —COOH, —COCl, etc., and is normally, but not always, a succession of condensation reactions. Consequently, the majority of polymers formed in this way differ slightly from the original monomers because a small molecule is eliminated in the reaction, e.g., the reaction between ethylene glycol and terephthalic acid produces a polyester better known as terylene. 

The second general pattern of polymerization is step-growth or condensation polymerization. While terminal alkenes are the most common monomers in chain-growth polymerization, bifunctional molecules are the characteristic monomers for step-growth polymerization. The polyester-, polyamide-, and polyurethane-forming reactions shown below are examples of step-growth polymerizations ... 

---