Other Cyclic Esters

Polymerization and copolymerization of the two l,4-dioxane-2,5-diones (dilactones), glyco-lide and lactide (XLIX with R = H and CH3, respectively) proceeds using anionic initiators  

Polymerization of a cyclic carbonate ester yields a linear polycarbonate [Kuhling et al., 1989 Rokicki, 2000], For example, the cyclic oligomer (m — 2-20 in Eq. 7-85) of the 

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Polymerization by a ring-opening reaction is confined to cyclic monomers which contain at least one heteroatom. The mechanism is very often a polyaddi-tion-type with a product which has a polycondensation-type character. For example, ethylene oxide and other cyclic esters can be polymerized into linear chains by this type of reaction. An even more complicated example of this type of polymerization reaction is the polymerization of e-caprolactam into Nylon 6 (PA 6). 

Meldrum s acid undergoes standard Knoevenagel condensaticms with aromatic and heteroaromatic aldehydes,with hindered aliphatic aldehydes R R CCHO and with reactive, simple aliphatic ketones. - Condensation with less reactive ketones can be performed in the presence of TiCU/amine 2 > 5 or by prior conversion of the ketone into the corresponding ketimine. The reaction with simple aliphatic aldehydes under standard conditions usually gives the corresponding Michael products as described in Section 1.11.2.6. Knoevenagel condensations of some other cyclic esters of ma-lonic acid have also been reported. 

In kinetics of this living polymerization, kp and fep were determined by using a Szwarc plot for simultaneous propagation on ions and ion pairs (illustrated in Figures 1 and 2). The same method was used in studies of anionic polymerization of other cyclic esters. Results are shown in Figure 4. 

One specially interesting synthesis of organotin substituted alcohols involves the insertion of epoxides and other cyclic esters into the tin-alkali bond, and subsequent hydrolysis of the sodium alkoxide formed254. 

Esters undergo the same kinds of reactions that we ve seen for other carboxylic acid derivatives, but they are less reactive toward nucleophiles than either acid chlorides or anhydrides. All their reactions are equally applicable to both acyclic and cyclic esters, called lactones. 

The mechanism of oxidation probably involves in most cases the initial formation of a glycol (15-35) or cyclic ester,and then further oxidation as in 19-7. In line with the electrophilic attack on the alkene, triple-bonds are more resistant to oxidation than double bonds. Terminal triple-bond compounds can be cleaved to carboxylic acids (RC=CHRCOOH) with thallium(III) nitrate or with [bis(trifluoroacetoxy)iodo]pentafluorobenzene, that is, C6F5l(OCOCF3)2, among other reagents. 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

Polyester syntheses have been achieved by enzymatic ring-opening polymerization of lactide and lactones with various ring-sizes. Here, we focus not only on these cyclic esters but also other cyclic monomers for lipase-catalyzed ringopening polymerizations. Figure 8 summarizes cyclic monomers for providing polyesters via lipase catalysis. 

FIGURE 6.1 Constitutional factors affecting the reactivity of functional groups. (A) The reactivity of W depends on the location of the residue. (B) The amino group of a dipeptide ester reacts with the ester carbonyl to form a cyclic dipeptide amino groups of other peptide esters do not react in this manner. (C, D) Reactions between residues of identical configuration do not occur at the same rates as reactions between residues of opposite configuration. 

Cyclic oligomers of condensation polymers such as polycarbonates and polyesters have been known for quite some time. Early work by Carothers in the 1930s showed that preparation of aliphatic cyclic oligomers was possible via distillative depolymerization [1, 2], However, little interest in the all-aliphatics was generated, due to the low glass transition temperatures of these materials. Other small-ring, all-aliphatic cyclic ester systems, such as caprolactone, lactide... 

Lactones (cyclic esters) undergo anionic and cationic polymerization to form polyesters (Eq. 7-75) [Duda and Penczek, 2001 Johns et al., 1984 Kubisa, 1996 Lundberg and Cox, 1969 Young et al., 1977]. The reactivity of different lactones generally follows the pattern for other cyclic monomers, except that the 5-membered lactone (y-butyrolactone) does... 

Copolymerizations between pairs of cyclic esters, acetals, sulfides, siloxanes, alkenes, lactams, lactones, /V-carboxy-a-amino acid anhydrides, imines, and other cyclic monomers... 

The fifth sapphyrin-phosphate structure to be solved is of the mixed chlo-ride/monobasic cyclic AMP salt of diprotonated sapphyrin 4 (Figure 9). While the general features of binding are similar to the other phosphate ester structures... 

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