Polymers From Cyclic Esters

Lactones, i.e. esters of hydroxyacids and their dimers, like glycolide and lactide, are two major groups of cyclic esters used in polymerization. These compounds are used on the large scale and polymerized mostly by anionic or coordinative mechanisms. Four, six-, and seven-membered lactones polymerize by both cationic and anionic mechanisms. Polyesters prepared in this way are however only a small fraction of the polyesters prepared by polycondensation. 

Both cationic and anionic polymerizations of e-caprolactone are accompanied by the formation of cycles 4 5) as discussed later in this section. Anionic polymerization leads to living systems whereas side reactions were observed in all cationic processes. 

The cationic and coordinative polymerizations are the only routes for polymerization of the six-membered dimers of the glycolic and lactic acids, i.e., glycolide and lactide, leading to polymers used in medicine. 

Cationic polymerization of lactones can be initiated by a wide variety of protonic and Lewis acids, and by oxonium and carbenium salts. 

---

Biodegradable polymers made from cyclic esters have received increasing attention as resorbable biomaterials as well as commodity thermoplastics. Although the CAGR amounts to 20-30%, it is still a niche market accounting for around 1% of the synthetic plastics. Among the monomers, lactic acid, either produced by fermentation or chemical processes, gives access to polylactides (Scheme 21.16)... 

The high level of interest in the ring-opening polymerization (ROP) of cyclic esters (lactones) stems from the biocompatibility and biodegradability of their polymers. Resorbable aliphatic... 

Experimentally obtained were an ester polymer from methylisopropylketene, two polymers from pentamethyleneketene (one with the ester the other with ketonic stmcture), and an ester polymer from 4-methylpentamethyleneketene (91). Their stereochemistry has not yet been definitely determined. From the last monomer two cyclic dimers of ketonic structure, cis and trans (92) were obtained they possess a close analogy to the aforementioned polymers. 

The production of polyesters from lactones (cyclic esters) avoids the problems usually encountered in traditional polyester synthesis due to esterification equilibrium and water production. In fact, they yield in a single step, in the presence of ROP catalysts, high molecular weight polymers endowed with optimum mechanical properties. This is particularly true when large lactones are used as starting monomers. On the other hand, one of the main drawbacks connected with their use lies in their high cost. 

Duda A Penczek S (2001) Thermodynamics, kinetics, and mechanisms of cyclic esters polymerization. In Polymers from renewable resources, vol 764. American Chemical Society, pp 160-198... 

Polymerization of cyclic esters of phosphoric acid (cyclic phosphates) is interesting from the synthetic point of view, because the resulting polymers have the sequence of atoms of the chain identical with this, which appears in important biopolymers such as nucleic or teichoic acids. Both 5- and 6-membered cyclic phosphates undergo polymerization. 

The thermal stability of this polymer has been studied, and by heating from ambient to 500° C the polymer generates butadiene, tetrahydrofuran, dihydrofuran, water, the cyclic ester of phenylphosphonic acid and 1,4-butanediol, phenylphosphonic acid, and butanediol [1]. The same report evaluates thermal decomposition of this polymer end capped with phenylisocyanate. In addition to the same compounds as for the main polymer, CO2 and aniline were detected from the polymer decomposition. 

FI/FD-MS may also help to determine if a/clic oligomers are present. Figure 6.8 is the FD spectrum of a toluene extract from a segmented polyurethane. Solvent extraction is often used to remove low molecular weight material from a polymer for analysis. In this case three series of cyclic ester/ urethane oligomers could readily be identified from the FD-MS molecular... 

---