Aliphatic polyesters mechanism

Aliphatic polyesters are low-melting (40-80°C) semicrystalline polymers or viscous fluids and present inferior mechanical properties. Notable exceptions are poly (a-hydroxy acid)s and poly (ft -hydroxy acid)s. 

In addition to the pathways depicted above, a 4-center concerted mechanism yielding ketenes has been reported during die vacuum pyrolysis of aliphatic polyesters (Scheme 2.4).89,90... 

PET, PTT, and PBT have similar molecular structure and general properties and find similar applications as engineering thermoplastic polymers in fibers, films, and solid-state molding resins. PEN is significantly superior in terms of thermal and mechanical resistance and barrier properties. The thermal properties of aromatic-aliphatic polyesters are summarized in Table 2.6 and are discussed above (Section 2.2.1.1). 

Mechanical properties. See also Dynamic mechanical analysis (DMA) of polyamides, 138 of polyester LCPs, 52 of polyurethanes, 242-244 of semicrystalline aromatic-aliphatic polyesters, 45 Mechanical recycling, 208 Medical applications, for polyurethanes, 207... 

Phthalazinone, 355 synthesis of, 356 Phthalic anhydride, 101 Phthalic anhydride-glycerol reaction, 19 Physical properties. See also Barrier properties Dielectric properties Mechanical properties Molecular weight Optical properties Structure-property relationships Thermal properties of aliphatic polyesters, 40-44 of aromatic-aliphatic polyesters, 44-47 of aromatic polyesters, 47-53 of aromatic polymers, 273-274 of epoxy-phenol networks, 413-416 molecular weight and, 3 of PBT, PEN, and PTT, 44-46 of polyester-ether thermoplastic elastomers, 54 of polyesters, 32-60 of polyimides, 273-287 of polymers, 3... 

These representative aliphatic polyesters are often used in copolymerized form in various combinations, for example, poly(lactide-co-glycolide) (PLGA) [66-68] and poly(lactide-co-caprolactone) [69-73], to improve degradation rates, mechanical properties, processability, and solubility by reducing crystallinity. Other monomers such as 1,4-dioxepan-5-one (DXO) [74—76], 1,4-dioxane-2-one [77], and trimethylene carbonate (TMC) [28] (Fig. 2) have also been used as comonomers to improve the hydrophobicity of the aliphatic polyesters as well as their degradability and mechanical properties. 

Polyesters offer multiple options to meet the complex world of degradable polymers. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET). Specific local environmental factors such as humidity, pH and temperature significantly influence the rate of degradation. 

The purpose of this review is to report on the recent developments in the macromolecular engineering of aliphatic polyesters. First, the possibilities offered by the living (co)polymerization of (di)lactones will be reviewed. The second part is devoted to the synthesis of block and graft copolymers, combining the living coordination ROP of (di)lactones with other living/controlled polymerization mechanisms of other cyclic and unsaturated comonomers. Finally, several examples of novel types of materials prepared by this macromolecular engineering will be presented. 

At the end of the 1990s, BASF commercialized Ecoflex F, a completely biodegradable statistical copolyester based on the fossil monomers 1,4-butanediol (BDO), adipic acid and terephthalic acid (see Fig. 3). Ecoflex F combines the good biodegradability known from aliphatic polyesters with the good mechanical properties of aromatic polyesters. 

Aliphatic polyesters occupy a key position in the field of polymer science because they exhibit the remarkable properties of biodegradability and biocompatibihty, which opens up a wide range of applications as environmentally friendly thermoplastics and biomaterials. Three different mechanisms of polymerization can be implemented to synthesize aliphatic polyesters (1) the ring-opening polymerization (ROP) of cyclic ketene acetals, (2) the step-growth polymerization of lactones, and (3) the ROP of lactones (Fig. 1). 

Another interesting example of lactones are the p-hydroxyalkanoates, whose ROP affords poly(p-hydroxyalkanoate)s (PHAs), a class of aliphatic polyesters naturally produced by bacteria (Fig. 3) [12, 13]. Poly(3-(R)-hydroxybutyrate) (PHB) is a typical example. PHB is a stiff thermoplastic material with relatively poor impact strength, but the incorporation of other monomers can improve the mechanical properties. 

This review aims at reporting on the synthesis of aliphatic polyesters by ROP of lactones. It is worth noting that lactones include cyclic mono- and diesters. Typical cyclic diesters are lactide and glycolide, whose polymerizations provide aliphatic polyesters widely used in the frame of biomedical applications. Nevertheless, this review will focus on the polymerization of cyclic monoesters. It will be shown that the ROP of lactones can take place by various mechanisms. The polymerization can be initiated by anions, organometallic species, cations, and nucleophiles. It can also be catalyzed by Bronsted acids, Lewis acids, enzymes, organic nucleophiles, and bases. The number of processes reported for the ROP of lactones is so huge that it is almost impossible to describe aU of them. In this review, we will focus on the more... 

The polymerization of substituted lactones is an attractive strategy for extending the range of aliphatic polyesters and for tailoring important properties such as biodegradation rate, bioadherence, crystallinity, hydrophilicity, and mechanical properties [100]. Moreover, the substituent can bear a functional group, which can be very useful for the covalent attachment of drugs, probes, or control units. 

A change of architecture is another route that enables diversification of the properties of aliphatic polyesters. This review will focus on star-shaped, graft, macrocyclic, and crosslinked aliphatic polyesters. It must be noted that the ROP of lactones has been combined with several other polymerization mechanisms such as ROP of other heterocyclic monomers, ionic polymerization, ROMP, and radical polymerization. Nevertheless, this review will not cover these examples and will focus on polymers exclusively made up of poly(lactone)s. 

Duda, A. and S. Penczek, Mechanisms of Aliphatic Polyester Formation, Chap. 12 in Biopolymers, Vol. 3b PolyestersII Properties and Chemical Synthesis, A. Steinbucheland Y. Doi, eds., Wiley-VCH, Weinheim, 2001. 

Aliphatic polyesters are the most economically competitive of the biodegradable polymers moreover, synthetic polyesters are expected to be degraded nonspecifi-cally by lipases. Although these polyesters are biodegradable, they often lack good thermal and mechanical properties. On the other hand, aromatic polyesters - such as... 

It is further attractive that polymer blending may offer opportunities not only to improve the processability and modify the physical properties of CEs, but also to alter the thermal instability and/or mechanical brittleness of the second component polymers, e.g., many aliphatic polyesters including bacterial poly(hydroxyalkanoate)s. Recent developments in the area of cellulose ester/polymer blends are reviewed below. 

Hyper-branched aliphatic polyesters are different from linear polyesters because of their unique mechanical and rheological properties, which can easily be tailored by changing the nature of the end group [156-159]. Both dendrimers and hyper-branched macromolecules are prepared from AB2 monomers, using dif-... 

It is believed that chain scission occurs through simple hydrolysis, but the kinetics of this hydrolysis are influenced by anions, cations, and enzymes [190]. The process is autocatalytic and the products of hydrolysis such as carboxylic groups participate in the transition state. Water preferentially enters the amorphous parts but crystalline domains are also affected [125]. The degradation of aliphatic polyesters is believed to be dominated by a hydrolytic mechanism but it is also promoted by enzymatic activities [4,7,191-193]. 

A controllable biodegradability, desirable mechanical properties, suitable gas permeability and selectivity would extend the potential application areas of aliphatic polyesters not only in agriculture or in the greenhouse or packaging industry but also as a substitute for human skin. There is a need for such focused studies in the future. 

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