Stereocomplex thermal properties

The above-mentioned chemical catalytic routes lead to racemic AHA mixtures. For the direct use of LA (or its esters) as a solvent or platform molecule for achiral molecules like acrylic acid and pyruvic acid, stereochemistry does not matter. The properties of the polyester PLA, the major application of LA, however, suffer tremendously if d and l isomers are built in irregularly [28]. This is exemplified by atactic PLA, made from racemic LA, which is an amorphous polymer with low performance and limited application. However, when l- and D-lactic acid are processed separately into their respective isotactic L- and d-PLA, as discovered by Tsuji et al., a stereocomplex is formed upon blending these polymers. This polymer exhibits enhanced mechanical and thermal properties [28, 164]. A productive route to D-Iactic acid is, however, missing today. If the chemocatalytic routes to LA are to become viable, enantiomer resolution of the racemate needs to be performed. Given separation success, a cheap source of o-lactic acid will be unlocked immediately, providing an additional advantage over the fermentation route (cfr. Table 2). 

Renewable sources such as starch-made PLAs are biodegradable and compostable. They usually have very low or no toxicity but possess high mechanical performance compared to those of commercial pol5miers. However, the thermal stability of PLAs is not sufficiently high enough to use them as an alternative in many commercial pol5nner applications. Various PIA hlends have been studied to improve their thermal properties. A stereocomplex is formed from enantiomeric PLAs, poly(L-lactic acid) (PLIA), and poly(D-lactide) (PDLA) due to the strong interaction between PLLA and PDLA chains. 

Spectra obtained for atactic PMEPL and the stereocomplex formed by equal molar mixtures of the two isotactic polymers of opposite absolute configuration are presented in Fig. 3. In contrast to the isotactic case, no dependence on thermal history is found. These spectra are unexpectedly similar, and resemble the solution cast isotactic spectrum of Fig. 1. NMR spectra were also recorded for polymers of intermediate tacticity. PMEPL of optical purity 75% shows the same dependence on sample preparation as is found for the isotactic polymer whereas samples of optical purity 25% behave as the atactic case. Similar observations were made by Grenier and Prud homme by the comparison of other properties, including x-ray patterns, solubility and morphology. 

Compared with traditional petroleum-based plastics, PLA is still more expensive and usually has less desirable mechanical and physical properties, which limit its commercialization and applications. The recent commercialization of D-(-)-lactic acid and lactide has the potential to improve the mechanical and thermal characteristics of PLA resins and blends by the crystallization of stereocomplex PLA, which allows application in previously unattainable high-end markets. 

An equivalent mixture of optically pure PLLA and PDLA having the same melting point of about 175°C [60] forms a stereocomplex (sc-PLA) that has a higher melting point ( 245°C) than those of pure PLLA and PDLA [61, 62]. It is well known that the sc-PLA has some properties that are superior to PL As. However, since PL As can easily be hydrolyzed and have poor thermal and photostability, the characteristic degradabilities of such PLAs are important in determining how molten sc-PLA should be processed without causing serious thermal decomposition. 

The use of stereocomplexes as thermally reversible crosslinkers in block copolymers containing syndiotactic PMMA blocks is unprecedented and may result in interesting new properties.7.8,9. This process is illustrated in Figure 1 for AB and ABA block copolymers in which B is an elastomeric block. In the following, we will very briefly describe the general methods of synthesis of these block copolymers and some of the characteristic of their mixtures with isotactic (I)-PMMA. 

In principle, rac-lactide, a racemic mixmre of d- and L-lactide, may be polymerized in a stereoselective fashion. Depending on the stereoselection as the ROP proceeds, the resulting polymer may thus exhibit different stereoregularities these directly influence the thermal and mechanical properties of the produced PLAs. In this regard, isotactic PLA stereoblocks and PLA stereocomplexes, which are of interest for their thermal and mechanical properties, may be produced via the ROP of rac-lactide initiated by an achiral derivative, provided the polymerization proceeds via a chain-end stereocontrolled mechanism i.e., the last inserted lactide unit stereo-controls the insertion of the incoming monomer. This strategy has been first validated using salen-based aluminum complexes such as 16 (Scheme 16, top) to produce PLLA-PDLA isotactic stereoblocks [95, 96]. Alternatively, the chiral racemic salen aluminum complex 17 was found to be suitable for the parallel stereoselective synthesis of isotactic poly(D-lactide) and poly(L-lactide) from rac-... 

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