Polymers, tactic stereocomplex formation

Since there was no pathway towards syndiotactic PHB or unnatural isotactic (5)-PHB available for a long time, a more detailed investigation on material properties with regards to tacticity and stereocomplex formation is stiU missing. To date, it is not known whether syndiotactic PHBs crystallize in a similar manner to isotactic stereoisomers and therefore possesses similar properties nor how they are influenced by blending of polymers with different stereochemistry. 

Liquori et al. [23] first discovered that isotactic and syndiotactic PMMA chains form a crystalline stereocomplex. A number of authors have since studied this phenomenon [24]. Buter et al. [25,26] reported the formation of an in situ complex during stereospecific replica polymerization of methyl methacrylate in the presence of preformed isotactic or syndiotactic PMMA. Hatada et al. [24] reported a detailed study of the complex formation, using highly stereoregular PMMA polymers with narrow molecular weight distribution. The effect of tacticity on the characteristics of Langmuir-Blodgett films of PMMA and the stereocomplex between isotactic and syndiotactic PMMA in such monolayers at the air-water interface have been reported in a series of papers by Brinkhuis and Schouten [27,27a]. Similar to this system, Hatada et al. [28] reported stereocomplex formation in solution and in the bulk between isotactic polymers of / -(+)- and S-(—)-a-methylbenzyl methacrylates. 

Association phenomena in dilute solutions of atactic (a) and stereoregular (i,s) PMMA in suitable solvents have been extensively studied in the past. The anomalies observed even in dilute solutions were explained by intermolecular association of complementary sequences of the stereoregular polymer chains, so-called stereocomplex formation. Besides the well known stereocomplex, remarkable effects in solutions of either i- or s-PMMA alone led to the assumption that associated structures can also be formed as a consequence of interactions between sequences of equal tacticity. It has been found that the solvent exhibits a striking influence on the association of PMMA. This paper deals predominantly with rheological investigations of dilute solutions of a-, i- and s-PMMA and their mixtures, as well as investigations on a stereoblock polymer. Results of calorimetric and electron microscopic investigations will be taken into account. 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

Recently stereocomplexes of PLA appeared on the market and lead to promising applications in durable devices (see below PLA applications). The stereocomplexes are defined as the association of polymers with different tacticity or conformation. Three synthesis routes are used to produce a PLA stereocomplex, either in solution or in melt state during pol)nnerization or hydrolysis. The complexe formation is possible with (i) two monomers (L-lactide and D-lactide), (ii) polymer and monomer or (iii) two polymers (PLLA and PDLA). This synthesis is often performed with stannous tin and 1-dodecanol (lauryl alcohol) as initiator or co-initiator of the reaction [36-38]. According to Tsuji et al. [38], some other parameters affect the formation of stereocomplexes ... 

PLA is a fascinating material and there are big differences in the thermomechanical properties of the polymer depending on the tacticity formed. For example, atactic and heterotactic PLA are amorphous and have no defined melting point. Polymerisation of either the pure l- or d- stereoforms produce isotactic PLA with a Tg of ca. 50 °C and a T , of 170-180 °C. However, a 50 50 mixture of PLLA and PDLA has a melting point of ca. 220-230 °C, this enhancement is due to the formation of a stereocomplex between opposite chiral chains. It is also possible, and indeed preferred, to produce the stereocomplexed polymer from the isoselective polymerisation of rac-LA forming stereoblock PLA. There are many examples of zirconium, and indeed group-4 metals as a whole, in the literature for the polymerisation of rac-LA, the next section will focus on the stereoselective examples of ROP. 

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