Stereocomplex formation

Poly(lactic acid)-based Stereocomplex formation pH [21-25]... 

Watanabe J, Eriguchi T, Ishihara K (2002) Stereocomplex formation by enantiomeric poly (lactic acid) graft-type phospholipid polymers for tissue engineering. Biomacromolecules 3 1109-1114... 

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. 

Acknowledgement Special thanks to Urs J. Haenggi (Biomer) for information about biotechnological PHB synthesis and literature on stereocomplex formation of poly(lactide). We are also grateful to Dr. Carly Anderson and Dr. Sergei Vagin for their help with this article, as well as Benedikt Simon Soller and Simon Meister for their extensive help with the literature research. 

Stereocomplexes are a special group of compounds which can be obtained as a result of template polymerization and stereocomplex formation, described many years... 

In connection with the aforementioned study on polymerization mechanism of MMA77,78), Miyamoto et al. developed a preparatory method of separating blends of isotactic and syndiotactic PMMA82 The principle was based on a competitive adsorption of these different stereoisomeric polymers from a nonpolar solution (chloroform) onto an adsorbent surface (silica gel). The procedure was quite simple, as described below A given polymer blend was dissolved in chloroform, in which no stereocomplex formation usually occurs, and silica gel was then dispersed in this solution for adsorptive equilibration with the polymer species. The isotactic species could be isolated as the adsorbed component. In practice, its purity was ca. 80—90%, which depended on the added amount of silica gel. By repeating the same procedure, the purity could be enhanced. 

It has been shown85 that the decrease of molecular weight of one or two components leads to the disappearance of the maximum of reduced viscosity reflecting the stereocomplex formation. 

In physically crosshnked gels, dissolution of the pol3nner network is prevented by physical interactions that exist between the polymer chains. Physical crosslinking can be estabhshed by, for instance, ionic, hydrophobic, or coiled-coil interactions (15). A novel physical method by which to create hydrogel is the use of stereocomplex formation. This method has been recently investigated by us and others (42—46). In this section, the residts obtained with these gels based on dex are summarized. 

Importantly, the hydrogel is formed in an all-aqueous environment in which the use of organic solvents is avoided. Crosshnking is estabhshed by stereocomplex formation... 

De Jong SJ, De Smedt SC, Wahls MWC, Demeester J, Kettenes-van den Bosch JJ, Hennink WE. Novel self-assembled hydrogels by stereocomplex formation in aqueous solution of enantiomeric lactic acid oligomers grafted to dextran. Macromolecules 2000 33 3680-3686. 

On the other hand, stereocomplexes and solvatophobic bonds are specific to macromolecular multimerization. Certain polymers with mutually complementary stereostructures form what are called stereocomplexes whose stoichiometry is dependent on the stereosequence length. Examples of such stereocomplexing are provided by the pairs poly(7-benzyl-D-glutamate) with poly(7-benzyl-L-glutamate) and isotactic with syndiotactic poly(methyl methacrylates) (PMMA). The specific enthalpy of stereocomplex formation with it- and st-PMMA is, for example, a linear function of the syndiotactic diad mass fraction, with a maximum at Wst = 0.58 (Figure 6-11). If, however. 

Figure 6-11. Negative specific enthalpy for the stereocomplex formation from st- and it-poly(methyl methacrylates) in o-xylol (O) or dimethyl formamide ( ) at 25°C as a function of the mass fraction of the syndiotactic polymer. The dashed line gives the relationship for a plot against the syndiotactic heptad content (after data from W. Siemens and G. Rehage).

As stated above, the hydrolytic degradation resistance of PLA-based materials was enhanced by stereocomplex formation. However, even in the molten state (i.e. above the I m of the stereocomplex crystallites), the PLLA/PDLA blend has a higher thermal resistance than neat PLLA or PDLA [275,276]. 

The stereocomplex formation of PLLA and PDLA is enhanced in the presence of stereoblock poly(lactic acid) [419], Stereocomplex formation was proposed to be a novel method of controlling the protein- and cell-adhesive properties of biodegradable matrices composed of PEG-PLA copolymers [420], A triangular growth mechanism was proposed for the PLLA/PDLA stereocomplex [95], and the nucleating mechanism is not altered by the PLLA/PDLA blending ratio [421],... 

Tsuji, H. (2005) Poly(lactide) stereocomplexes Formation, structure, properties, degradation, and applications. Macromolecular Bioscience, 5, 569-597. 

Ikada, Y., Jamshidi, K., Tsuji, H. and Hyon, S.-H. (1987) Stereocomplex formation between enantiomeric poly(lactides). Macromolecules, 20, 904-906. 

Tsuji, H. and Okumura, A. (2009) Stereocomplex formation between enantiomeric substituted polydactide)s blends of poly[(S)-2-hydroxybutyrate] and poly[(R)-2-hydroxybutyrate]. Macromolecules, 42, 7263-7266. 

Tsuji, H., Shimizu, K., Sakamoto, Y. and Okumura, A. (2011) Hetero-stereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s. Polymer, 52, 1318-1325. 

Tsuji, H. and Ikada, Y. (1992) Stereocomplex formation between enantiomeric polyflactic add)s. 6. Binary blends from copolymers. Macromolecules, 25, 5719-5723. 

Brizzolara, D., Cantow, H.-J., Diederichs, K. et al. (1996) Mechanism of stereocomplex formation between enantiomeric poly(lactide)s. Macromolecules, 29, 191-197. 

Fukushima, K., Chang, Y.-H. and Kimura, Y. (2007) Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid). Macromolecular Bioscience, 7, 829-835. 

Nagahama, K., Nishimura, Y., Ohya, Y. and Ouchi, T. (2007) Impacts of stereoregularity and stereocomplex formation on physicochemical, protein adsorption and cell adhesion behaviors of star-shaped 8-arms poly(ethylene glycol)-poly(lactide) block copolymer films. Polymer, 48, 2649-2658. 

Tsuji H (2005) Poly(lactide) Stereocomplexes Formation, Structure, Properties, Degradation, and Applications. Macromol Biosci 5 569-597... 

If the compound B adds on to poly(A) without formation of covalent bonds or elimination of D, it is called complex formation. But this only rarely involves geniune chemical reaction such as, of course, complex formation by coordinate or electron-deficient bonds. Most cases involve purely physical bonds such as hydrogen bonding or hydrophobic bonding. B may also occur as a polymer in complex formation. If the complex is only formed on the basis of configurational differences in poly( A) and poly(B) with otherwise identical constitution, it is called stereocomplex formation. 

Figure 6 Schematic illustration of a preferred-handed helicity induction in achiral st-PMMA in the presence of Ceo with (S)- or (R)-PEA, memory of the induced helicity after removal of PEA, and subsequent optically active stereocomplex formation after the addition of it-PMMA, resulting from replacement of the encapsulated Ceo molecules by it-PMMA strands. Reproduced with permission from Yashima, E. Maeda, K. lida, H. etal. Chem. Rev. 2009, 109, 6102. Copyright 2009 American Chemical Society.

Table 10.2.4. Solvents classification as a function of compiexing power for PiMiMA stereocomplexes formation (after reference 26)...

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