Stereocomplex structure

PLA is well known as a degradable material at higher temperatures. However, the mechanisms of the thermal degradation are complex. To clarify and control the thermal degradation of PLA, many efforts have been made. As a result, the effects of some important factors, such as polymerization catalyst residues, chain-end structures, depolymerization catalysts, stereocomplex structure, racemization, blends with other polymers, and so on, have been clarified. Highly active and selective depolymerization cat-... 

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

PMMA added previously would depend on their structure. Indeed, when polymerization was carried out in the presence of isotactic PMMA, it influenced the formation of new macromolecules, affecting the rate of polymerization, the molecular weights of the polymer formed, and the structure of its molecules. It is natural to assume that these effects are caused by the appearance of a stereocomplex during polymerization between the polymer added beforehand and the growing macroradical. It is also characteristic that the ratio of polymerization rates in the presence and in the absence of the polymer is independent of the concentrations of monomer and polymer added, depending only on their ratio. Viscosity investigations revealed that these solutions are highly crosslinked. 

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

T. Kobayashi, M. Todoki, M. Fujii, T. Takeyama and H. Tanzawa, Permeability and structure of PMMA stereocomplex hollow fiber membrane for hemodialysis, in E. Drioli and M. Nakagaki (Eds.), Proc. Eur.-fpn Cong. Membr. Membr. Processes, Plenum, New York, NY, 1986, pp. 507-513. 

In toluene solution, syndiotactic block copolymers of methyl methacrylate (PMMA) and allyl methacrylate were formed using triphenylphosphine and triethylaluminum as initiator [76]. In acetone solution, syndiotactic poly(methyl methacrylate) forms a stereocomplex with other syndiotactic polymers. The complex formed with syndiotactic poly(allyl methacrylate), upon separation from the reaction mixture and drying had a melting point of 141.5°C by DSC thermogram. From X-ray powder patterns of this and related complexes of PMMA with other polymethacrylates, the authors postulate that a double-stranded helix may represent a model of the structure of these complexes [77]. 

Zhang, J., Sato, H., Tsuji, H., Noda, I. Ozaki, Y. (2005). Differences in the CH3"0=C interactions among poly(L-lactide), poly(L-lactide)/poly(D-lactide) stereocomplex, and poly(3-hydroxybutyrate) studied by infrared spectroscopy. Journal of Molecular Structure. Vol. 735-736, No. 14, pp. 249-257... 

Ishii D, Lee WK, Kasuya KI, Iwata T (2007) Fine structure and enzymatic degradation of poly[(R)-3-hydroxybutyrate] and stereocomplexed poly(lactide) nanofibers. 1 Biotechnol 132 318-324... 

Furthermore for the formation of a stereocomplex which blends of poly(L-lactide) and poly (D-lactide) could be found the interesting phenomena. The PLA stereocomplex was found to possess a racemic crystalline structure, where PDLA and PLLA chains were packed side hy side with a D monomer unit to L monomer unit... 

Figure 8.7 Crystal structure of PLA stereocomplex [33]. The lines between PLLA and PDLA chains were added to original figure. (Reproduced with permission from ref [33]. Copyright 1991, Taylor Francis.)...

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

Okihara, T, Tsuji, M., Kawaguchi, A. et al. (1991) Crystal structure of stereocomplex of poly(L-lactide) and poly(D-lactide). Journal of Macromolecular Science Physics, B30, 119-140. 

Hirata, M. and Kimura, T. (2010) Structure and properties of stereocomplex-type polydactic acid), in Poly(Lactic Acid) Synthesis, Structures, Properties, Processing, and Applications (eds R. Auras, L.-T. Lim, S.E.M. Selke and H. Tsuji), John WUey Sons, Inc., NJ, pp. 59-65. 

Abstract Stereocomplexes have been formed by mixing the two isotactic poly(a-methyl-a-cthyl-p-propriolactones) (PMEPL) of opposite chirality. This leads to an insoluble complex exhibiting a melting transition which is 40 C above that of the initial isotactic components. Structural differences between these samples have been determined by nuclear magnetic resonance spectroscopy and electron diffraction. It was found by NMR that the stereocomplex crystallizes in a 2 helical conformation whereas the corresponding isotactic chains exhibit a helical or extended chain conformation depending upon the method of sample preparation. Electron diffraction confirms these measurements with the determination of a,b and c dimensions of the orthorombic unit cells. 

In the present paper, differences between isotactic PMEPL and the stereocomplex are examined by solid state nuclear magnetic resonance (NMR) spectroscopy. These studies reveal new polymorphic behaviour of the isotactic polymer and differences in crystal structure which depend on tacticity. Crystal structures of the various polymorphs were also determined by electron and x-ray diffraction studies. 

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