Stereocomplex composition

As mentioned earlier, the stereocomplex composition of the lactide produced is dependent on the initial crude lactic acid... 

By studying mixtures of L- and D-LA of varying composition, Spassky et al. have demonstrated that (256) yields PLA containing long isotactic sequences, with a ratio of homo cross propagation of 2.8.796 Hence, an 80/20 L D mixture when polymerized to 70% conversion displayed an optical purity of 87%. Even at the relatively low optical purity of 65/35 L D, isotactic block lengths of >12 repeat units were reported. Achiral (259) converts rac-LA into highly isotactic PLA a Tm of 192 °C indicates that the chains of P(L-LA) and P(D-LA) form a stereocomplex.792 797-799... 

The analytical methods of the determination of stereocomplexes are the same as those mentioned previously, especially the utilization of the hydrodynamic properties of the complex solution and the physicochemical properties of the products. In polar organic solvents, the complex is obtained as precipitate and its composition is 1/2 according to the molar ratio of the units [iso-PMMA]/[synd-PMMA]. In contrast, in non-polar aromatic solvents, the com-... 

Fig. 36. Composition of the stereocomplex in the system of isotactic poly-(methyl methacrylate) (iso-PMAA) and syndiotacticPMMA (synd-PMMA)376). Time after mixing O 1 h, 4h,A 8h...

The permeabilities of the stereocomplex membranes to water and NaCl are shown in Fig. 38 as a function of the water content or the composition (isotactic mol%). The permeability ratios of urea to NaCl are summarized in Table 20315). The dependence of water permeability on the water content is very strong whereas that of the permeability to solutes (in this case NaCl) is weak. The water permeability of the PMMA membrane is extremely high compared to that of cuprophan whereas its permeability to solute is almost the same. This characteristic permeability accounts for the effect of the hyd-... 

One of the cases of such complementarity may be the atactic polymer which can be considered as a copolymer of isotactic and syndiotactic links. Indeed, the composition and properties of the polyccanplex depend on the chain stereoregularity, for example in the case of poly(methyl methacrylate) stereocomplexes and poly(ethylene glycol) with isotactic or atactic PAA complexes ... 

Tsuji, H. and Ikada, Y. (2008) Stereocomplex between Enantiomeric Poly(lactide)s, in Biodegradable Polymer Blends and Composites from Renewable Resources (ed. L. Yu), Wiley, NJ, pp. 165-190. 

If stereocomplex formation were to occur only between I-PMMA and S-PMMA, then the mixing of isotactic-atactic-isotactic (lAtl)- and syndiotactic-atactic-syndiotactic (SAtS)-PMMA stereoblock polymers in complexing solvents should result in stereocomplexes imbedded in an atactic matrix (Figure 2). Here the stereocomplexes would act as physical cross-links providing a network structure from a single chemical composition. From an application viewpoint, such a material should possess interesting properties compared to conventional PMMA. 

The equilibrium melting temperature of the stereocomplex remains the same over the whole range of composition, except at high poly-R excesses where it decreases slightly. 

At the same time, the crystallization of the excess poly-R is observed only at enantiomeric excesses between 63% and 100%. At other compositions, the crystallization of the excess poly-R is hindered by the crystallization of the stereocomplex. Indeed, the crystallization of the stereocomplex occurs first, leaving an excess of uncrystallized Isotactic pol)nner which is dispersed in a rigid matrix of crystalline material. The crystallization of the excess pol3nner is then impeded when the excess is lower than 63% at... 

The hydrolytic degradation behaviour of aliphatic polyesters depends on many factors such as matrix morphology, stereocomplexation, PLA configuration, chemical composition, size and porosity, dmg loading, etc. (Li, 1999,2006 Li and Vert, 1999). The influence of the factors on the degradation is discussed in the following sections. 

Keywords modification, copolymerization, stereocomplex, blending, composites, additives, modifiers, mechanical properties, crystallization... 

Table 3. Effects of Composition and Annealing Temperature on the AH of Stereocomplex Decomposition in Mixtures of PBD-S-PMMA and I-PMMA. 

The majority of compositionally different poly(meth)acrylates are immiscible. Nevertheless, there are some examples noted in the literature of miscible combinations. One of the most interesting and well-studied cases involves isotactic PMMA (iPMMA) with syndiotactic PMMA (sPMMA).[149] The miscibility observed may be expected, but the formation of a stereocomplex offered an interesting system to study. The tacticity of PMMA blends with poly(vinyl pyrrolidone) (PVP) affected the blend phase behavior as atatic and syndiotactic PMMA were miscible with PVP, but isotactic PMMA was phase sepa-rated.[150,151] Miscibility with upper critical solution temperature (UCST) behavior was observed for PnBA blends with poly(propylene glycol) (linear and three arm star) oligomers.[152] Poly(vinyl buty-ral)/PMMA blends were found to be phase separated at high MW PMMA.[153] At low MW PMMA (2000-5000 g/mol), miscibility with UCST behavior was observed. The PMAA/PVAc miscibility was... 

The vast majority of compositionally different poly(meth)acrylates are immiscible. However, there are examples noted in the literature of miscible combinations. One of the more interesting and well-studied cases involves isotactic PMMA (iPMMA) with syndiotactic PMMA (sPMMA) [780-782 ]. The miscibility observed may be expected, but the formation of a stereocomplex offered an interesting system to study. 

Besides, dextran-graft-poly(L-lactide) (Dex-g-PLLA) and dextran-graft-poly(D-lactide) (Dex-g-PDLA) amphiphilic copolymers have been synthesized with well-defined compositions in a dilute aqueous solution."" Such a system displayed tuneable degradation properties by varying the number of grafted PLA chains as well as applying stereocomplexation. 

In some cases, the template has the same chemical composition as daughter polymer. In the first case with the polymerization of methyl methacrylate (MMA) onto isotactic poly (methyl methacrylate) (PMMA), it was found that daughter polymer is preferentially syndiotactic. This interesting fact was explained by earlier known possibility of stereocomplex formation between iso- and syndiotactic PMMAs. Moreover, very interesting kinetic effects were observed in this system. Comparison of results obtained for blank and template... 

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