Poly isotactic polymer stereocomplexes

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 ... 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

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]. 

In bulk we may not forget the so called stereocomplexes between stereoregular polymers. In 1961 Watanabe reported that mixing syndiotactic poIy(methylmetha-crylate) (j-PMMA) and isotactic poly(methylmethacrylate) (/t-PMMA) leads to the... 

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. 

Similarly, as shown in Figure 6, the morphology of poly-R/poly-S binary mixtures of PMEPL is controlled by the formation of the stereocomplex at high temperatures, followed by the crystallization of the excess polymer at lower temperatures. It is believed that the excess polymer is trapped between the spherulite lamellae of the stereocomplex, and at their boundaries. Large spherulites are then observed for the racemate sample, and for all binary mixtures in which the stereocomplex can be formed, but very small spherulites are found for pure isotactic chains. 

The stereocomplex formed from complementary strands of isotactic and syndiotactic poly(methyl methacrylate)s (it- and st-PMMAs) with an it st-stoichiometry of 1 2 represents another class of unique, polymer-hased helical supramolecules suitable for AFM studies. Although this stereocomplex has heen known for half a century, the molecular basis of the stracture and the mechanism of complex formation are still under dehate. In 1989, Schomaker and Challa proposed a double-stranded helix model for the stereocomplex based on the wide angle X-ray scattering (WAXS) analysis of the stretched fiber, which is composed of a 9i it-PMMA helix (nine repeating MMA units per turn) strrrormded hy an 18i st-PMMA helix, resulting in a double-stranded helix with a helical pitch of 1.84 nm. To verify the stmcture of the PMMA stereocomplex by the AFM technique, Kumaki et al. prepared a mixed monolayer of it- and... 

The presence of a third transition between the Tg and Tm in the case of the isotactic poly(methyl chloroacrylate) has been commented on previously and evidence has been presented for the assignment of this phenomenon to the glass transition of a stereocomplex of isotactic and syndiotactic polymer molecules (3). It is not possible to confirm or reject this hypothesis on the basis of available evidence. 

The relaxation labelled a for the isotactic poly(methyl o chloroacrylate) in Table II is thought to correlate with the transition observed by DSC at 150° and is tentatively assigned to motions accompanying the glass transition of the "stereocomplex" postulated to occur in these polymers, composed of isotactic and syndiotactic chains. 

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... 

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-... 

SEC-NMR for the analysis of polymethacrylates. He studied isotactic PMMA with different molar masses to analyze the end-groups and the number average molar mass as well as the CCD of (methyl methacrylate)-co-(butyl methacrylate) copolymers. These polymers were studied at slow flow rates in fully deuterated solvents. Blends of isotactic and syndiotactic PMMA were also studied regarding the stereocomplexation in non-deuterated solvents. ° Kramer et reported about SEC-NMR for the chemical composition analysis of poly... 

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