Poly Stereo complex

The synthesis of monodisperse high molecular weight syndiotactic poly(methyl methacrylate) mentioned above was possible at a polymerization temperature as low as — 95 °C [3]. However, methods to synthesize it at a relatively high temperature or to synthesize its isotactic counterpart remain to be found. When the latter is made available, the synthesis of stereo-complexes of poly(methyl methacrylate) will become the focus of considerable interest. 

NMR spectroscopy can be also used in investigations of polymer aggregates in solution, gels etc, i.e. in systems which are between liquids and solids. As an example, studies of the formation and structure of a stereo-complex of poly(methyl methacrylate) (PMMA) by Spevacek and Schneider (1974, 1975) can be mentioned. The formation of ordered associated structures of the stereo-complex upon mixing solutions of isotactic and syndio-tactic PMMA manifests itself by the broadening of all polymer lines and results in disappearance of HR-NMR signals wide-lines then can be detected. 

Fig. 6. Preparation of stereo-complex between isotactic and syndiotactic poly(MMA)...

The latter form can be prepared at a high draw ratio and a high drawing temperature [28]. The 7-form is formed by epitaxial crystallization [29]. It has been observed that a blend with equivalent poly(L-lactide) PLLA and poly(D-lactide) PDLA contents gives stereo-complexation (racemic crystallite) of both polymers. This stereocomplex has higher mechanical properties than those of both PLAs, and a higher melting temperature of 230°C. The literature reports different density data [4] for PLA, with most values for the crystalline polymer around 1.29 compared with 1.25 for the amorphous material. 

Tsuji, H. and Ikada, Y. (2009) Stereo-complexation between enantiomeric poly(lactide)s. Biodegrad. Polym. Blends Compos. Renewable Resour., 165—190. 

S.H. (2007) Crystal density and heat of fusion for a stereo-complex of poly(L-lactic acid) and poly(D-lactic acid). J. 

PLA homo-stereo-complexation can take plaee both in enantiomerie mixture and in stereo-bloeks of PLA-based polymers. The key parameter affecting stereo-complexation is the mixing ratio of PLLA to PDLA sequenees, the equimolar mixture yielding the optimal degree of stereo-eomplexation. PLA stereo-complex is usually prepared from eo-preeipitation of PLLA and PDLA in solution or through eooling from a melted mixture of both poly-mers. " Hetero-stereo-eomplexation between PLA and other optically active materials was also reported. ... 

Interestingly, PLA stereo-complex can also be obtained during in vitro degradation of poly(DL-lactide), which is an intrinsically amorphous polymer. In fact, poly(DL-lactide) contains isotactic l-LA and d-LA se-... 

H. Tsuji, Poly(lactide) stereo complexes Formation, structure, properties, degradation, and applications, Macromol Biosci., 5 (7), 569-597,2005. 

Tsuji, H., 2000. In vitro hydrolysis of blends from enantiomeric poly (lactide)s Part 1. WeU-stereo-complexed blend and non-blended films. Polymer 41, 3621—3630. 

It has been attempted to perform template polymer syntheses without using biological sources. Concepts focus on the formation of a complex between monomer molecules and a present macromolecule [4,480], This way the monomer will get preorganized and the polymerization is supposed to follow a zip mechanism controlled by the length and the configuration of the template polymer, offering replication of the molecular weight and control of the stereo structure. Polymerization of acrylic acid in the presence of poly(ethyleneimine), N-vinylimidazole/ poly(methacrylic acid) or acrylonitrile with poly(vinylacetate) have been described [469,470,471,472,473]. Recently the preparation of solid polyelectrolyte complexes from chitosan and sodium-styrenesulfonate has been reported [481]. 

Figure 7.71 illustrates the change of the glass transition for poly(styrene-co-acrylonitrile) as a function of the run number, defined in Fig. 7.70. The branches of the Barton equation (mA > mg, and mA < mB) are symmetric to the line which extends to the alternating copolymer. This treatment of T can also be applied to triads, and one can use the other equations of Fig. 7.69 as base, but with an increase in complexity. Stereo-specific copolymers consisting of meso and racemic dyads can be treated if the samples of different tacticity have different glass transitions [32].

N.m.r. has been used to probe various aggregation phenomena. Heatley and Begum observed that mieellization of styrene-butadiene-styrene tri-block copolymers in non-solvents for butadiene is accompanied by marked hindrance to butadiene mobility. Bovey et a/. have shown that C Ti s for poly(vinylidene fluoride) and PMMA are the same in a mixed solution as in single solutions. The unusual compatibility of these two polyn rs therefore does not arise from complex formation. Self-association of stereo-r ular PMMA s in dilute solution has been studied. In toluene, isotactic PMMA is 10—17% associated at 27 C whereas a highly syndiotactic sample is 76% associated. 

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-... 

In 1928, Otto Diels and Kurt Alder reported that ring forming reactions are possible between various dienes (47t-compo-nents) and double bonded groups (27t-components) [24]. The Diels-Alder reaction is an example of a process that can be used to increase molecular complexity in a rapid, stereo-specific, and stereoselective manner. It has found extensive use in the syntheses of (poly)cyclic structures and continues to be one of the most reliable methods for the introduction of multiple stereogenic centers [12]. 

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