Stereocomplex solvent

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 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-... 

Stereocomplexes are obtained in suitable dilute and concentrated solutions and in bulk. The formation of the complexes is strongly affected by the kind of solvent used. The solvents can be divided into three classes299 ... 

Table 19. Classification of solvents according to their ability of promoting the formation of stereocomplex between iso-PMMA and synd-PMMA299)...

Fig. 33a-d. Solvent effect on the formation of the stereocomplex of isotactic polyfmethyl methacrylate) (iso-PMMA) with syndiotactic PMMA (synd-PMMA)300. (a) In chloroform, (b) in benzene, (c) in dimethylformamide, (d) in acetone... 

A secondary stereocomplex gel with the ratio [iso-PMMA]/[synd-PMMA] = 1/1 obtained from o-xylene solution showed an endothermic peak at 110 °C which was attributed to the melting point of the stereocomplex. The degree of crystallization was about 5-6%. Since this gel contained over 80% solvent, micro-Brownian motions of PMMA chains in the amorphous region were relatively unhindered. When the PMMA gel was cooled rapidly after melting above its Tm (i.e. above 145°C), first the loss modulus (c") changed relative to the viscosity of the system whereas the storage modulus began to increase after some time. This result confirms the assumption that first the polymer chains cannot be cross-linked but are weakly associated with one other, and then a... 

Moreover, the solvent and the blending mode are also parameters which strongly influence the stereocomplexation [36]. 

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... 

Electrospinning of PLA and copolymers [12] and the PLA stereocomplex [145,146] has been used to prepare nanofibres or nanomats. Similar to the effect of pores on the mechanical properties of PLA-based materials, the formation of nanomats effectively lowers the E and elevates the 8b value. The morphology of the fibres and nanomats and the mechanical properties are affected by the applied voltage, the effluent rate and concentration of the solution, the type of solvent and so on (Figure 8.12 [147]). 

X 10 g mol ) crystallize in the a -form during solvent evaporation this reorganizes into the a-form at 120 °C, melts at 180 °C and further recrystallizes into stereocomplex crystallites [99],... 

The above-mentioned chemical catalytic routes lead to racemic AHA mixtures. For the direct use of LA (or its esters) as a solvent or platform molecule for achiral molecules like acrylic acid and pyruvic acid, stereochemistry does not matter. The properties of the polyester PLA, the major application of LA, however, suffer tremendously if d and l isomers are built in irregularly [28]. This is exemplified by atactic PLA, made from racemic LA, which is an amorphous polymer with low performance and limited application. However, when l- and D-lactic acid are processed separately into their respective isotactic L- and d-PLA, as discovered by Tsuji et al., a stereocomplex is formed upon blending these polymers. This polymer exhibits enhanced mechanical and thermal properties [28, 164]. A productive route to D-Iactic acid is, however, missing today. If the chemocatalytic routes to LA are to become viable, enantiomer resolution of the racemate needs to be performed. Given separation success, a cheap source of o-lactic acid will be unlocked immediately, providing an additional advantage over the fermentation route (cfr. Table 2). 

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

Several works have shown that the aggregation of isotactic and syndiotactic chains leads to the formation of stereocomplexes for which the iso/syndio stoichiometry is found equal to 1/2, probably with a structure composed of a double-stranded helix of a 30/4 helicoidal isotactic chain surrounded by a 60/4 helicoidal syndiotactic chain. Syndiotactic PMMA self-aggregates exhibit similar structures, with conformations close to extended chains. Experimental data indicate that, in self-aggregated syndiotactic PMMA in solution, some of the ester groups are close in contact, probably in a double helix structure with solvent molecules included in the cavities of inner- and inter-helices. Isotactic PMMA self-aggregates also exhibit conformational helix structures. 

PMMA stereocomplexes and self-aggregates exhibit some similar aspects, particularly the complexing power of the solvent or the helical structure of paired chains. 

Association phenomena in dilute solutions of atactic (a) and stereoregular (i,s) PMMA in suitable solvents have been extensively studied in the past. The anomalies observed even in dilute solutions were explained by intermolecular association of complementary sequences of the stereoregular polymer chains, so-called stereocomplex formation. Besides the well known stereocomplex, remarkable effects in solutions of either i- or s-PMMA alone led to the assumption that associated structures can also be formed as a consequence of interactions between sequences of equal tacticity. It has been found that the solvent exhibits a striking influence on the association of PMMA. This paper deals predominantly with rheological investigations of dilute solutions of a-, i- and s-PMMA and their mixtures, as well as investigations on a stereoblock polymer. Results of calorimetric and electron microscopic investigations will be taken into account. 

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. 

It is known for some time that in suitable solvents, mixtures of syndiotactic and isotactic PMMA form stereocomplexes. The parameters that govern stereocomplex formation have been extensively studied by Challa, Spevacek and others and were recently reviewed by Spevacek.4 Stereocomplex formation is believed to involve the formation of a type of double helix in which the larger (60/4) pitched syndiotactic PMMA helix is coiled around the smaller (30/4) pitched isotactic helix.5 Formation of the complex is exothermic ( -10.2 kJ/base mole) and is believed to be due to a sterically optimal fit between the two helices. Stereocomplex formation has been shown to be reversible both in solution and in solids. An approximate decomposition ("melting ) temperature of 200°C has been demonstrated in solids.5... 

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