Stereocomplexes, PMMA

Sterocomplexation between two layers can occur between two stereoregular polymers, as in the case of multilayers obtained with isotactic and syndiotactic PMMA and enantiomeric (l- and D-) polylactides. This type of interaction for LbL assembly has not been extended to many stereoregular materials however, because of the advantages of stmctural regularity observed within stereocomplexed PMMA LbL films, they have found the incorporation of other materials, such as enzjunes and antibodies, within the films important. 

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. 

E. Schomaker in The Process of Stereocomplexation between it- and st- PMMA, Thesis, State University Groningen, 1988. 

Ryan and Fleischer, Jr. (21) found that if isotactic and syndiotactic poly (methyl methacrylate) (PMMA) are mixed, they form a stereocomplex. It might have been supposed that the course of polymerization of methyl methacrylate in the presence of preformed stereoisomers of... 

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 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-... 

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... 

Fig. 34 a, b. Effect of the mixing ratio of isotactic po y(methyi methacrylate) (iso-PMMA) and syndiotactic PMMA cm the melting temperature (Tm) and specific area of the melting endotherms299, (a) Relationship between Tm and crystallization temperature (Tc) for different mixtures of stereoregular PMMA [iso-PMMA]/[synd-PMMA] 1/2, O 1/1, C 2/1, 4/1, (b) Specific areas of melting endotherms of stereocomplexes with different [iso-PMMA]/[synd-PMMA] ratio in bulk mixtures. Ciystallization time O 4 h, O 16 h, 4 days, 2 weeks... 

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-... 

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...

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... 

When matrix polymerization of methyl methacrylate monomer was performed in the presence of iso-PMMA or of the stereocomplex ([iso-PMMA]/ [synd-PMMA] = 1/1), the tan 6 peaks of the a-process (caused by the micro-Brownian motions of main chain) and the / -process (caused by the motions of the ester side chains of PMMA) shifted to temperature ranges which were higher by 5-10°C and 30-40 °C, respectively, as compared with synd-PMMA312. From these results, Tanzawa et al. concluded that the stereospecific association drastically restricted the motion of the ester side chains of the two stereoregular PMMAs. In addition, the mutual interlocking of the side chains of PMMA also hindered, to a certain extent, the motion of the main chains of PMMA. Furthermore, they studied the matrix polymerization of methyl acrylate (MA) and ethyl acrylate (EA) in the presence of stereoregular PMMA and found ... 

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-... 

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... 

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]. 

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