Aggregation of stereoregular PMMA

2 EFFECT OF POLYMER/SOLVENT INTERACTIONS ON AGGREGATION OF STEREOREGULAR PMMA 

Strongly complexing solvents CCI4, acetonitrile, DMF, DMSO, THF, toluene, acetone 

Weakly complexing solvents benzene, o-dichlorobenzene, dioxane 

Several works have shown that the aggregation of isotactic and syndiotactic chains leads to the formation of stereoeomplexes 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 surroxmded 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 slructure with solvent molecules included in the cavities of inner- and inter-helices. Isotactic PMMA self-aggregates also exhibit conformational helix structures. 

These results prove that the presence of a complexing solvent leads to the formation of well-ordered structure made of paired PMMA chains. 

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Toluene, THF and acetone are described as basic solvents. The residual solvent molecules can therefore only weakly interact with the acrylate groups. Interactions between chains are then favored and self-associations between PMMA chains appear. The rotation of the aery late groups becomes more difficult compared to the bulk PMMA, probably due to self-aggregation of some PMMA chains (even if the studied polymer is not stereoregular). 

To resume this part, it is possible to eorrelate the complexing power of solvents towards stereoregular PMMA with flieir acid-base character. It appears that complexing solvents exhibit basic character, like PMMA. Moreover, some of them are strongly self-associated. Solvent/solvent and chain/chain interactions are consequently favored, leading to the formation of PMMA aggregates. 

These works have shown that acid-basic character of solvent is able to have a major influence on polymer film properties. Acid-base interactions between stereoregular PMMA and some solvents can lead to the formation of aggregates, which modify the solution properties. Elsewhere, residual solvent molecules trapped in solid conventional PMMAfilmshave an effect on the polymer chains mobility, depending on the acid-base character of the solvent. Both solvent effects, due to acid-base interactions, are able to modify the wetting and the adhesion properties ofthe films, but also the mechanical and the durability behavior of the final film. It is then necessary to take into account, in the choice of a solvent, its acid-base properties, in addition to its solubility parameter, especially for PMMA. REFERENCES... 

It is well-known that syndiotactic poly(methyl methacrylate) (sPMMA) could crystallize in the presence of some solvents (35, 34). This phenomenon is strongly dependent on the PMMA stereoregularity and the solvent used. The aggregation takes place in some solvents, such as o-dichlorobenzene, butyl acetate and o-xylene, even in dilute solutions as result of interactions between long parallel syndiotactic sequences. 

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