Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.2 Effect of polymer/solvent acid-base interactions 579
vor the formation of aggregates and non-complexing solvents hinder the formation of aggregates.
Table 10.2.4 presents the solvents classification as a function of complexing
power for PMMA stereocomplexes formation.
Table 10.2.4. Solvents classification as a function of complexing power for PMMA
stereocomplexes formation (after reference 26)
Strongly complexing solvents CCl4, acetonitrile, DMF, DMSO, THF, toluene, acetone
Weakly complexing solvents benzene, o-dichlorobenzene, dioxane
Non-complexing solvents chloroform, dichloromethane
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, 28,29 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. 30 Syndiotactic
PMMA self-aggregates exhibit similar structures, with conformations close to extended
chains. 31 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 32 with
solvent molecules included in the cavities of inner- and inter-helices. 33 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.
A high degree of stereoregularity is needed to allow the formation of aggregates.
Syndiotactic and isotactic sequences lengths have to be larger than a critical length in order
to be involved in paired association. As the aggregation power of the solvent increases, the
critical sequence length decreases.
Many authors explain aggregation as resulting from interactions between PMMA
chains, probably through their ester functional groups. 34 The incidence of the nature of the
solvent in the aggregate formation is not yet totally clarified in the literature. Apparently,
there is no obvious relationship between the solvent polarity and the ability to induce aggregation
of PMMA chains.
10.2.2.2 Relation between the complexing power of solvents and their
acid-base properties
A tentative explanation of the effect of solvent has been proposed recently by using the
Lewis acid-base concept. 35 The acid or basic character of a given solvent was studied simultaneous
with its strength of complexation in order to establish a relationship between both
parameters.
PMMA stereocomplexes and self-aggregates exhibit some similar aspects, particularly
the complexing power of the solvent or the helical structure of paired chains.
PMMA is classified as a basic polymer (electron donor, according to the Lewis concept),
due to the presence of ester functional groups, where the carbonyl oxygen atom is the
basic site. 36 PMMA is therefore able to exchange strong acid-base interactions with an
acidic solvent, such as, chloroform, which is also a non-complexing solvent. According to
Fowkes works, PMMA and chloroform can form acid-base complexes, resulting from the