Handbook of Solvents - George Wypych - ChemTech - Ventech!

580 S. Bistac, M. Brogly
interaction between the ester basic group of PMMA and the hydrogen acid atom of chloroform.
According to Drago’s classification, it appears therefore in Table 10.2.2 that chloroform
exhibits an acid character. 37 PMMA presents a basic character, with E b=0.68 and
C b=0.96.
The fact that an acidic solvent, such as, chloroform is able to exchange strong
acid-base interactions with PMMA ester groups is able to explain its non-complexing behavior.
Strong acid-base interactions between PMMA and chloroform molecules hinder the
aggregation of PMMA chains, reflecting greater PMMA/solvent interactions compared to
PMMA/PMMA interactions. In the presence of basic solvents, the lower PMMA/solvent
acid-base interactions lead to inter-chains associations (and consequently aggregation), energetically
favorable. Moreover, solvents, such as, acetone or THF possess a high degree of
self-association (31%, 27% respectively) and Fowkes has shown that the contribution of
solvent/solvent acid-base interactions to the heat of vaporization is proportional to the degree
of self-association. 38 Acetone and THF are therefore strongly acid-base self-associated
solvents. As a consequence, they would develop preferentially self-associations (between
solvent molecules) rather than PMMA/solvent interactions. Hence the PMMA chains aggregation
would be also favored.
Table 10.2.5. AN and DN numbers for
complexing and non-complexing
solvents (after reference 19)
AN DN, kcal/mol
DMF 16.0 26.6
Acetone 12.5 17.0
THF 8.0 20
DMSO 20.4 29.8
Benzene 8.2 0.1
Chloroform 23.1 0
Gutmann’s numbers of complexing and
non-complexing solvents are listed in Table
10.2.5.The results indicate also that chloroform is
an acidic solvent, with a high Acceptor Number
(acidic character), and a Donor Number (basic
character) equal to zero. The advantage of
Gutmann’s approach is to consider the potential
amphoteric character. As an example, benzene,
which is classified as a basic solvent by Drago,
exhibits an amphoteric character with a low AN,
but also a DN close to zero. This result could explain
that, in some cases, benzene (which is generally
a complexing solvent) is described as a
non-complexing solvent for syndiotactic
PMMA. 31 In this case, amphoteric character of
benzene favors acid-base interactions with
PMMA.
To resume this part, it is possible to correlate the complexing power of solvents towards
stereoregular PMMA with their 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.
On the contrary, chloroform is an acidic solvent, and the development of acid-base interactions
with PMMA ester groups helps to hinder the formation of chains aggregates.
However, this original view on the relationship between complexing power and solvent
acid-base character does not take into account the way of action of the solvent in terms of
steric effect and macromolecular conformations of aggregates.