Methods based on the formation of complexes of CDAs with metal cations seem
especially interesting for testing using PPAs, due both to the possibility of controlling the conformational equilibria at the pendants that will transmit further effects
to the backbone, and to the potential role that metal ions could play in the
establishment of supramolecular networks between the polymer chains (interchain
bonding), giving birth to new types of nanostructures (i.e. helical polymer–metal
complexes, HPMCs).
The results of the work based on the above hypotheses and the incorporation of
CDAs 1–3 as pendants in PPAs are presented in the next section.
2 Reversible Helical Inversion by Metal Ion Complexation
The previously described PPAs poly-(R)-1 and poly-(S)-1 containing (R)- and (S)phenylglycine methyl ester pendants (PGME) [1] were the first polymers chosen to
test the potential control of the helicity by complexation with metal cations. Due to
the presence of the stereogenic centre, each of these “enantiomeric” polymers had
shown by circular dichroism (CD) a defined, but not assigned, backbone helicity,
determined by the pendant.
Once synthesised [9], atom force microscopy (AFM) gave important insights
into the helicity and morphology of those polymers. In the first place, their helical
senses could be assigned. Thus, poly-(R)-1 adopted a right-handed helical conformation (P) and poly-(S)-1 a left-handed one (M ) in CHCl 3 (positive and negative
Cotton effects, respectively, at 375 nm), thus establishing the correlation between
the configuration of the asymmetric carbon at the pendant and the axial chirality of
the main chain.
Next, the effect of the complexation with metal cations and the consequent
formation of HPMCs were investigated. After the addition of a series of perchlorates of mono- and divalent metal cations (Li
+ , Na
+
, Ag
+ , Mg
2+ and Ba
2+ ) to the
polymers in CHCl 3 , the CD spectra showed, in all cases, that inversion of the
helicity had taken place (opposite CD signs), with Ba
2+ giving the strongest
response; M
2+ to mru (mol/mol) ratios ranging from %6 (THF) to %1 (CHCl 3 )
were typically employed. The addition of a scavenger (acetylacetone) reversed the
helicity, causing recovery of the original CD spectra (Fig. 2).
These results proved the feasibility of the working hypothesis. Both experimental (AFM, variable-temperature CD,
13 C NMR, Fourier transform infrared spectroscopy) and theoretical (discrete Fourier transform, DFT, calculations) evidence
point to the mechanistic scenario that follows.
In the absence of metal cations (i.e. Ba
2+ ), the pendants present conformational
equilibria in which a certain form (synperiplanar 1, sp1) predominates. This
conformational preference of the pendants is transmitted to the polyene backbone,
which adopts the most stable form [i.e. left-handed in the case of poly-(S)-1, Fig. 2].
When an appropriate salt [i.e. Ba(ClO 4 ) 2 ] is added, coordination takes place
between the metal cations and the two carbonyl groups (amide/ester) at the
126
F. Freire et al.
especially interesting for testing using PPAs, due both to the possibility of controlling the conformational equilibria at the pendants that will transmit further effects
to the backbone, and to the potential role that metal ions could play in the
establishment of supramolecular networks between the polymer chains (interchain
bonding), giving birth to new types of nanostructures (i.e. helical polymer–metal
complexes, HPMCs).
The results of the work based on the above hypotheses and the incorporation of
CDAs 1–3 as pendants in PPAs are presented in the next section.
2 Reversible Helical Inversion by Metal Ion Complexation
The previously described PPAs poly-(R)-1 and poly-(S)-1 containing (R)- and (S)phenylglycine methyl ester pendants (PGME) [1] were the first polymers chosen to
test the potential control of the helicity by complexation with metal cations. Due to
the presence of the stereogenic centre, each of these “enantiomeric” polymers had
shown by circular dichroism (CD) a defined, but not assigned, backbone helicity,
determined by the pendant.
Once synthesised [9], atom force microscopy (AFM) gave important insights
into the helicity and morphology of those polymers. In the first place, their helical
senses could be assigned. Thus, poly-(R)-1 adopted a right-handed helical conformation (P) and poly-(S)-1 a left-handed one (M ) in CHCl 3 (positive and negative
Cotton effects, respectively, at 375 nm), thus establishing the correlation between
the configuration of the asymmetric carbon at the pendant and the axial chirality of
the main chain.
Next, the effect of the complexation with metal cations and the consequent
formation of HPMCs were investigated. After the addition of a series of perchlorates of mono- and divalent metal cations (Li
+ , Na
+
, Ag
+ , Mg
2+ and Ba
2+ ) to the
polymers in CHCl 3 , the CD spectra showed, in all cases, that inversion of the
helicity had taken place (opposite CD signs), with Ba
2+ giving the strongest
response; M
2+ to mru (mol/mol) ratios ranging from %6 (THF) to %1 (CHCl 3 )
were typically employed. The addition of a scavenger (acetylacetone) reversed the
helicity, causing recovery of the original CD spectra (Fig. 2).
These results proved the feasibility of the working hypothesis. Both experimental (AFM, variable-temperature CD,
13 C NMR, Fourier transform infrared spectroscopy) and theoretical (discrete Fourier transform, DFT, calculations) evidence
point to the mechanistic scenario that follows.
In the absence of metal cations (i.e. Ba
2+ ), the pendants present conformational
equilibria in which a certain form (synperiplanar 1, sp1) predominates. This
conformational preference of the pendants is transmitted to the polyene backbone,
which adopts the most stable form [i.e. left-handed in the case of poly-(S)-1, Fig. 2].
When an appropriate salt [i.e. Ba(ClO 4 ) 2 ] is added, coordination takes place
between the metal cations and the two carbonyl groups (amide/ester) at the
126
F. Freire et al.