helicity adopted by the polymer and the associated chiroptical response is determined by the valence of the metal cation. It is should be noted that the external
stimulus is achiral in this amplification process.
The versatility of the polymer for helix sense selection and chiral amplification
was demonstrated in a number of experiments performed in CHCl 3 . For instance,
once the chiral amplification was obtained with a divalent cation (i.e. cation to mru
ratio of 0.1), the helicity could be reversed by addition of a monovalent cation at a
higher concentration (i.e. cation to mru ratio of 1.0). The opposite addition
sequence (first monovalent, second divalent) acted in a similar way: the axial
chirality induced by the cation in excess was predominant. CD spectra analogous
to those obtained by step-by-step additions were recorded when mixtures of monoand divalent cations were added simultaneously.
Cation scavenger resins established the reversibility of the above processes, and
the recovered polymers were apt to be reused in the formation of new HPMCs.
5 Nanostructures of Helical Polymer–Metal Complexes
During the studies on HPMCs (see previous sections), the possibility of the metal
ions acting as intercalating agents between polymer chains, and thus establishing
interchain linkages, was considered. It was observed that HPMCs from the polymers bearing MPA pendants, poly-(R)-2 and poly-(S)-2, could originate
nanostructures such as nanospheres with tuneable size and helicity when perchlorates of divalent metal ions [Mg
2+ , Ca
2+ , Mn
2+ , Co
2+ , Ni
2+ , Ba
2+ , Hg
2+ , Pb
2+ ] were
added in the presence of donor solvents (i.e. THF, acetone) [20].
In general, stable and homogeneous spherical particles with very good polydispersity index (PDI) were formed in solution (in many cases stable for more than
one month) in sizes ranging from 60 to 200 nm [i.e. poly-(R)-2 and Ca
2+ in THF, at
a M
2+ to mru (mol/mol) ratio of 1.0:1.0, generated 100 nm nanospheres; polymer
concentration was 0.1 mg/mL]. It was found that larger nanostructures were
unstable once generated: progressive aggregation phenomena led to precipitation
of the HPMC material.
A very interesting characteristic of the HPMC nanospheres was that their size
could be easily modulated. Their dimensions depend on the following three factors:
1. The solvent system: Donor solvents such as THF allowed better control of the
particle size because the metal cations were partially coordinated to solvent
molecules and thus prevented collapse of the nanostructures. In non-donor
solvents (i.e. CHCl 3 ), the cations acted as crosslinking agents that eventually
led to HPMC-insoluble materials. This problem could be avoided by adding
donor co-solvents (i.e. MeOH, acetone) that stopped both the growth and the
collapse of the nanospheres.
2. The element forming the cation: The M
2+ to mru (mol/mol) ratio needed to form
nanospheres of a given size was characteristic for each metal ion. For example,
Helical Polymer–Metal Complexes: The Role of Metal Ions on the Helicity and. . .
133
stimulus is achiral in this amplification process.
The versatility of the polymer for helix sense selection and chiral amplification
was demonstrated in a number of experiments performed in CHCl 3 . For instance,
once the chiral amplification was obtained with a divalent cation (i.e. cation to mru
ratio of 0.1), the helicity could be reversed by addition of a monovalent cation at a
higher concentration (i.e. cation to mru ratio of 1.0). The opposite addition
sequence (first monovalent, second divalent) acted in a similar way: the axial
chirality induced by the cation in excess was predominant. CD spectra analogous
to those obtained by step-by-step additions were recorded when mixtures of monoand divalent cations were added simultaneously.
Cation scavenger resins established the reversibility of the above processes, and
the recovered polymers were apt to be reused in the formation of new HPMCs.
5 Nanostructures of Helical Polymer–Metal Complexes
During the studies on HPMCs (see previous sections), the possibility of the metal
ions acting as intercalating agents between polymer chains, and thus establishing
interchain linkages, was considered. It was observed that HPMCs from the polymers bearing MPA pendants, poly-(R)-2 and poly-(S)-2, could originate
nanostructures such as nanospheres with tuneable size and helicity when perchlorates of divalent metal ions [Mg
2+ , Ca
2+ , Mn
2+ , Co
2+ , Ni
2+ , Ba
2+ , Hg
2+ , Pb
2+ ] were
added in the presence of donor solvents (i.e. THF, acetone) [20].
In general, stable and homogeneous spherical particles with very good polydispersity index (PDI) were formed in solution (in many cases stable for more than
one month) in sizes ranging from 60 to 200 nm [i.e. poly-(R)-2 and Ca
2+ in THF, at
a M
2+ to mru (mol/mol) ratio of 1.0:1.0, generated 100 nm nanospheres; polymer
concentration was 0.1 mg/mL]. It was found that larger nanostructures were
unstable once generated: progressive aggregation phenomena led to precipitation
of the HPMC material.
A very interesting characteristic of the HPMC nanospheres was that their size
could be easily modulated. Their dimensions depend on the following three factors:
1. The solvent system: Donor solvents such as THF allowed better control of the
particle size because the metal cations were partially coordinated to solvent
molecules and thus prevented collapse of the nanostructures. In non-donor
solvents (i.e. CHCl 3 ), the cations acted as crosslinking agents that eventually
led to HPMC-insoluble materials. This problem could be avoided by adding
donor co-solvents (i.e. MeOH, acetone) that stopped both the growth and the
collapse of the nanospheres.
2. The element forming the cation: The M
2+ to mru (mol/mol) ratio needed to form
nanospheres of a given size was characteristic for each metal ion. For example,
Helical Polymer–Metal Complexes: The Role of Metal Ions on the Helicity and. . .
133