267
methacrylate-terminated siloxane macromonomer (MTSM) and tris(trimethylsiloxy)-3-methacryloxpropylsilane. These authors reported that the properties of
interest for contact lens applications were influenced and adjusted by modifying the
length of the siloxane chain in MTSM.
Iron oxide NPs (IO NPs) are increasingly used in various biomedical applications, as was recently reviewed by Iacob et al. (2019). The non-toxicity and biocompatibility of magnetic NPs can be further improved by a special coating with organic
or inorganic molecules. One of the most used techniques is based on covering the
IO NPs with a functional silica shell (Santra et al. 2001; Kyeong et al. 2015; Wu
et al. 2015). Encapsulation of hydrophobic IO NPs or a combination of these with a
hydrophobic drug (nystatin) in biocompatible siloxane surfactants made magnetic
NPs promising for biomedical applications (Racles et al. 2014c, e).
11.7 Reactive and Functional Siloxanes as Ligands
for Metals
Short siloxanes, disiloxanes or cyclosiloxanes generally commercially available,
have been chemically modified by attaching electron-donating groups able to coordinate metal ions. Both ligands as such and their complexes are of great interest in
biology, chemistry and materials science (catalysis, gas separation and storage,
luminescence, etc.).
Original and valuable siloxanes containing suitable carboxyl groups as ligands
have been prepared (Table 11.5, entries 1–6), which constitute alternatives to classic
organic dicarboxylic acids. One of them is 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX, Table 11.5, entry 1). Although initially only synthesized in the laboratory (Cazacu et al. 1999, 2019), it has now become commercial. Being a ligand with
multiple binding sites, it generally leads to coordination polymers with single metal
ions or clusters as secondary building units (SBUs) or even to their subclass of
porous materials known as metal-organic frameworks (MOFs) (Cazacu et al. 2019)
which are of great interest for both research and industry with a wide range of applications: catalysis, drug release, gas sorption, luminescence, magnetism, etc. (Zhu
et al. 2017). The high length of the siloxane bond and the flexibility of the siloxane
angle reducing the conformational stiffness lead to the formation of coordination
polymers with new structural and topological forms (Davies et al. 2008). However,
since it is very flexible, in general, to obtain a well-defined porous crystalline structure, it is necessary to add a co-ligand to increase stiffness and stabilize the network.
It has been found that the acid itself forms supramolecular structures by means of
H-bonds that can be isolated in the crystalline state with 1,2-(4-bipyridyl)ethylene,
4,4′-bipyridine (Racles et al. 2013b) and 4,4′-azopiridine (Vlad et al. 2013a). The
architecture of the metal coordination structures formed is influenced by the coligand, the particularities of the metal ion and the reaction conditions. By reacting
this diacid with Zn(II) perchlorate in the presence of 4,4′-azopyridine, a 2D MOF is
11 Reactive and Functional Silicones for Special Applications
methacrylate-terminated siloxane macromonomer (MTSM) and tris(trimethylsiloxy)-3-methacryloxpropylsilane. These authors reported that the properties of
interest for contact lens applications were influenced and adjusted by modifying the
length of the siloxane chain in MTSM.
Iron oxide NPs (IO NPs) are increasingly used in various biomedical applications, as was recently reviewed by Iacob et al. (2019). The non-toxicity and biocompatibility of magnetic NPs can be further improved by a special coating with organic
or inorganic molecules. One of the most used techniques is based on covering the
IO NPs with a functional silica shell (Santra et al. 2001; Kyeong et al. 2015; Wu
et al. 2015). Encapsulation of hydrophobic IO NPs or a combination of these with a
hydrophobic drug (nystatin) in biocompatible siloxane surfactants made magnetic
NPs promising for biomedical applications (Racles et al. 2014c, e).
11.7 Reactive and Functional Siloxanes as Ligands
for Metals
Short siloxanes, disiloxanes or cyclosiloxanes generally commercially available,
have been chemically modified by attaching electron-donating groups able to coordinate metal ions. Both ligands as such and their complexes are of great interest in
biology, chemistry and materials science (catalysis, gas separation and storage,
luminescence, etc.).
Original and valuable siloxanes containing suitable carboxyl groups as ligands
have been prepared (Table 11.5, entries 1–6), which constitute alternatives to classic
organic dicarboxylic acids. One of them is 1,3-bis(carboxypropyl)tetramethyldisiloxane (CX, Table 11.5, entry 1). Although initially only synthesized in the laboratory (Cazacu et al. 1999, 2019), it has now become commercial. Being a ligand with
multiple binding sites, it generally leads to coordination polymers with single metal
ions or clusters as secondary building units (SBUs) or even to their subclass of
porous materials known as metal-organic frameworks (MOFs) (Cazacu et al. 2019)
which are of great interest for both research and industry with a wide range of applications: catalysis, drug release, gas sorption, luminescence, magnetism, etc. (Zhu
et al. 2017). The high length of the siloxane bond and the flexibility of the siloxane
angle reducing the conformational stiffness lead to the formation of coordination
polymers with new structural and topological forms (Davies et al. 2008). However,
since it is very flexible, in general, to obtain a well-defined porous crystalline structure, it is necessary to add a co-ligand to increase stiffness and stabilize the network.
It has been found that the acid itself forms supramolecular structures by means of
H-bonds that can be isolated in the crystalline state with 1,2-(4-bipyridyl)ethylene,
4,4′-bipyridine (Racles et al. 2013b) and 4,4′-azopiridine (Vlad et al. 2013a). The
architecture of the metal coordination structures formed is influenced by the coligand, the particularities of the metal ion and the reaction conditions. By reacting
this diacid with Zn(II) perchlorate in the presence of 4,4′-azopyridine, a 2D MOF is
11 Reactive and Functional Silicones for Special Applications