Chemistry Reference
In-Depth Information
respectively. Block copolymer 2 utilizes a thymine and a nitrile functional unit cova-
lently linked to the polymer side chains with 2,6-diaminopyridine with palladated
SCS pincer recognition units being used as the complementary recognition units.
The Weck group demonstrated for these systems that the degree of polymerization,
block copolymerization, block copolymer composition, and metal coordination
have no profound effect on the strength of the hydrogen bonding interactions
along the polymer backbones. This suggests that the metal coordination and hydrogen
bonding are also orthogonal to each other in block copolymers.
Next, Nair et al. (2006) investigated the effect of the nature and placement of the
hydrogen bonding group either covalently attached to the polymer backbone or used
as a complementary recognition unit. They found that the placement of the hydrogen
bonding group had a limited impact on both the strength of the hydrogen bonding
interactions and the solubility of the copolymers. For example, the hydrogen
bonding unit attached to the polymer backbone is switched in block copolymers 1
and 2 from 2,6-diaminopyridine for copolymer 1 to thymine for copolymer 2.
Nevertheless, both terminal hydrogen bonding units are complementary to each
other and consist of identical ADA-DAD units; hence, it is expected that the associ-
ation constant (K
a
) values for all polymers will be similar. However, the K
a
values of
all block copolymers based on thymine are significantly lower (about 10-30%) than
those based on 2,6-diaminopyridine, which can be attributed to the dimerization of
the thymine groups attached to the polymer backbone. In contrast, block copolymers
based on 2,6-diaminopyridine have a low propensity to dimerize, resulting in higher
association constants.
The effect of metal placement along the copolymers was also investigated.
Diblock copolymer 1 is based on the SCS palladated pincer complex whereas
diblock copolymer 2 is based on nitrile groups. The nitrile functionalized polymers
in essence act as a “macroligand” that can coordinate with the small molecule Pd
pincer, whereas the covalent functionalization of the pincer complex onto the
polymer can be seen as a “polymeric metal center” that needs to be activated prior
to functionalization. When using an appropriate solvent, quantitative metal coordi-
nation took place and no interference of the hydrogen bonding moieties during the
metal coordination steps were observed for either case. These studies proved that
the hydrogen bonding and metal coordination are orthogonal and independent of
the composition and the macromolecular architecture, whereas the placement of the
functional groups covalently attached to the polymer or as a small molecule affects
the strength of the hydrogen bonding interactions in a limited manner.
5.3.2. Combination of Hydrogen Bonding and Coulombic Charges
The Weck research group reported the combination of hydrogen bonding
and coulombic interactions in the same system (Nair et al. 2008). Random
polynorbornene-based copolymer 3 containing both terminal hydrogen bonding
and coulombic sites along the polymer side chains was synthesized via ROMP.
The charged coulombic sites on the polymer are quaternary ammonium iodides,
whereas 2,6-diaminopyridines serve as the hydrogen bonding sites. All copolymers
