Chemistry Reference
In-Depth Information
were below the entanglement molecular weight (M
c
). In contrast, the
polystyrene blocks possessed molecular weights above M
c
, which allowed some
degree of entanglement with the polystyrene side of the interface.
Self-association and multiple modes of association of hydrogen bonding groups
frequently leads to complexity in the behavior of hydrogen bonded systems in
which complementary hydrogen bonding groups are involved. Han et al. (2000) syn-
thesized poly(styrene-b-vinylphenyldimethylsilanol) polymers via anionic polymer-
ization of silane (Si-H) containing monomers and subsequent postpolymerization
oxidation with dimethyldioxirane The stronger acidity of the silanol group compared
to the hydroxyl group was expected to result in greater hydrogen bonding donating
capabilities. However, self-association of the silanol residues was found to play a
large role in the behavior of these copolymers. Block copolymers with a range of
11-33% conversion to silanol groups were miscible with the weak hydrogen bond
accepting homopolymer poly(n-butyl methacrylate) whereas higher or lower conver-
sions led to macrophase separation. Blends of the 21% silanol block copolymer with
the stronger hydrogen bond acceptor poly(4-vinylpyridine) or poly(N-vinylpyrroli-
done) formed clear films in all cases and exhibited hydrogen bonding through
changes in the FTIR spectrum.
4.3.2. Nucleobase Containing Hydrogen Bonding Block Copolymers
Nucleobase containing hydrogen bonding block copolymers are typically synthesized
via techniques that are amenable to protic functionality of the hydrogen bonding
groups such as living radical or methathesis polymerization techniques. In addition,
nucleobase-functional polymers are synthesized via postpolymerization techniques,
although these are typically hampered by incomplete conversion and side reactions
(Pollino and Weck 2005). Nucleobase associations possess strengths in the range
of 100 M
21
(A-T, two hydrogen bonds) to 10
4
M
21
(C-G, three hydrogen bonds),
which places them between simple single hydrogen bonds and synthetic quadruple
or higher multiple hydrogen bonding interactions (Kyogoku et al. 1967b; Thomas
and Kyogoku 1967). Recent computational studies on the energy of hydrogen
bonding interactions in DNA duplexes revealed values of
14 kcal/mol for the
A-T pair and
27 kcal/mol for the C-G pair (Sponer et al. 2004). For adenine-
uracil (A-U) hydrogen bonds in chloroform, the association energy was determined
experimentally from an Arrhenius plot of the association constants, yielding a
value of 6.2 kcal/mol (Kyogoku et al. 1967a). Nucleobases are complementary in
nature, so that specific pairs such as adenine and thymine or uracil (A-T, A-U) or
cytosine and guanine (C-G) are typically employed. Sivakova et al. (2005) noted
that the behavior of the isolated nucleobases in synthetic polymers is quite different
from their behavior in DNA where they bond to a complementary base. Multiple
association modes are also possible for these nucleobases, including the primary
Watson-Crick mode that is present in DNA and the less commonly observed
Hoogsteen association mode (Ghosal and Muniyappa 2006), as well as several
self-association modes. The self-association modes of typical nucleobases are
extremely weak in nature and exhibit association constants of less than 10 M
21
