Chemistry Reference
In-Depth Information
diminishes with increasing molecular weight; however, in low molecular weight
systems even weak hydrogen bonding groups lead to significant changes in properties
(Lillya et al. 1992). Recently, attention has centered on strong telechelic hydrogen
bonding, because the degree of association of telechelic polymers depends on the
association constant raised to the one-half power (Xu et al. 2004). Meijer (Cates
1987; Hirschberg et al. 1999; Brunsveld et al. 2001; Ligthart et al. 2005) and
Long (Yamauchi, Lizotte, Hercules, et al. (2002); McKee et al. 2004) have exten-
sively studied the self-complementary quadruple hydrogen bonding unit 2-uriedo-
4[1H]-pyrimidone (UPy). This DDAA quadruple hydrogen array forms very strong
self-associated dimers in solution, with association constants near 6
10
7
M
21
in
chloroform and dimer lifetimes of 170 ms in chloroform (S¨ntjens et al. 2000).
Telechelic functionalization of molecules with UPy was shown to result in the for-
mation of hydrogen bonded extended chain structures, where the degree of polymer-
ization (as determined from solution viscosity) depended on the solution
concentration (Cates 1987). Much like a step-growth condensation polymerization,
this leads to sensitivity of the noncovalent degree of polymerization on monofunc-
tional end cappers and the formation of cycles are observed in cases where the
spacer between the UPy groups is bent (Sijbesma et al. 1997).
Some of the earliest telechelic UPy containing polymers were based on oligomeric
(DP ¼ 2, 100) poly(dimethylsiloxane) (Hirschberg et al. 1999). The degrees of associ-
ation observed through melt rheological measurements ranged from 100 to 20 and
decreased with increasing molecular weight. The activation energy also decreased
with increasing molecular weight and was lower for a benzyl protected precursor
(37 kJ/mol). A rubbery plateau was observed in melt rheological experiments for
the short (DP ¼ 2) telechelic polymer, indicating entanglements in themelt and associ-
ated structures with molecular weight above M
c
. Solution rheological measurements
indicated that the solution viscosity scaled with the concentration at a value of 3.9,
whereas a benzyl protected analogue produced a scaling value of 1.06 (Fig. 4.11).
Figure 4.11 Solution viscosity of (a) telechelic UPy functional poly(dimethylsiloxane) and
(b) a benzyl protected analog in chloroform at 20 8C. Reprinted from Cates (1987).
Copyright 1987 American Chemical Society.
