Published on Web 12/24/2004
Complementary Quadruple Hydrogen Bonding in Supramolecular Copolymers G. B. W. L. Ligthart, Haruki Ohkawa, Rint P. Sijbesma,* and E. W. Meijer* Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB, EindhoVen, The Netherlands Received October 24, 2004; E-mail:
[email protected] (R.P.S.);
[email protected] (E.W.M.)
This paper describes a supramolecular copolymer with continuously tunable composition from pure homopolymeric to strictly alternating, based on a combination of complementary and selfcomplementary hydrogen-bonding units (Scheme 1a). Due to the high binding strength of quadruple hydrogen-bonding motifs, high degrees of polymerization are obtained. Furthermore, simply mixing components in solution results in different degrees of comonomer incorporation. Scheme 1
Figure 1. (a) (9, black): Specific viscosity of 25 mM p-THF1000bisUPy 4 in chloroform as a function of added bisNapy 3. (1, red): Specific viscosity of 40 mM bisUPy 5 in chloroform as a function of added bisNapy 3. (b): Specific viscosity of 40 mM bisUPy 6 in chloroform as a function of added monofunctional Napy 2 (b, black) and UPy 1 (2, red).
Among several noncovalent interactions used in the self-assembly of polymers,1 multiple hydrogen bonds have taken a prominent position.2 The first supramolecular polymers, as well as more recent examples, are based on complementary hydrogen-bonding units,3,4 while complementarity and self-complementarity were combined in calixarene urea-based polymers.5 Supramolecular polymers based on self-complementary 2-ureido-4[1H]-pyrimidinone (UPy) units, 1, were shown to have high degrees of polymerization (DP) in bulk as well as in solution,6 due to the very high binding strength (Kdim ) 6 × 107 M-1 in CDCl3).7 Recently, Li and co-workers8 have reported the strong and selective complexation of the 6[1H] tautomeric form of 1 with 2,7diamido-1,8-naphthyridines,9 2 (Napy), via quadruple hydrogen bonds between ADDA and DAAD arrays (Scheme 1b). One equivalent of Napy in CDCl3 disrupts the UPy dimers. The high selectivity and strength render the UPy-Napy heterodimer very attractive for constructing complementary supramolecular copolymers. Therefore, we decided to study heterodimerization between bifunctional molecules in more detail. The study of UPy-Napy supramolecular copolymers required the synthesis of bifunctional Napy derivatives on a multigram scale. To this end, an efficient route based on Buchwald amidation10 of 2-amido-7-chloro-1,8-naphthyridine with hexanedioic amide was developed to obtain bifunctional Napy derivative 3, which contains 2-ethylhexamido substituents for added solubility. 810
9
J. AM. CHEM. SOC. 2005, 127, 810-811
Subsequently, UPy-Napy complexation in supramolecular polymers was studied using 3 and bifunctional UPy derivatives 4-6. Titration of a 25 mM solution of UPy telechelic polytetrahydrofuran11 4 (Mn ) 103 g/mol; UPy end-group functionality > 1.98)12 with 3 resulted in a viscosity plot with two distinct regions (Figure 1a). Upon addition of
