Self-Threading-Based Approach for Branched and/or Cross-linked Poly(methacrylate rotaxane)s Caiguo Gong and Harry W. Gibson* Contribution from the Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virgina 24061 ReceiVed March 3, 1997 X Abstract: Physically branched and cross-linked polymeric structures were produced for the first time by rotaxane formation during reaction of a pendant group of a preformed macromolecule. The rotaxane structure is believed to form from a hydrogen-bonded bimolecular complex of 5-(hydroxymethyl)-1,3-phenylene-1′,3′-phenylene-32-crown- 10 (16) by esterification of the hydroxy group of one macrocycle through the cavity of the second in its reaction with poly(methacryloyl chloride) (12). For esters formed in model reactions of 12 with methanol and with 5-(hydroxymethyl)-1,3-phenylene-16-crown-5 (14), which is too small to be threaded, the degrees of polymerization were identical; however, the polymer from reaction of 12 and 16 under the same conditions had a significantly higher degree of polymerization and polydispersity, i.e., was highly branched via rotaxane formation. Increasing the concentration in the reaction of 12 with 16 led to the formation of a gel fraction along with a high molecular weight sol fraction; the gel represents a novel network structure based on mechanical interlocking via rotaxane structures. 2D NOESY NMR experiments clearly demonstrated the rotaxane structure as manifest in the through- space correlation of the benzylic protons of the “thread” with the intra-annular protons of the “bead”. Introduction Polyrotaxanes, in which rotaxane units are incorporated into macromolecules, because of their novel architectures, have received world-wide attention. 1-6 Several types of polyrotax- anes as illustrated in Scheme 1 can be constructed by proper procedures. To date, most of the reported polyrotaxanes are of the first and third types. To our knowledge, no examples of the second type, side chain polyrotaxanes formed from polymers with pendant macrocycle units, have been reported! One of the objectives of this work was to explore this class of polyrotaxanes. The abilities of polyether macrocycles to complex with metal ions and organic cations and to form hydrogen bonds with hydroxy groups provide the driving forces 7 necessary for preparation of these new materials, and numerous main chain polyrotaxanes of the first type (Scheme 1) have been prepared with crown ethers as the macrocyclic components. 1,5,6 In order to prevent the slippage of the cyclic molecule from the linear chain, blocking groups (BG) are often introduced at the chain ends and/or as in-chain units. Recently, it was found that the introduction of difunctional BG can increase the threading efficiency (m/n, the average number of macrocycles per repeat unit in the polyrotaxane) by as much as 15 times! 6a,c Hydrogen bonding between hydroxy groups and the crown ether is proposed as a driving force for the threading; an endo esteri- fication of the complexed structure 1 taking place through the cavity of the crown ether yields a main chain polyrotaxane structure (3), while an exo esterification yields an unthreaded structure (2, Scheme 2). When this concept is extended, therefore, a crown ether bearing a hydroxy group is expected to self-associate as in structure 8. Endo esterification of 8 with a poly(acid chloride) (Scheme 3) will lead to a novel rotaxane structure (9) along with side chain polymacrocycle units (10) via exo esterification. Since a macrocycle will not readily pass through an identical macrocycle, it can play the same role as a BG, and thus, 9 is expected to be stable to dethreading. This process should ultimately yield the branched and/or cross-linked structure 11. In this work, this novel self-threading method for formation of physically linked networks is investigated Results and Discussion I. Preparation of Poly(methacrylate)s. Poly(methacryloyl chloride) (12) was prepared by free radical polymerization with 2,2′-azobisisobutyronitrile (AIBN) as initiator in toluene (Scheme 4). 12 was then reacted in pyridine with methanol, 5-(hy- droxymethyl)-1,3-phenylene-16-crown-5 (hydroxymethyl-MP- 16C5, 14) and 5-(hydroxymethyl)-1,3-phenylene-1′,3′-phenylene- 32-crown-10 (hydroxymethyl-BMP32C10, 16) to afford poly- X Abstract published in AdVance ACS Abstracts, June 15, 1997. (1) (a) Gibson, H. W.; Bheda, M. C.; Engen, P. T. Prog. Polym. Sci. 1994, 19, 843-945. (b) Ambalino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725-2828. (c) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154-1196. (d) Gibson, H. W. In Large Ring Molecules; Semlyen, J. A., Ed.; J. Wiley and Sons: New York, 1996; pp 191-262. (2) (a) Wenz, G.; Keller, B. Angew. Chem., Int. Ed. Engl. 1992, 104, 201-204. (b) Wenz, G.; Keller, B. Angew. Chem., Int. Ed. Engl. 1992, 31, 197-199. (c) Wenz, G.; Wolf, F.; Wagner, M.; Kubik, S. New J. Chem. 1993, 17, 729-738. (d) Wenz, G. Macromol. Symp. 1994, 87, 11-16. (e) Weickenmeier, M.; Wenz, G. Macromol. Rapid Commun. 1996, 17, 731- 736. (f) Steinbrunn, M. B.; Wenz, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 2139-2142. (3) (a) Born, M.; Ritter, H. Acta Polym. 1994, 45, 68-72. (b) Born, M.; Ritter, H. Angew. Chem., Int. Ed. Engl. 1995, 107, 342-345. (c) Born, M.; Ritter, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 309-311. (d) Born, M.; Ritter, H. AdV. Mater. 1996, 8, 149-151. (e) Born, M.; Ritter, H. Macromol. Rapid Commun. 1996, 17, 197-202. (4) (a) Harada, A.; Li, J.; Kamachi, M. Nature 1993, 364, 516-518; 1994, 370, 126-129. (b) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 26, 5698-5703. (c) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1994, 27, 4538-4543. (d) Harada, A.; Okada, M.; Li, J.; Kamachi, M. Macromolecules 1995, 28, 8406-8411. (5) (a) Gibson, H. W.; Marand, H. AdV. Mater. 1993, 5, 11-21. (b) Shen, Y. X.; Xie, D.; Gibson, H. W. J. Am. Chem. Soc. 1994, 116, 537-548. (c) Gibson, H. W.; Liu, S.; Lecavalier, P.; Wu, C.; Shen, Y. X. J. Am. Chem. Soc. 1995, 117, 852-874. (6) (a) Gong, C.; Gibson, H. W. Macromolecules 1996, 29, 7029-7033. (b) Gong, C.; Gibson, H. W. Macromol. Chem. Phys. 1997. In press. (c) Gong, C.; Ji, Q.; Glass, T. E.; Gibson, H. W. Submitted. (d) Gibson, H. W.; Liu, S.; Gong, C.; Ji, Q.; Joseph, E. Macromolecules In press. (7) Izatt, R. M.; Bradshaw, J. S.; Pawlak, K.; Bruening, R. L.; Tarbet, B. Chem. ReV. 1992, 92, 1261-1354. 5862 J. Am. Chem. Soc. 1997, 119, 5862-5866 S0002-7863(97)00673-2 CCC: $14.00 © 1997 American Chemical Society