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40958-GB7
Studying Structural Parameters Important to the Topochemical Polymerization of Butadiene

Brian D. Dinkelmeyer, Western Carolina University

The objective of this project is to design and study the topochemical 1-4 polymerization of butadienes within crystals and gain insight into structural influences on the polymerization process.  The outcome of these reactions depend more on the molecular packing than on the inherent reactivity of the molecules making up the crystal.  As a result, topochemical reactions have not gained wide spread use as a synthetic tool.   To be successful, butadiene monomers must be organized into stacks with the proper orientation and spacing between reactive atoms for polymerization to occur.  In order to increase the scope and general utility of topochemical and solid state reactions it is necessary to have general supramolecular tools that can be transferrable to the design of a wide range of substrates.  Our previous work has demonstrated that we can successfully employ a supramolecular strategy that has been used in the design of topochemical diacetylene polymerizations[1] for diene polymerizations.

The co-crystal strategy employed involves crystallizing host and guest molecules where host molecules (1-4) organize diene guest molecules (EE-1-4) for solid state reactivity (scheme 1). The oxalamide host molecules form predictable and persistent 1-D H-bonded networks.  These host molecules were chosen since the H-bonded networks they form have a characteristic lattice spacing of 4.8-5.1 Å.  This spacing matches the molecular repeat of the desired 1-4 polymer.   The co-crystals that result will contain guest molecules organized into stacks with a lattice repeat equivalent to that of the host network (4.8-5.1 Å).  This spacing of monomers insures that the polymer product can fit within the host lattice. We believe that this strategy will minimize the lattice strain associated with the polymerization process (fig.1).

Current work is focused on 2,3-disubsituted butadiene guests molecules.  There is currently nothing in the literature on the solid-state reactivity of internally substituted dienes.  We believe these systems will make excellent candidates for the topochemical polymerization.  Topochemical polymerizations require that reactions occur with the least amount of atomic motion. 

It has been hypothesized that the requirement for least atomic motion can be met by rotation about a monomers center of gravity.  We proposed that internally substituted dienes can meet this requirement more easily since the reactive diene functional group can rotate about the C2 and C3 substituents in a ‘turnstile' type motion (fig 2.).  Terminally substituted dienes require that the entire molecule rotate about their center's of gravity.  The ‘turnstile' like motion that can occur for internally substituted dienes allows the diene moiety to rotate while leaving the C2 and C3 substituents in place.  We proposed that this ‘turnstile' motion will result in less lattice strain for the process making internally substituted dienes more reactive in the solid state.

We have synthesized 3,4-bis(methylene)hexanedioic acid 1[2] and made attempts at co-crystallizing it with host 1-4.  However, the X-ray crystal structure of compound 1 was ideally organized to undergo a topochemical 1-4 polymerization.  We initiated the polymerization of 1 by uV irradiation and followed the progress of the polymerization with IR, NMR and PXRD.  We also obtained a crystal structure of the polymerized product (fig. 3).  The crystal structure showed that the crystal-to-crystal polymerization was indeed a topotactic process.  The polymer crystal retained the same spacegroup and symmetry relationships between components as the mother crystal (fig. 3).  Polymerization occurs along the b-axis and results in contraction of the crystal in the unit cell by 0.832 Ǻ (fig. 4).   

           

We are currently correlating changes in IR spectra and PXRD in an attempt to be able to correlate changes in molecular structure (IR data) with the movement of miller planes in the PXRD.  It is hoped that this will allow us to track the small atomic movements involved in topochemical processes.  We think this will be especially useful for solid state reactions where a crystal structure for the product is not obtainable.

	Monomer P21/c a = 4.7483 Ǻ b = 9.729 Ǻ            β = 100.45 c = 8.9186 Ǻ V =  405.172 Ǻ3		Polymer P21/c a = 4.803 Ǻ b = 8.897 Ǻ            β = 100.3 c = 9.016 Ǻ V =  379.066 Ǻ3

Figure 4. (top left) monomer crystal viewed down a-axis.  (top right) polymer viewed down a-axis.  (bottom left) monomer crystal viewed down b-axis.  (bottom left) polymer viewed down b-axis. (● indicate the position of inversion centers)

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[1] a) Ouyang, X.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc.; (Communication); 2003; 125(41); 12400-12401.  b) John J. Kane, Ruey-Fen Liao, Joseph W. Lauher, Frank W. Fowler
J. Am. Chem. Soc.; 1995; 117(48); 12003-12004.  c) Lauher, J. W.; Fowler, F. W.; Goroff, N. S.
Acc. Chem. Res.; (Article); 2008; 41(9); 1215-1229.
[2] A. Srikirishna, S. Nagaraju, P. Kondaiah. Tetrahedron Vol.51, No. 6. pp. 1809-1816, 1995.

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