O O H H RO OR H H H H H H H H n–1 1 n–1 2 3 4 5 Polymers and oligomers with transverse aromatic groups and tightly controlled chain conformations Roger W. Alder,*† Kevin R. Anderson, Paul A. Benjes, Craig P. Butts, Panayiotis A. Koutentis and A. Guy Orpen School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS Anionic ring-opening polymerisation of spiro[cyclopropane- 1,9A-fluorene] with fluorenyl anion as initiator yields poly- mers 1 with chains which are essentially all-anti and have transversely-oriented fluorenyl groups, as shown by strong upfield shifts for the mid-chain methylene and 1,8-fluorenyl protons; synthetic routes to some specific oligomers are also reported. Controlling the secondary structure of acyclic molecules and polymers is important for many applications. We have shown that quaternary centres exert extensive conformational effects on adjacent chains. 1,2 Combining these effects with the rigid side groups provided by transversely-oriented fluorene rings should lead to a polymer 1 with a highly-constrained all-anti (extended chain) secondary structure; any gauche bonds in the chain create strong interactions between CH 2 groups in the chain and the 1,8-CH groups on the fluorene rings. Calculations on oligomers using the MM3 force field 3,4 predict that all-anti structures are 9–10 kJ mol 21 more stable than the next best conformation, which has one gauche bond in the terminal section of the chain. Introduction of gauche bonds in the interior parts of these oligomers costs 13.5–14 kJ mol 21 , corresponding to a > 200 : 1 preference for all-anti conformations at ambient temperature in these parts of the chain. In structures like 1, anti conformations can be readily recognised from the upfield shifts in 1 H NMR spectra due to aromatic ring current effects. Thus the CH 2 OH protons in diol 2 (n = 1, R = H), whose X-ray structure‡ is shown in Fig. 1, resonate at d 2.85, whereas in acetal 3 the corresponding protons occur at d 3.95. Anionic ring-opening polymerisation is a common process for heterocyclic monomers like epoxides, but it has barely been explored for hydrocarbon monomers like cyclopropanes. [1.1.1]Propellane 5 and some bicyclobutanes 6 (which have much higher strain energies) and a few vinylcyclopropanes 7 have been polymerised by this process. Simple 1,1-disubstituted cyclopropanes with two anion-stabilising groups appear good candidates, but the only example in the literature is with CO 2 Me as the stabilising group. 8 We find that polymerisation of spiro[cyclopropane- 1,9A-fluorene] 9 4 can indeed be initiated by an S N 2 reaction with fluorenyllithium. Temperatures > 100 °C are required; we have used DMPU as solvent, but HMPA is also effective. During polymerisation reactions with monomer : initiator ratios > 3 : 1, a precipitate appears after about 10 min at 150 °C, raising concern that chain growth may be limited by the insolubility of the product. This insolubility precludes any simple determina- tion of the degree of polymerisation (DP), but NMR data (see below) indicate that oligomers below the octamer 1 (n = 8) are almost absent in products from reactions with monomer- : initiator ratios > 12 : 1, so polymerisation probably continues in spite of the precipitation. Further optimisation of this polymerisation will be undertaken and we intend making modified monomers with flexible chains on the periphery of the aromatic rings to improve solubility. We found a much improved synthesis of 4 in > 90% yield from fluorene, BuLi and ethylene sulfate. 10 Ethylene sulfate is much more efficient in this reaction than 1,2-dibromoethane, which gives mixtures of dimer 1 (n = 2), 11 4 and cyclohexane derivative 5,§ even when fluorenyllithium is added slowly to excess dibromide. Ethane-1,2-diyl dimesylate only gives 4, without 1 (n = 2) or 5, but is relatively unreactive. We are investigating the reasons for the effectiveness of ethylene Fig. 1 X-Ray structure of diol 2 (n = 1, R = H) showing the all-anti aliphatic chain Chem. Commun., 1998 309 Downloaded on 18 September 2012 Published on 01 January 1998 on http://pubs.rsc.org | doi:10.1039/A707389E View Online / Journal Homepage / Table of Contents for this issue