45698-AC7
Highly Fluorinated Diels-Alder Polyphenylenes
Part 1. Monomer Synthesis. We have now optimized our monomer syntheses so that we can attain approximately 50% overall yields in the four-step syntheses of the butylated monomers and in the three-step syntheses of our perfluorinated monomers. Fluorinated CPD monomer synthesis is no longer a primary issue for us. The next step here is to work on our fluorous ponytail monomer systems. We have not made much progress on our ponytail systems because our collaborator has had trouble purchasing sulfur tetrafluoride.
Part 2. Cyclopentadiene Oxidation Chemistry. In our last report, we indicated that the final oxidation step in the synthesis of our monomers is conducted by one of two methods. The first method is air-oxidation, generally using a ligated soluble copper catalyst such as copper(II) bromide pyridine complex. The other method, which is used primarily for our fully-fluorinated monomers, is selenium dioxide. It turned out that selenium dioxide was problematic for a variety of reasons, but chief among those were (a) it’s high toxicity, and (b) difficulty removing the colloidal selenium(0) byproduct. Traces of selenium were left in the monomer even after a chromatographic separation on silica gel. These impurities caused rapid decomposition of the dialkynylbenzene monomer in the subsequent polymerization.
Therefore we revisited the oxidation of our fully-fluorinated monomers once again. This time the student found that oxidation using tert-butyl hydroperoxide and catalytic (0.01 mol %) selenium dioxide was nearly as effective (75% yield instead of 90% yield). The reaction was also much cleaner. The hydroperoxide reoxidizes selenium(0) in situ, and the selenium dioxide is easily removed as selenic acid by aqueous extraction.
Part 3. Polymerization Reactions. We have conducted extensive experiments to optimize a solvent and a set of reaction conditions for polymerization. The best solvents now seem to be chlorinated aromatic compounds like 1,2,4-trichlorobenzene and 1-chloronaphthalene. The most delicate issue seems to be an as-yet unidentified side-reaction that takes place between the CPD monomer and possibly a decomposition byproduct arising from the dialkynylbenzene monomer. GPC analysis suggests that these small decomposition pathways are leading to some branching or cross-linking of our polymers (based on high Mz/Mn values). A suite of control experiments is needed to nail down the source of the problem. Thus at present, our polymerization systems are limited to molecular weights (Mn) of about 15 kD to 25 kD. Unfortunately these molecular weights are not quite high enough to form films. We experimented with different terminal alkynes such as 4,4'-diethynyldiphenyl ether but did not see any significant improvement.
Part 4. Characterization issues. Although our molecular weights are still too low to cast good films, we have started to conduct physical characterization on our systems. TGA analysis of the butylated polymers (ca. n = 20) show loss of the tert-butyl group at around 250 C (that is a typical temperature for debutylation) but the polymers are stable otherwise up to over 450 C. That is a promising result. GPC analysis can be problematic because we have seen evidence of column “sticking” and aggregation. So we are rather reluctant to make claims based on that data. When a student shows you a GPC trace for a condensation polymer and the Mn value is one million, something is not right. We are presently exploring light-scattering data to troubleshoot this issue.
Part 5. Widening the Search. Because of the problems that we have encountered in using terminal alkynes in our polymerizations, we have spent a good part of the last year synthesizing special internal alkynes that we can use for polymerization experiments. Internal alkynes are less reactive, but they also do not decompose nearly as easily. Thus even though the polymerization temperatures will be much higher, the reaction might be more successful. This same approach has been used elsewhere (commercially) for non-fluorinated analogues and it works well. Monomers that we have prepared include 4,4'-bis(phenylethynyl)diphenyl ether, 3,5-bis(phenylethynyl)-1-fluorobenzene, 1,4-bis(phenylethynyl)-tetrafluorobenzene, among others. Preliminary experiments (conducted in sealed glass tubes in a tube furnace!) show that model reactions with internal alkynes can be quite clean, so we are very hopeful that this modification will give us a higher molecular weight polymer that we can use to cast robust films.
