Cyclopentadiene-Maleimide Platform for Thermally Reversible Polymers
Stegall, Jeremy Brent
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This dissertation describes a new platform for the synthesis of thermally reversible polymers, based on Diels-Alder reactions of bis-cyclopentadienes (bis-CPDs) and bis-maleimides (bis-MIs), that meets two main objectives. First, the bis-CPD must resist characteristic self-coupling. Second, the CPD-MI adducts should undergo the retro-Diels-Alder (rDA) reaction (i.e., thermal depolymerization) in a temperature regime that is comparable or slightly higher than that of the freely reversible bis-furan/bis-MI polymers (rDA between 80 �[BULLET]C and 130 �[BULLET]C) but much lower than that of bis-CPD homopolymers (rDA above 160 �[BULLET]C). Structure-reactivity relationships gleaned from the literature and from related but as yet unpublished work in our own laboratories led to our main hypothesis that a CPD moiety bearing one sterically encumbering substituent such as isopropyl (iPr) or tert-butyl (tBu) and one electron-withdrawing substituent such as perfluoroaryl would have the desired reactivity and adduct stability in combination with an N-substituted maleimide. Synthetic considerations led to a bis-CPD monomer design in which two alkylcyclopentadiene groups (alkyl = iPr or tBu) are connected by an octafluorobiphenylene linker. As an initial fundamental step (Chapter 3), 1-(nonafluorobiphenyl-4'-yl)-4-tert-butylcyclopentadiene (1) was synthesized to provide a monofunctional model for the proposed difunctional CPD monomer. Reactions of 1 and N-(4-fluorophenyl)maleimide (FMI) afforded up to five regio- and stereo-isomeric adducts (of fourteen possible). Variable-temperature reactivity studies combined with NMR spectroscopic analysis, X-ray crystallography, and computational modeling enabled product distributions to be understood according to a conventional kinetic-vs- iii thermodynamic framework. These studies also predicted the microstructure of polymers derived from the proposed bis-CPD monomer, which is structurally analogous to 1, and bis-MIs. Moreover, 1 does not undergo DA self-coupling under ordinary conditions (T < 180 �[BULLET]C). Thermolysis studies of the major adducts revealed that the rDA becomes observable on a laboratory timescale (hours) at about 140 �[BULLET]C, which is at the upper end of the temperature range reported for furan+MI adducts but well below that of CPD+CPD adducts. In contrast, adducts formed from either of the analogous monosubstituted cyclopentadienes (tBuC5H5 and C6F5C5H5) do not undergo rDA below 180 �[BULLET]C. These results strongly support the proposed bis-CPD monomer design. In a second fundamental step (Chapter 4), the hypothesis that an electron-withdrawing CPD substituent would destabilize a CPD-MI adduct was further tested by reacting N-(4-fluorophenyl)maleimide with a series of triarylated cyclopentadienes (1,2,3-Ar3C5H3 and 1,2,4-Ar3C5H3, Ar = C6F5, C6F4CF3, and Ar = C5F4N). The perfluorophenyl- and perfluorotolyl-substituted compounds were previously reported, but the perfluoropyridyl-substituted cyclopentadienes were prepared for this study using SNAr reactions of pentafluoropyridine and sodium cyclopentadienide. The least electron deficient cyclopentadiene in each series (Ar = C6F5) reacted the most quickly and with the highest ultimate equilibrium binding constant, confirming the electron-effects hypothesis as well as the underlying presumption that DA reactions of even relatively electron-poor CPDs with MI would behave according to normal-electron-demand principles. In the main section of this dissertation (Chapter 5) the proposed bis(cyclopentadiene)s reacted with a series of previously reported bis(maleimides) to form linear polymers having molecular weights (Mn) up to 40 kDa. Relationships among the length and flexibility of the bis-MI linker iv (C6H12, C12H24, C6H4OC6H4, and (C2H4O)2), the identity of the CPD alkyl substitutent (CHMe2, CMe3 and CMe2Ph) and the glass transition temperature (Tg) as measured by differential scanning calorimetry (DSC) were understood in terms of a general model of local segmental mobility and free volume. Solution thermolysis of a model polymer system (bis-MI linker = C6H12 (7), CPD alkyl substituent = tBu) showed a rapid decrease in molecular weight at 160 C as determined by size exclusion chromatography (SEC). Solution thermolysis in the presence of excess FMI (as a trap for free CPD moieties) revealed that the onset temperature for rDA on a laboratory time scale (hours) was as low as 120 �[BULLET]C. In the bulk, thermolysis above 250 �[BULLET]C under vacuum led to recovery of a small portion of the bis-CPD monomer, but bulk thermolysis at 200 �[BULLET]C did not reveal a change in molecular weight as determined by SEC. The current interpretation of these observations is that limited mobility in these glassy polymers prohibits retro-DA decoupling. These findings largely validate the main hypothesis of this dissertation.
- Doctoral Dissertations