Di(cyanate Ester) Networks Based on Alternative Fluorinated Bisphenols with Extremely Low Water Uptake

A new polycyanurate network exhibiting extremely low moisture uptake has been produced via the treat‐ ment of perfluoroyclobutane‐containing Bisphenol T with cyanogen bromide and subsequent thermal cyclotrimerization. The water uptake, at 0.56 ± 0.10% after immersion in water at 85 °C for 96 hours, represents some of the most promising moisture resistance observed to date in polycyanurate networks. This excellent performance derives from a near optimal value of the glass transition at 190 °C at full cure. Superior dielectric loss characteristics compared to commercial polycyanu‐ rate networks based on Bisphenol E were also observed. Polycyanurate networks derived from this new monomer appear particularly well‐suited for applications such as radomes and spacecraft where polycyanurates are already widely recog‐ nized as providing outstanding properties. Manuscript Received Date: Among thermosetting polymers, polycyanurates (also known as “cyanate ester resins”) offer an outstanding com‐ bination of physical properties that have made them highly desirable for applications ranging from printed circuit boards and radomes to magnet casings for thermonuclear fusion reactors and support structures for interplanetary space probes. These properties include excellent flame, smoke, and toxicity characteristics, glass transition temper‐ atures of up to 400 oC in fully cured networks, low moisture uptake, and a very low coefficient of hygroscopic expansion, as well as an unusually low dielectric constant and dielectric loss factor compared to other thermosetting resins of similar expected polarity. In addition, cyanate ester monomers typically afford low melting points and melt viscosities, mak‐ ing them suitable for use in a wide variety of processing op‐ erations from nano‐molding to filament winding. Because the cyclotrimerization chemistry that forms polycyanurates involves a single, well‐defined end product with few side reactions, and because the conversion is relatively easy to quantify via methods such as FT‐IR spectroscopy, investiga‐ tions of structure‐property relationships in polycyanurate networks enable straightforward optimization of properties, while often yielding insights into thermosetting networks that can be applied to more complex cure chemistries. Many of the desirable physical properties of cyanurate networks, such as low dielectric constant and chemical in‐ ertness, are shared with fluorinated polymers. A logical way to further optimize the performance of polycyanurate net‐ works has been to pursue incorporation of fluorinated chem‐ ical moieties. For instance, AroCy F®, a cyanate ester synthe‐ sized from Bisphenol AF (the hexafluoroisopropylidene ana‐ log of Bisphenol A), provides a reduction in moisture content of around 30% on a concentration per unit weight basis (but only 15% on a concentration per unit volume basis) compared to AroCy B®, which is synthesized from Bisphenol A. A variation on the AroCy F® monomer in an oligomeric liquid form has also been studied by Yameen et al. Recently, the use of polymers containing Bisphenol A has emerged as a public health concern in many countries, due to the ability of some bisphenol compounds to function as endocrine disruptors under certain conditions. Additional investigations have shown that bisphenol compounds with a molecular shape similar to Bisphenol A, including Bisphenol AF, can also interact with human hormone receptors in a similar fashion. Although densely cross‐linked polymers, such as cyanate esters, severely limit the mobility of small molecules, trace quantities of precursor bisphenols may be found in cyanate ester resins, either as residuals from syn‐ thetic precursors (as in epoxies) or as products from hydro‐ lytic or thermal degradation. The availability of fluorinated cyanate esters based on alternative fluorinated bisphenol precursors, particularly those with a molecular geometry substantially different than Bisphenol A, could drastically reduce such concerns by eliminating the “lock and key” fit to hormone receptors that facilitates at least some modes of endocrine disruption. Loudas and Vogel, and later Snow and co‐workers, synthesized a variety of fluoroaliphatic dicyanate ester mon‐ omers. These monomers were liquids at room temperature, and cured into either elastomers or glassy polymer networks with moisture uptake as low as 0.68% after 96 hours in boil‐ ing water (equal to about a 55% reduction in moisture uptake on a molar basis compared to AroCy B®). Although such cyanate esters do not involve bisphenol precursors, they lack the high‐temperature resistance and mechanical stiffness associated with aromatic content, which limits their poten‐ tial for high‐performance applications. An interesting alternative to Bisphenol A is Bisphenol T, which contains a perfluorocyclobutane moiety and, being bridged by four atoms between phenyl groups, is significant‐ ly different in shape compared to Bisphenol A and Bisphenol AF (both of which feature bridges with a single atom). Per‐ fluorocyclobutanes have already emerged as an established fluorinated moiety in polymer thermoplastic materials. These cyclic fluoroaliphatic structures offer good thermo‐ chemical stability, and have recently been shown to exhibit thermally‐induced self‐healing and mechanophore char‐ acteristics. Although thermosetting resins containing per‐ fluorocyclobutane groups have been formed from tri‐ functional perfluorovinyl ethers, to date there have been no reports of cyanate esters that incorporate this relatively new and interesting class of fluorine‐containing monomer. Herein we present the synthesis and physical properties of a new dicyanate ester monomer (“PFCBCy”) containing a perfluorocyclobutane group, as well as its cure characteristics and the physical properties of the resultant polycyanurate networks. We discovered that the combination of hydro‐ phobic character and the optimal degree of flexibility in the network chain segments led to an extremely low moisture uptake of only 0.56 ± 0.10% in the polycyanurate network at full cure, a 70% decrease on a moles per unit volume basis compared to AroCy B®, and among the lowest values meas‐ ured for any polycyanurate network. Such low moisture up‐ take, along with a high level of fluorination, also led to signif‐ icant improvements in dielectric performance compared to more conventional polycyanurate networks. The PFCBCy therefore offers both the high performance of an aromatic fluorinated cyanate ester along with the potential for new and exciting properties such as the mitigation of some health and safety concerns and/or thermal mending characteristics. Synthesis of the PFCBCy monomer was accomplished in a single, straightforward step via treatment of commercially available Bisphenol T with 3.5 equiv. cyanogen bromide at ‐78 °C, using 3.0 equiv. triethylamine added dropwise over 15 min. followed by 2 hours of additional stirring. These condi‐ tions are typical for the synthesis of cyanate esters using cy‐ anogen bromide (complete details with characterization are provided in Supporting Information). Following purification by washing in methanol, PFCBCy was obtained in 56% yield at the 1 g scale as a white powder with a melting point of 103 °C. The identity of the product was confirmed by High Reso‐ lution Mass Spectroscopy, along with ATR‐FTIR spectrosco‐ py, which showed the characteristic doublet at 2237 cm and 2270 cm for cyanate ester groups, and F NMR, which re‐ vealed the characteristic signature of perfluorocyclobutyl groups. Scheme 1 shows the synthesis and cure of the PFCBCy to form a macromolecular network. A DSC thermogram of the PFCBCy (Figure 1) exhibited the expected thermal properties of a high‐purity, uncatalyzed cyanate ester monomer (cure exotherm of 97 kJ/eq. with a peak above 300 °C). A second DSC scan revealed the post‐cure glass transition temperature (TG) near 190 °C. The purity of the monomer as determined by automated analysis of the melting curve with the van ‘t Hoff equation was only around 85 mol%, a likely reflection of the fact that the monomer exists as a pair of stereoisomers. The lower value of the post‐cured TG compared to fully‐cured networks derived from more common dicyanates (such as the dicyanate esters of Bisphenol A and Bisphenol E, at around 300 °C) likely arises from flexible –O‐ linkages in the network segments along with a roughly 25% lower number density of cross‐links (a rough estimate of cross‐link density is derived in Supporting Information). In contrast, dicyanate networks containing cyclohexyl moieties but not ether link‐ ages in the backbone, which also feature a lower cross‐link density compared to the dicyanate esters of Bisphenol A and E, show TG values as high as 300 °C. SCHEME 1.