In‐situ Robust Polymeric Networks Prepared via Facile Uncatalyzed Huisgen Cycloaddition of Alkyne‐terminated Polyurethane with Terminal Azides

Abstract

We carried out catalyst – and solvent‐free Huisgen azide‐alkyne cycloadditon (AAC) to develop the polyurethane‐based networks crosslinked through triazole moieties at chain‐ends. An asymmetrical divalent compound carrying the hydroxyl and electron‐deficient alkyne as terminal groups was newly synthesized to prepare the alkyne‐terminated polymer containing urethane moieties within polymer backbone. Additionally, three kinds of terminal‐azide crosslinkers were introduced to react with alkyne‐terminated polyurethanes without any promoters. Kinetics of the uncatalyzed AAC were monitored by using real‐time FT‐IR technique at 50 °C temperature. The rate of AAC using aromatic azide dipole was faster than other dipoles. Compared to the non‐polyurethane‐based networks crosslinked via either urethane or triazole moieties at chain‐ends, triazole chain‐ends crosslinked polyurethane networks conveyed the excellent mechanical properties. However, thermal properties at low temperature were not as good as those of non‐polyurethane networks. Especially, Tg of polyurethane networks formed by an oligomeric azide crosslinker was much higher than others.

Propiolate‐Terminated Polybutadiene‐Based Network Binders for Solid Propellants via Catalyst‐Free Azide‐Alkyne Cycloaddition

“…The conversion reached to about 85 % in a week although the fast kinetics was observed at the inital stage in catalyst-free AAC. This is noticeably slow compared to our other works on polycaprolactone ether (or polycaprolactone)-based catalystfree AAC which is almost completed in a day at 50 °C- [14,16,17]. The reason why catalyst-free AAC(AzÀ Ak, Az: azide, Ak:alkyne) of azide-terminated polymers(Az) with alkyne curatives(Ak) is slower than catalyst-free AAC(AkÀ Az) of alkyne-terminated polymer(Ak) with azide curative(Az) is attributable to the reactivity of alkynes which is the key rate determining factor in catalyst-free AAC rather than that of azides.…”

“…HTPB was purchased from ATOCHEM under the trade name of R‐45 M. Isophorone diisocyanate (IPDI) from Mobay and dibutyl tindilaurate (DBTDL) were used as a urethane curative and a curing catalyst, respectively. 4‐Hydroxybutylpropiolate (HBP), a divalent agent, for obtaining an asymmetric propiolate‐ and isocyanate‐terminated chain‐linker was synthesized and used, as described elsewhere [17]. Trivalent terminal‐azide compounds, trimethylolpropane triazide (TMPTA) and 1,3,5‐triazido ethoxybenzene (TAEB), were synthesized as described in our earlier work [17].…”

“…4‐Hydroxybutylpropiolate (HBP), a divalent agent, for obtaining an asymmetric propiolate‐ and isocyanate‐terminated chain‐linker was synthesized and used, as described elsewhere [17]. Trivalent terminal‐azide compounds, trimethylolpropane triazide (TMPTA) and 1,3,5‐triazido ethoxybenzene (TAEB), were synthesized as described in our earlier work [17]. Two kinds of propiolate‐terminated polybutadienes as well as the bubbled PB‐based binder were synthesized as follows; Propiolate‐terminated polybutadiene (PTPB) : Propiolic acid (7.5 g, 107.1 mmol) and p‐toluene sulfonic acid (5.0 mmol) were added to the solution of HTPB (100 g, 35.7 mmol) in benzene (1000 ml) followed by refluxing using Dean‐stark trap to extract H 2 O.…”

“…When it comes to triazole curing system for solid propellants, glycidyl azide polymer (GAP)-based solid propellants have been primarily studied by using bis-propargyl compounds to crosslink polymer chains through the triazole moieties formed between polymer side-backbone chains [13]. Since 2010s, some works on azide-terminated polyether-based network binders available for nitrate ester plasticized polyether (NEPE)-typed propellants have also been carried out [14][15][16][17]. On the other hand, just a few studies on TCS based on polybutadiene mostly used in solid propellants has been conducted.…”

Eco‐friendly chemically crosslinked solid composite propellants via catalyst‐free azide‐alkyne cycloaddition

“…When it comes to catalyst-free AAC, our previous works have mainly focused on the polymeric networks using various polymers like polycarolactone (PCL), polyethylene glycol (PEG), polycaprolactone ether (PCE) and glycidyl azide polymer (GAP) available for solid composite propellants. Recently, we conducted a study on PBbased networks using PB modified with an electrically activated alkyne, propiolate (=C=C=C(=O)=O=) at terminals, distinguishing it from other polymers terminated with azides [27][28][29][30]. Despite these investigations into polymeric networks via catalyst-free AAC, no research on solid propellants cured through catalyst-free ACC have been reported.…”