Abstract
Oxidation responsive polymers with triggered degradation pathways have been prepared via attachment of self-immolative moieties onto a hydrolytically unstable polyphosphazene backbone. After controlled main-chain growth, postpolymerization functionalization allows the preparation of hydrolytically stable poly(organo)phosphazenes decorated with a phenylboronic ester caging group. In oxidative environments, triggered cleavage of the caging group is followed by self-immolation, exposing the unstable glycine-substituted polyphosphazene which subsequently undergoes to backbone degradation to low-molecular weight molecules. As well as giving mechanistic insights, detailed GPC and 1H and 31P NMR analysis reveal the polymers to be stable in aqueous solutions, but show a selective, fast degradation upon exposure to hydrogen peroxide containing solutions. Since the post-polymerization functionalization route allows simple access to polymer backbones with a broad range of molecular weights, the approach of using the inorganic backbone as a platform significantly expands the toolbox of polymers capable of stimuli-responsive degradation.
Highlights
Chemistry7b,9 have been used to prepare poly(caprolactone) and poly(carbonate)s (PCs),[10] which undergo a chainshattering process in response to a variety of stimuli including enzymatic,7b photochemical,[8] and oxidative3b environments
After controlled main-chain growth, postpolymerization functionalization allows the preparation of hydrolytically stable poly(organo)phosphazenes decorated with a phenylboronic ester caging group
In oxidative environments, triggered cleavage of the caging group is followed by self-immolation, exposing the unstable glycine-substituted polyphosphazene which subsequently undergoes to backbone degradation to low-molecular weight molecules
Summary
Letter degradation products shown to be a benign buffered mixture of amino acid (ester), phosphates, and ammonium salts.16d It is well-established that the degradation is acid-catalyzed17c,d and that the presence of acidic groups in proximity to the backbone phosphorus accelerate hydrolysis rates.17b in early studies into poly(amino acid ester)phosphazenes, Allcock and co-workers described the inability to isolate the glycine-substituted polyphosphazene [NP(NHCH2COOH)2]3 due to its extremely rapid hydrolysis.17b we proposed that through essentially caging poly(glycine)phosphazene via the addition of stimuli-responsive protection groups, it should be possible to prepare a stable polymer that, upon removal of the caging moiety, produces this hydrolytically unstable glycinesubstituted polyphosphazene, which will spontaneously and rapidly disintegrate into small molecules (Scheme 1), an effect similar to that of a chain-shattering polymer. Studies of the sample by 31P NMR spectroscopy (Figure S15) indicated the stability of poly(glycine ethyl ester)phosphazene even to a higher concentration of H2O2 (100 mM), confirming that any H2O2 triggered degradation effect is exclusively due to the presence of the self-immolative boronate ester moiety. A new type of polymer based on a polyphosphazene with phenylboronate moieties along the main chain has been prepared While such unique boroncontaining polymers may have many interesting properties,[28] the linkage of the boronate group via a self-immolative motif allowed the preparation of polymers stable in ambient conditions but with a stimulus-responsive degradation pathway in oxidative environments. Proton NMR studies showed that, while boron oxidation and phosphazene main chain degradation are both rapid, the rate-limiting step is the self-immolation of the phenol to present the free acid. Experimental section, additional data for polymer characterization, and full NMR spectra (PDF)
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