The stability of enamine crosslinks formed from acetoacetate/amine in synthetic hydrogels

Functional pH-responsive polymers containing dynamic enaminone linkages for the release of active organic amines

“…S27†), showed appearance of two peaks at δ = 6.66 ppm and δ = 8.51 ppm which were assigned for the –N H protons of the E -isomer and the Z -isomer at a ratio of [ E ] : [ Z ]-isomer = 38 : 62. This ratio of isomers was in agreement with a previous report, 67 where the higher percentage of Z -isomer was attributed to the formation of a stable six-membered ring as a result of hydrogen bonding interactions between the –N H and the CO groups of the enaminone. The extent of modification was determined using 1 H-NMR by a comparison of integrations of the two α-protons at δ = 3.6 ppm (e, Fig.…”

“…S15, see ESI†). As previously reported, 67 acetoacetate groups are susceptible to hydrolysis leading in our case to the formation of HEMA units. This was also supported by a GC-FID analysis of the AEMA monomer proving the presence of a small amount (∼7%) of HEMA available for copolymerisation (Fig.…”

“…As already mentioned, previous studies have shown that pAEMA units may hydrolyse to HEMA when left for several weeks at physiological pH and temperature. 67 To assess that, mPEG 43 - b -pAEMA 25 was dispersed in water for a period of two weeks and the peak integral –C H 2 of HEMA at δ = 3.9 ppm was monitored. Results showed no change in the integrals proving the stability of the acetoacetate groups towards hydrolysis for a period of two weeks.…”

“…The M n, NMR values were determined using 1 H-NMR spectroscopy (Fig. S18, see ESI †) by comparing the proton integrals of the initiator -C 6 H 5 peaks (δ = 7.24 ppm) (g + h + i, As previously reported, 67 acetoacetate groups are susceptible to hydrolysis leading in our case to the formation of HEMA units. This was also supported by a GC-FID analysis of the AEMA monomer proving the presence of a small amount (∼7%) of HEMA available for copolymerisation (Fig.…”