Macro-azo-initiators having cinnamate end groups:
synthesis, characterization and photopolymerization with 2-hydroxyethyl methacrylate
Polimery 2001, No 11-12, 812
Three macro-azo-initiators (MAI, Mn = 1583, 4520, 8427) prepared (after ) by reacting poly(ethylene oxide) (PEO, Mn = 600, 2000, and 4000) with AIBN (2:1 by mole), were modified by esterification with cinnamoyl chloride to replace the original hydroxyl end-groups by the cinnamate end-groups. The MAI were used as initiators (2-10 wt. % based on monomer weight) in photoinitiated (UV quartz tube, 37.5 W, distance 20 cm) polymerization of 2-hydroxyethyl methacrylate (HEMA) coupled with photocrosslinking (Scheme I). The structures suggested for the MAI oligomers were confirmed in terms of FTIR and 1H-NMR spectra (Figs. l, 2). Photocrosslinking was monitored at 282 nm; as the irradiation time was protracted, the absorbance fell (Fig. 3). For the three MAI examined, crosslinking percentage was studied in relation to irradiation time; crosslinking rates were highest within the first 2-4 min (Fig. 4); in 20 min, the cinnamate groups participated in 77-82% in the crosslinking (Fig. 4). The conversion of photosensitive groups (-CH=CH-) was not related to the M of MAI. A DSC thermogram for MAI-2000 (Fig. 5) exhibited an endotherm at 54°C (melting of PEO crystal phase) and a broad exotherm at 90-137°C (decompn. of azo groups). Photocrosslinked PHEMA prepared with MAI-2000 having cinnamate end-groups attained the equilibrium degree of swelling (in deionized H2O) in 20 h (Fig. 6). The swelling degree rose as the amount of MAI used was raised, presumably on account of the rising content of polyoxide segments; it also rose as the Mn of the modified MAI was increased (Fig. 7).
Keywords: macro-azo-initiators with cinnamate end-groups, photopolymerization, photocrosslinking, hydrogels of poly(2-hydroxyethyl methacrylate)