New stimuli-responsive copolymers of N-acryloyl-N 0 -alkyl piperazine and methyl methacrylate and their hydrogels L.H. Gan * , G. Roshan Deen, X.J. Loh, Y.Y. Gan Division of Chemistry, School of Science, Nanyang Technological University, 469, Bukit Timah Road, Singapore, Singapore 259756 Received 22 November 1999; received in revised form 18 February 2000; accepted 29 March 2000 Abstract Water-soluble copolymers of N-acryloyl-N 0 -alkylpiperazine (alkyl: methyl, ethyl) with methyl methacrylate were synthesized and the lower critical solution temperatures (LCSTs) of the copolymers which depend on the compositions and pH of the aqueous solutions are described. The effects of cationic surfactants and simple inorganic salts on the LCSTs are also reported. The influences of pH and temperature on the swelling of the ionizable and thermosensitive hydrogels are studied.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Water-soluble polymers; Piperazine; Lower critical solution temperature 1. Introduction Water-soluble polymers which show a phase transition in response to external stimuli such as temperature [1,2], pH [3,4], electrolytes [5,6], illumination [7,8], electric field [9], magnetic field [10] have attracted much interest because of their versatile applications such as in the fields of purifica- tion of chemicals or as bio-active agents [9], in the devel- opment of artificial muscles [9], and in controlled drug delivery systems [11,12]. These polymers are soluble in water at low temperatures but separate from solution when the temperature is raised above the lower critical solu- tion temperatures (LCSTs). Poly(N-isopropyl acrylamide) (PNIPAM) is one of the most widely studied thermo-respon- sive polymer, which undergoes phase separation at around 32°C. Currently, polymer systems that respond to more than one stimulus such as pH and temperature are extensively researched [13–16]. In this paper we report the synthesis of a series of new piperazine-containing copolymers. These copolymers are water-soluble and they exhibit interesting LCST phenom- ena. The coil-to-globule phase transition of the copolymer is sensitive to pH, temperature, salts and surfactants. The responsive behaviour of the corresponding hydrogels is described. 2. Experimental 2.1. Synthesis of monomers The synthesis of N-acryloyl-N 0 -methyl piperazine (AcrNMP) has been described previously [17]. N-Acry- loyl-N 0 -ethyl piperazine (AcrNEP) was synthesized using the same procedure and was obtained as a pale yellow liquid with a yield of 60%. 1 H NMR (400 MHz in ppm): 0.98– 1.04 (CH 3 , t, 3H), 2.36–2.40 (–CH 2 , q, 2H), 3.5 (–N(CH 2 ) 2 –, b, 4H), 3.65 ((CH 2 ) 2 –N–CO–, b, 4H), 6.45– 6.52 (y CH, q, 1H, J cis  16.63 Hz, J trans  10.62 Hz), 6.15– 6.25 (CH 2 y , q, 1H, J cis  16.69 Hz), 5.61–5.63 (CH 2 y , q, 1H, J trans  10.63 Hz). 13 C NMR (ppm): 127.6 (CH 2 y ), 128.9 (y CH), 165.3 (– Cy O), 41.9 (–CO–N(CH 2 ) 2 r), 45.7 (s(CH 2 ) 2 –N–), 52.2 (sN– CH 2 ), 11.8 (– CH 3 ). 2.2. Synthesis of copolymers Copolymers of AcrNMP and AcrNEP with MMA (Scheme 1) were synthesized in dioxane using different monomer feed ratios and AIBN as initiator. The monomer feed ratios are given in Table 1. 2.3. Synthesis of hydrogels Hydrogels of AcrNMP/MMA and AcrNEP/MMA with various percentage of EGDMA as crosslinkers were prepared by photo-initiation polymerization in bulk (Table 1). The solid transparent pale yellow polymer was immersed in diethyl ether for 3 weeks to remove any unreacted Polymer 42 (2001) 65–69 0032-3861/01/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(00)00296-2 * Corresponding author. Tel.: +65-4605337; fax: +65-4698928. E-mail address: lhgan@nie.edu.sg (L.H. Gan). www.elsevier.nl/locate/polymer