Mode of crosslinking of degradable poly(vinylpyridine N-oxide) gels N. Has,rc, Department of Chemistry, Middle East Technical University, Ankara, Turkey I. L. Kamel Department of Materials Engineering, Drexel University, Philadelphia, PA 19104, USA V. N. Haslrcl* Department of Biological Sciences. Middle East Technical University, Ankara, Turkey (Received 3 August 1989; revised 9 November 1989; accepted 18 December 1989) The degradation mechanism of gels from poly(vinylpyridine N-oxide) was investigated by differential scanning calorimetry , C, H, N analysis, infra-red spectroscopy and degradability tests. It was found that poly(2-vinyl pyridine N-oxide) gels, regardless of the persulphate initiator used, were weaker than poly(4-vinyl pyridine N-oxide) gels. C, H, N analysis and infra-red spectroscopy indicated that the initiator becomes an integral part of the gels. A mechanism for the formation of the gels is proposed. (Keywords: poly(vinylpyridlne N-oxide); hydrogels; degradable gels; crosslinking) INTRODUCTION Products of synthetic polymeric origin are almost always designed to have service lives of at least several years. In the biomedical field, however, some requirements can best be solved if the products have predetermined and relatively short service lives. Biodegradable sustained- release drug systems constitute an appropriate example for such cases. The temporary presence of drug depot can be achieved if it is constructed from a biodegradable material. If the residence time in the body can be controlled, then it is possible to use the same well characterized system for various applications. In order to achieve these, one has to find a biocompatible polymer, characterize it and understand the underlying mechanism of degradation. Poly(2-vinylpyridine N-oxide) (P2VNO) satisfies the requirement of biocompatibility. It has been put to use in clinical trials as a chemotherapeutic agent against silicosis 1. It has been shown to be non-toxic, and is non-carcinogenic as long as its monomer or dimer do not remain in the polymer2. It has also been shown that it is not degradable unless it is subjected to rigorous conditions, which are not encountered in the body 3. In its preparation poly(2-vinylpyridine) (P2VN), a hydro- phobic polymer, is converted to its oxide P2VNO, which is hydrophilic. A crosslinked hydrophilic polymer always has a very good potential for use as a biomaterial due to its high water retention. Thus P2VNO becomes an excellent candidate as a starting material for construction of a degradable product if a degradable form can be obtained. It was previously reported that a series of polymer networks can be synthesized from P2VNO with varying rates of degradation4. The nature of the degradation was, however, not known. Understanding and control of the * To whomcorrespondenceshouldbe addressed process of degradation was thought to be important both for the sake of fundamental chemical research as well as to tailor-make materials with predetermined service life. In that work degradable gels were synthesized from (a) P2VNO and (b) P2VNO and poly(1-vinylpyrrolidone) (PVP), through the action of ammonium persulphate. It was observed that, in both gel types, an increase in PVNO content led to an increase in instability. Generally, a covalent bond will not result in a degradable network unless it contains bonds in the polymer backbone that are easily hydrolysable. Since P2VNO is known to be quite stable, that possibility for gel degradation had to be excluded. The biodegradation had to result from an interaction that takes place only in the presence of ammonium persulphate during the process of gel formation. One possibility was the retention of ammonium persulphate in the formed network through bond for- mation that in strength lies somewhere between covalent and hydrogen bonds. If this hypothesis was correct, then ammonium persulphate or its ions could be detected in the network and possibly in the initial degradation products. The following sections contain the results of the experimental work, including analyses aimed at resolving the mode of crosslinking of hydrogels of the 2- and 4-isomers of PVNO (Figures la and lb) formed under the action of two different persulphates. EXPERIMENTAL METHODS P2VNO and P4VNO were prepared according to the procedure published previouslya, dried in vacuum oven at 45°C overnight and stored in vacuum. Three different types of gels were prepared in order to investigate the effects of ingredients on the properties of the gels. The gels contained the following: P2VNO and 0032-3861/90/122393-04 © 1990Butterworth-Heinemann Ltd. POLYMER, 1990, Vol 31, December 2393