Polymerization of linear aliphatic diamine-based benzoxazine resins under inert and oxidative environments Douglas J. Allen, Hatsuo Ishida * Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States Received 19 July 2007; received in revised form 24 August 2007; accepted 2 September 2007 Available online 5 September 2007 Abstract A series of linear aliphatic diamine-based benzoxazine monomers have been studied. Reaction times and purification procedures have been optimized for each individual diamine. The structure of these diamine-based benzoxazine monomers has been characterized by 1 H and 13 C NMR, and infrared spectroscopy. The rate of polymerization has been studied by Fourier transform infrared spectroscopy as a function of the chain length of the aliphatic amines. The glass transition temperatures (T g ) of the polybenzoxazines from these monomers are also studied. The short chain amine polybenzoxazine exhibits the T g of around 170  C. The influence of the polymerization environment for these linear aliphatic diamine-based series of benzoxazine monomers has been studied under air and inert atmosphere. Differential scanning calorimetry is used to determine the melting points of these benzoxazines and the temperature of the peak polymerization exotherm. An anomalous polymerization behavior of ethylene diamine-based polybenzoxazine is also reported. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Polybenzoxazine; Linear amine-based benzoxazine; FTIR 1. Introduction Polybenzoxazines are a class of thermosetting phenolic resins that have been developed over the last decade as an attractive alternative to epoxies and traditional phenolic resins [1e10]. Benzoxazine resins are readily synthesized, either in solution or by a melt-state reaction using a combination of a phenolic derivative, formaldehyde, and a primary amine [11,12]. The unique chemistry of benzoxazines is responsible for a number of inherent processing benefits, including low melt viscosity, no volatile release upon cure, rapid develop- ment of properties, and low overall shrinkage. Thermally acti- vated ring-opening polymerization results in a high modulus thermosetting material with excellent thermal, mechanical, and electrical properties [10]. Thermally activated polymerization of monofunctional benzoxazines typically leads to a linear or branched structure with a molecular weight in the order of 500e2000 and is characterized by the presence of a Mannich base bridge [13]. The polyfunctionality required to form an infinite net- work structure upon polymerization may be achieved through monomer synthesis utilizing either a multifunctional phenolic molecule with a monoamine, or with a multifunctional amine paired with a mono-phenol, examples of which are shown in Fig. 1. However, of these two approaches, the overwhelming majority of polybenzoxazine research published to date has fo- cused almost exclusively on materials in which the phenolic compound, typically a bisphenol, provides this multifunctional core. Despite their additional molecular design opportunities, the subclass of multifunctional amine-based materials has largely been ignored. With a vast number of suitable starting compounds available, multifunctional amine-based polybenz- oxazines have tremendous untapped potential in the area of tailoring molecular structure for specific applications. Recently, we reported the synthesis and properties of linear aliphatic di- amine-based benzoxazines [14]. Also, an attempt to synthesize aromatic diamine-based benzoxazines has been reported [15]. As is the case with traditional phenolic resins, the brittle nature of polybenzoxazines can sometimes limit their potential applications. This characteristic is a consequence of the rigid * Corresponding author. E-mail address: hxi3@cwru.edu (H. Ishida). 0032-3861/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.09.003 Polymer 48 (2007) 6763e6772 www.elsevier.com/locate/polymer