Synthesis and degradation behavior of poly(ethyl cyanoacrylate)

https://doi.org/10.1016/j.polymdegradstab.2008.04.012Get rights and content

Abstract

Poly(ethyl cyanoacrylate) was synthesized using N,N′-dimethyl-p-toluidine as an initiator through an anionic/zwitterionic pathway. The degradability and the degradation mechanism of the prepared polymer were examined from various viewpoints. A combination of TGA and GPC analysis allowed us to confirm that the thermal degradation of this polymer was predominantly due to an unzipping depolymerization process initiated from the polymer chain terminus. The polymer was inherently unstable and exhibited interesting degradation behavior in solution with basic reagents. The degradation in solution was also found to be attributed to the unzipping of the monomer from the chain end. However, the degradation behavior of the polymer could be controlled by changing solvents, temperatures, and additives. These findings give an insight into the degradation behavior of poly(alkyl cyanoacrylate)s, which is a crucial point in utilizing these polymer homologues for various applications.

Introduction

Alkyl-2-cyanoacrylates (ACAs) were first synthesized in 1949 [1] and have been known as one of the most reactive monomers. Although these monomers can undergo polymerization through both free radical and anionic polymerization mechanisms, the anionic pathway has attracted more interest owing to the ease of initiation and rapid rate of polymerization, even by traces of nucleophiles or weak bases such as water, amines, alcohols or phosphines. The effectiveness of anionic polymerization of the ACAs originates from the monomer property of the 1,1 di-substituted strong electron withdrawing groups, nitrile (CN) and ester (COOR). The presence of the attacking nucleophile leads to strong electromeric effects which make the nitrile and the ester group highly negative, causing the polarization of the double bond and activating the monomer to the nucleophilic attack. Then, the propagation takes the form of the addition of an electron deficient monomer to an anionic chain end, while the substituents stabilize the negative charge carried by growing polymers through delocalizing the negative charge formed at the α-carbon. The mechanism for the anionic/zwitterionic polymerization of ACAs is shown in Fig. 1.

In addition to the aforementioned merits, the excellent wetting and binding properties of ACAs to a wide range of substrates have resulted in their successful application and marketing as “superglue”. Moreover, the feature of strong binding power of ACAs to the skin through the initiation and polymerization by amino acids of proteins, in combination with their biocompatibility and biodegradability, has facilitated the use of these monomers for several applications such as tissue adhesives for the closure of skin wounds and surgical glues [2], [3]. Poly(alkyl cyanoacrylate)s (PACAs) also have been widely proposed as promising nanoparticle drug delivery materials due to their ability to entrap a variety of biologically active compounds (drugs) followed by in vivo delivery and release through biodegradation [4]. Other applications of these polymer homologues include the detection of latent fingerprints in crime investigations [5], as an electrolyte matrix for dye-sensitized solar cells [6], and other medical applications [7], [8].

It has been reported that these polymers have weak stability and are easily degraded in contact with water [3], [9], at elevated temperatures [10], [11], [12], or even in solutions [13], [14]. Hence, the poor stability of these polymers at elevated temperatures generally limits their applications as adhesives and polymer composites due to their limited operating temperatures. In addition, the utilization of these polymer homologues in humid conditions or dissolving them in solvent for processing deteriorates the properties due to rapid molecular weight degradation. The degradability of nanoparticles of these polymers has been studied extensively in terms of biodegradation of drug delivery systems [15], [16], [17]. On the other hand, only limited works have been carried out on the degradation behavior of bulk phase PACAs [13], [14], [18], [19], [20]. The reason for this might be due to lack of a simple, general, and controllable polymer system for analyzing the properties.

In terms of degradation by chemical species in solution phase, researchers have found that the PACAs are inherently unstable, thus easily degraded by basic species. Ryan and McCann [13] first reported that PACA exhibited a depolymerization–repolymerization (DPRP) reaction in THF solution after adding a strong base, tetrabutyl ammonium hydroxide (TBAOH). The polymer backbone in solution underwent rapid depolymerization followed by the simultaneous repolymerization of the degraded monomer to produce lower molecular weight “daughter” polymer by means of residual initiator or other basic chemical species. Later, Swanson et al. [14] discovered that the addition of base was not necessary for the depolymerization of the polymer in acetonitrile. They also reported that the degradation of the polymer was observed even in solid phase of the polymer although the degradation rate in that case was very slow.

In this paper, we report on the synthesis, degradation behavior, and degradation mechanism of poly(ethyl cyanoacrylate) (PECA). The degradation behavior of the synthesized polymer was systematically observed from several points of view in an effort to possibly manipulate the degradability of this polymer. A tertiary amine (N,N′-dimethyl-p-toluidine, DMPT) was used as an anionic/zwitterionic initiator for the polymerization of the ethyl cyanoacrylate (ECA) monomer, which is presumed to lead to the zwitterionic polymerization process, propagating anionically without termination [21], [22], [23] as was proposed in Fig. 1.

Section snippets

Materials

Ethyl cyanoacrylate monomer (E–Z bond, viscosity; 5 cps, contains 0–0.5% hydroquinone as an inhibitor) was purchased from K&R International. Acetone (HPLC grade, >99.9%), methanol (HPLC grade, >99.9%), hydrochloric acid (37%, ACS reagent grade), and methanesulfonic acid (>99.5%) were purchased from Sigma–Aldrich and used without further purification. Nitromethane (HPLC grade, Fluka), N,N′-dimethyl-p-toluidine (99%, Alfa Aesar), and acetone-d6 (99.8% D atom, Acros Organics) were also used without

Results and discussion

The bulk radical polymerization was carried out by introducing calculated amounts of anionic/zwitterionic initiator (DMPT) diluted in acetone (1/1000 v/v) in a reaction vessel containing 10 mL of ECA with rapid initial mixing. DMPT is a tertiary amine and is commonly used as a catalyst or accelerator for polymer synthesis. In this report, DMPT was used as an effective initiator for the synthesis of PECA. Tertiary amines are known to rapidly initiate ECA polymerization with a strong exotherm and

Conclusions

Poly(ethyl cyanoacrylate) was successfully synthesized by DMPT using an anionic/zwitterionic pathway through both bulk and solution polymerization processes. The degradation behaviors of this polymer were observed in many aspects. The polymer had relatively poor thermal stability; the polymer starts losing weight at about 160 °C and completely degraded at around 300 °C at a heating rate of 20 °C/min by TGA. The calculated activation energy for degradation based on the Arrhenius equation was 37.4 

Acknowledgment

This research was funded by Biotechnology Research and Development Corporation (BRDC). The allowance and assistance of TGA experiments of Dr. Abdellatif Mohamed and Mr. Jason Adkins are greatly appreciated. We also thank Dr. Karl Vermillion for his NMR experiment and helpful comments. We are also in debt to Ms. Sheila Maroney for her careful proofreading of this manuscript.

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