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Synthesis and degradation behavior of poly(ethyl cyanoacrylate)
Polymer Degradation and Stability 93 (2008) 1243–1251
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Synthesis and degradation behavior of poly(ethyl cyanoacrylate)q Moon Gyu Han*, Sanghoon Kim, Sean X. Liu Cereal Products and Food Science Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United Sates Department of Agriculture 1815 N. University Street, Peoria, IL 61604, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2008 Received in revised form 23 April 2008 Accepted 25 April 2008 Available online 2 May 2008
Poly(ethyl cyanoacrylate) was synthesized using N,N0 -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. Published by Elsevier Ltd.
Keywords: Degradation Poly(ethyl cyanoacrylate) Unzipping Gel permeation chromatography
1. 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 a-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
q Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. * Corresponding author. Tel.: þ1 309 681 6259; fax: þ1 309 681 6685. E-mail address: [email protected] (M.G. Han). 0141-3910/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2008.04.012
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 [10–12], 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 [15–17]. On the other hand, only limited works have been carried out on the degradation behavior of bulk phase PACAs [13,14,18–20]. The reason for this might be due to lack of a simple, general, and controllable polymer system for analyzing the properties.
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Fig. 1. Chemical structures and the schematic illustration of the zwitterionic polymerization process.
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,N0 -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 [21–23] as was proposed in Fig. 1.
were then dissolved in acetone to obtain molecular weight distribution data by gel permeation chromatography (GPC). A portion of the sample was precipitated in acidified methanol, washed twice with methanol and water, and dried under vacuum before conducting analysis by NMR and FT-IR. To evaluate the in situ degradation mechanism, polymerization was carried out in acetone at three different temperatures (5, 25, and 45 C) and the change in molecular weight was monitored in situ by GPC at fixed intervals during degradation at each temperature. The detailed procedure for solution polymerization is as follows: 1 mL of ECA was dissolved in 100 mL acetone in a 500 mL three-necked flask immersed in a water bath equipped with condenser. To the solution under nitrogen was introduced 0.2 mL of initiator DMPT dissolved in acetone (1/1000, v/v) with constant stirring. The polymerization of the monomer took place immediately after the introduction of the initiator, presumably in several seconds. While the reaction conditions were preserved, 1.5 mL of the reaction solutions were periodically collected and transferred to GPC in order to obtain molecular weight distribution curves of the synthesized polymer. A portion of the reaction solution was precipitated by pouring it into an excess amount of acidified MeOH solution. The precipitated polymer was filtered and repeatedly washed with MeOH and deionized water, then finally dried in vacuo. The yields of the produced polymers at all reaction temperatures were over 93%.
2. Experimental
2.3. Characterization
2.1. Materials
Reaction temperatures of the exothermic bulk polymerization were monitored by an IR thermometer (HIOKI HITESTER 3443) interfaced to a laptop computer. A program recorded the temperature changes every 2 s. Molecular weights of the polymers were measured with GPC using a Shimadzu model equipped with auto injector (SIL-10AD VP), column oven (TO-10AS VP), liquid chromatograph pump (LC-10AT VP), and GPC column (Showdex, GF-510 HQ) at 25 C using acetone as an eluent at a flow rate of 0.3 mL/min. Two detectors, refractive index (Optilab DSP Interferometric Refractometer, Wyatt Technology) and light scattering (DAWN EOS Enhanced Optical System, Wyatt Technology) were used to detect and record GPC output. Poly(methyl methacrylate) (PMMA) standards were used for molecular weight calibration. Specific refractive index increment (dn/dc) values were measured to be 0.134 in acetone at 690 nm by refractive index detector and this value was used to determine absolute molecular weights and molecular weight distributions of PECA. 13C and 1H NMR spectra were obtained on a Bruker Avance 500 MHz using a 5 mm BBI probe. All polymer samples for NMR experiments were dissolved in
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,N0 -dimethyl-p-toluidine (99%, Alfa Aesar), and acetone-d6 (99.8% D atom, Acros Organics) were also used without further purification. 2.2. Polymer synthesis Bulk polymerization of ECA monomer was carried out by introducing various amounts of initiator, N,N0 -dimethyl-p-toluidine (DMPT) diluted in acetone (1/1000, v/v), into 10 mL of monomer ECA, followed by rapid mixing for 10 s. Transparent, hard, and glassy solid polymer products were obtained after completion of the exothermic polymerization process. The polymerized samples
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acetone-d6 to make an approximately 5 wt% (w/v) solution. FT-IR spectra were collected using a Varian 3100 FT-IR spectrometer by dropping a small amount of sample solution onto a KBr plate and allowing the solvent to evaporate. Thermal degradation profiles and the resulting weight loss of the samples were obtained by thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) experiments were carried out under nitrogen purge from 20 to 800 C at a rate of 20 C/min with a TGA 2050 Thermogravimetric Analyzer (TA Instruments). Isothermal degradation experiments were also conducted by TGA at fixed temperatures of 150, 162.5, 175, 185, and 200 C. 3. 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 produce high molecular weight polymers [21]. Fig. 2(a) shows the exotherms of polymerization as a function of reaction time at six different synthetic recipes using different initiator amounts (monomer/initiator ratio ranging from 27,000 to 135,000). After introducing initiator into the reaction vessel, each sample exhibited a certain period of induction time followed by the exothermic polymerization. The induction period may arise from the presence of hydroquinone inhibitor in the commercial monomer. Therefore, polymerization is inhibited until enough anionic species are generated to neutralize the reaction mixture, whereupon rapid polymerization of the monomer commences. As is shown in the figure, both induction time and the time at maximum exothermic temperatures are shorter with increasing initiator amount. The relationship between reaction time at peak temperatures and monomer/initiator ratios is summarized in Fig. 2(b). The time at maximum temperature is linearly decreasing with increasing initiator amount. The number average molecular weights of the resulting bulk polymers were all high and were not significantly different with initiator amount, ranging from 851,700 to 111,900 g/mol as shown
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in Table 1. As an example, weight average molecular weight (Mw) of the polymer synthesized using 0.2 mL of DMPT and 10 mL of ECA was 1,030,000 g/mol and the molecular weight distribution was fairly narrow (Mw/Mn ¼ 1.09). This is due to the peculiar characteristics of the monomer ECA for the anionic/zwitterionic polymerization. Johnston and Pepper [21] reported that the reaction rates and polymer molecular weights of PECA were usually unaffected by the amount of initiator due to extremely high stability of the propagating cyanoacrylate anion. Klemarczyk [23] also reported that high molecular weight polymer could be obtained by using tertiary amines as initiators for the polymerization of ECA monomer. The growing species have been believed to be macrozwitterions when the ACA was polymerized by tertiary phosphates or amines [24], propagating anionically without termination (see Fig. 1). Therefore, the counter ions are attached to the termini of the growing polymer chains, not independently mobile as in the conventional anionic polymerization. Although it was difficult to tailor the molecular weight and molecular weight distribution due to the above-mentioned peculiar properties of the monomer and the limitations of the bulk polymerization, such as difficulty in controlling reaction exotherm, the polymerization of ECA using DMPT has been successfully performed in the wide range of polymer/initiator ratios. Fig. 3(a) presents representative thermal degradation and derivative thermogravimetry (DTG) curves of the bulk-polymerized poly(ethyl cyanoacrylate) (PECA) prepared using 0.2 mL of DMPT (sample no. 3 in Table 1). The TGA curves of the bulk-polymerized samples exhibited similar degradation profiles irrespective of the initiator amounts. The polymer starts losing weight at about 160 C, shows maximum degradation at around 265 C, and completely degraded at 300 C. The trend of degradation is similar to that observed by other researchers [20]. It has been suggested that the stability of the PACA depends on the nature of the initiator, polymerization method, and the length of alkyl side chains, and that the degradation occurs by unzipping of the polymer chain to produce monomer without significant molecular weight loss [12,20]. Later, the thermal degradation of the anionically polymerized poly(butyl cyanocrylate) (PBCA) was further identified to be initiated at the chain end containing polymerization initiator [10]. Isothermal TGA experiments were also conducted to investigate the degradation mechanism for our samples prepared using DMPT as an initiator
Fig. 2. (a) Exotherms of polymerization for bulk anionic/zwitterionic polymerization of 10 mL of ECA initiated by DMPT at different initiator amounts (mL). (b) Reaction time taken from the maximum temperature for the respective synthetic recipes from Fig. 2(a) as a function of monomer/initiator ratios.
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Table 1 GPC data of obtained PECA using various amounts of initiator in the polymerization of 10 mL of ECA Sample no.
Initiator amount (mL)
Monomer/initiator [M]/[I] 103
106 Mn
Mw/Mn
1 2 3 4 5 6
0.1 0.15 0.2 0.3 0.4 0.5
135 90 67.5 45 33.8 27
1.051 1.119 1.032 0.857 1.016 0.852
1.35 1.28 1.09 1.13 1.13 1.12
(Fig. 3(b)). The weight loss from PECA during isothermal heating was measured at five different temperatures between 150 and 200 C under nitrogen environment in order to observe the degradation trend more deeply and to determine the degradation kinetic constants. A faster degradation rate of the polymer was observed with increasing isothermal temperatures. Furthermore, the isothermal degradation phase is almost linear as a function of the time except when the isothermal heating at 200 C is employed and in the very late stage at 185 C (after 60 min). In order to determine the degradation kinetic constants based on weight loss, rates of weight loss were calculated from the data shown in Fig. 3(b). From the calculated rates of weight loss against the percentage weight loss, the initial degradation was determined to be a first-order reaction with respect to the sample weight loss [25]. Then, the degradation rate constants (k) based on a first-order reaction were calculated from the initial slopes for each temperature from Fig. 3(b). These values were used to calculate activation energy for thermal degradation using the Arrhenius equation, ln(k) ¼ ln(A) Ea/RT. Arrhenius plots of ln (k) versus 1/T fall on almost straight line fitting (R2 ¼ 0.95) in Fig. 4. From the slope of this figure, the calculated activation energy over the entire temperature range (150–200 C) of the polymer degradation was 37.4 kcal/mol. This value is lower than those obtained for poly(methyl methacrylate) (PMMA) [26] and most of the reported ones for other polymers, but is comparable to that observed for the poly(butyl cyanoacrylate) (PBCA) using various initiators [20] and PECA [12]. The thermal degradation mechanism of the PACAs has been considered to be a chain unzipping with a zip length greater than the degree of polymerization [20] and the activation energy for the degradation increased with increasing the size of the alkyl side chain [12]. Although the degradation behavior of the PACAs has been known to occur by an unzipping process, a question arises: what happens if the degraded product, in this case monomer, is not eliminated from the system? One of the possible solutions for this question is to carry out the degradation experiments in solution. Generally, liquid phase degradation occurs at lower temperature than bulk phase due to several factors including viscosity, thermal conductivity, heat capacity, density, or interaction between solvents and polymers. We have examined the degradation behavior with GPC by tracing the molecular weight change of PECA in acetone. For this experiment, polymers were dissolved in acetone, preserved at room temperature in a closed system, and sampled for GPC periodically. Fig. 5 shows the change of the GPC curve of a PECA sample during extended time of storage in dilute solution of acetone at room temperature. Interestingly, the intensity of the high molecular weight PECA peak gradually decreased, while a new peak corresponding to lower molecular weight appeared and evolved over the time period of 5 weeks. Ryan and McCann [13] and Robello et al. [14] have reported a similar phenomenon and they concluded that this phenomenon was due to rapid depolymerization of PACAs in solvents (by adding extra initiator in THF [13] and without adding extra initiator in acetonitrile [14]), and simultaneous formation of lower molecular weight polymers from the depolymerized monomer due to residual or added
initiators. In addition, they have coined the high molecular intrinsic polymer as ‘‘parent’’ polymer and the newly produced lower molecular weight polymer as ‘‘daughter’’ polymer. According to the terms they used, the GPC peak position of the originally synthesized ‘‘parent’’ PECA remains almost constant. However, the intensity of the GPC peak of original high molecular weight ‘‘parent’’ polymer gradually diminished, with simultaneous growing of the peak of ‘‘daughter’’ polymer. The observed weight average molecular weight (Mw) of the ‘‘daughter’’ polymer was ca. 68,000 g/mol (Mw/Mn ¼ 1.15). However, the degradation rate was much slower than the previous researchers’ results [13,14]. This is presumed to be related to the different environmental conditions in solution such as solvent, initiator, end group of polymer chain, and the chemical impurity. This experiment has been performed exactingly because trace amount of any chemical species, even in a glass vial, can affect the degradation behavior. To further characterize the degradation behavior, the changes of calculated normalized area of the ‘‘parent’’ polymer and the ‘‘daughter’’ polymer obtained from GPC data are plotted against aging time. The resulting curvature of normalized area of the growing ‘‘daughter’’ polymer and the diminishing ‘‘parent’’ polymer in Fig. 5(b) are almost opposite. This indicates that most of the degraded monomer from the ‘‘parent’’ polymer chain participated in creating new ‘‘daughter’’ polymer. The degradation process in solution has been believed to occur not by random scission mechanism along the chain but by unzipping mechanism from chain end because there should be a gradual
Fig. 3. (a) TG and DTG curves of poly(ethyl cyanoacrylate) synthesized in bulk phase under nitrogen obtained at a scanning rate of 20 C/min. (b) Isothermal TGA weight loss curves of PECA under nitrogen at five different temperatures (150, 165, 175, 185, and 200 C).
M.G. Han et al. / Polymer Degradation and Stability 93 (2008) 1243–1251
Fig. 4. Arrhenius plot for the isothermal degradation of PECA. Circle symbol indicates data points, and solid line indicates linear fit of the data.
decrease of the molecular weight with the appearance of the polymers of intermediate size in the former case. But if the chain degradation occurs through unzipping from the terminus of the polymer chain and this process is a random concomitant process of chain-end initiation and unzipping, the molecular weight of the parent chain should be decreased and must be observable when the degradation is prominent. This fact lead us to suppose that this interesting degradation behavior may be due to the much faster rate of chain unzipping reaction compared to the chain-end activation; hence the chain-end activation process must be a ratedetermining step for the degradation as was discussed by Ryan and McCann [13]. There is another possibility that the degradation might have occurred by random chain scission of the polymer as a ratedetermining step followed by rapid unzipping depolymerization of the scission fraction and simultaneous formation of ‘‘daughter’’ polymer. That, however, is unlikely because there was no observable
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‘‘daughter’’ polymer when a small amount of methanesulfonic acid was added to the polymer solution to cap the chain end with methane sulfate, or at least to prevent repolymerization of the degraded monomers. In order to compare the degradation behavior in solution with thermal degradation, the molecular weight change after thermal degradation was studied. For this purpose, the molecular weights of the residual polymers were traced by GPC after isothermal degradation from TGA under nitrogen at 200 C for various durations (10, 20, 40 and 80 min). This experiment also eliminates the possibility of repolymerization of the monomer because the degradation products are volatilized and eliminated in the TGA chamber through nitrogen flow. In this case, the molecular weight of the polymer determined by GPC decreased with increasing isothermal degradation time as is shown in Fig. 6A. However, we found out that our result was not consistent with Birkinshaw and Pepper’s results on PBCA [20], where they mentioned that the molecular weights of the degraded samples were essentially unchanged after half of their weights were degraded in open pan under nonisothermal degradation. This may be due to differences in experimental methods and materials used. As was discussed earlier, however, the molecular weight of the degraded polymer should be changed when the degradation was prominent [26]. Otherwise, there must be an inhomogeneous thermal degradation process occurring such as preferential degradation of a lower molecular weight polymer or limitation of the diffusion of the degraded monomer through a polymer layer. To further understand the thermal degradation mechanism, the number average molecular weights of the residual polymers were compared with residual weight % remained after thermal degradation by TGA as a function of degradation time (Fig. 6B). Interestingly, the plots between molecular weight (Mn) change and the residual polymer weight % are almost fit. In addition, the normalized molecular weight, MN (¼ 100 Mt/M0) of the residual polymer is also plotted against weight loss % (in the inset of Fig. 6B), where Mt is number average molecular weight of the residual polymer after being heated at 200 C for time t, and M0 is the initial number average molecular weight of the polymer before thermal degradation. The relationship between normalized molecular
Fig. 5. (a) Change in the GPC curves of PECA during storage at room temperature in acetone. (b) Calculated area from GPC curves of ‘‘parent’’ polymer (square) and ‘‘daughter’’ polymer (triangle) as a function of storage time.
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Fig. 6. A. GPC curves after thermal degradation at 200 C for 0 min (a), 10 min (b), 20 min (c), 40 min (d) and 80 min (e). B. Number average molecular weight (Mn) change taken from GPC and the residual weight % obtained by TGA as a function of isothermal degradation time. Circle plot is the weight remaining and star plot is the number average molecular weight. Normalized number average molecular weight versus lost weight % is shown in the inset.
weight, MN, and the weight loss % of the polymer after thermal degradation is approximately linear with a slope close to one except for the sample degraded for 80 min. Based on these results, the thermal degradation process could be concluded to be a concomitant initiation over the entire polymer chain and the simultaneous unzipping of the monomer from chain end, which again is responsible for the strong correlation between residual weight % and number average molecular weight (Mn) of the isothermally degraded polymer. If the degradation was occurring predominantly by random scission process, the curve of weight residue recorded on TGA and that of the molecular weight changes observed by GPC would not match so closely. In addition, there was no intermediate state or bimodal distribution in the GPC output. This indicates that the degradation process is a very homogeneous unzipping process through all polymer chains and that the chain unzipping process is slow relative to chain-end activation. A similar phenomenon was reported by Munoz-Guerra and coworkers for their thermal decomposition analysis of poly(b, L-malic acid) [27]. In the GPC curves of samples (d) and (e) of Fig. 6A, ‘‘daughter polymer’’-like peaks were observed. It is not very clear about the origin of these peaks; it can be due to either random scission degradation in later stages of degradation or fast unzipping degradation after being dissolved in acetone with simultaneous production of the ‘‘daughter’’ polymer during the GPC experiment. The latter hypothesis is considered more plausible because a production and evolution of the ‘‘daughter’’ polymer together with the decrease of the peak intensity of the ‘‘parent’’ polymer were observed in every isothermally degraded sample during storage (data is not shown), a tendency similar to the pristine sample shown in Fig. 5. From these findings, it is concluded that the isothermal degradation of the polymer is a homogeneous chain-end initiated unzipping process. It should be again noted that the molecular weight decrease of the thermally degraded sample lies in the very fast chain end initiation process and the removal of the unzipped monomer. These differences compared to the degradation in solution resulted in the discrepancy between the two different experiments in the degradation behavior, although the degradation was carried out by unzipping processes in both cases. For example, the degree of degradation of the sample aged in solvent for 4 days (about 43% by calculated GPC curve area in Fig. 5) is similar to the sample thermally degraded for 20 min (ca. 50%, see Fig. 6(c)). But the average
molecular weight of the thermally degraded sample is about half that of the ‘‘parent’’ polymer of the solution degraded sample. FT-IR and NMR spectroscopy results are shown in Fig. 7 for studying the possible structural change of the polymer before and after degradation in solvents. FT-IR spectra of the monomer ECA, bulk-polymerized sample using 0.2 mL of DMPT, and the same sample after 2 weeks of degradation in d-acetone are presented in Fig. 7(a)–(c), respectively. The observed FT-IR spectra of ECA and PECA are similar to those of the reported typical ECA and PECA [28,29]. Our particular interests are in the peaks of C]C bond, because this functional group is only observed in the monomer and disappear in the polymer due to the formation of polymer chain as suggested in Fig. 1. The peak at 1616 cm1 in Fig. 7(a) is ascribed to C]C stretching vibration of the monomer ECA and disappears in the polymer samples (Fig. 7(b) and (c)). The peak at 3131.0 cm1 is due to ]C–H stretching vibrations of vinyl structures (]CH2, ]CH–) and is also only observed in Fig. 7(a). An interesting feature is that the peaks due to the existence of monomer do not appear in Fig. 7(c). In addition, the FT-IR spectra of (b) and (c) were almost identical, which means that there was no isolated monomer when the polymer was degraded in solvent. The peaks at around 3080– 2800 cm1 are from symmetric and asymmetric C–H stretching vibrations of –CH2 and CH3 groups in the ethyl substituent, which are internal standards that are not changed after polymerization. The CN stretching vibration peak shifts from 2239.4 to 2248.8 cm1 along with the diminution in peak intensity when the monomer is polymerized. This might be due to the conjugation effects between –CN and C]O/C]C bonds after polymerization [28]. The strong C]O peak at 1735.8 cm1 shifted to 1743.7 cm1 after polymerization. 1H nuclear magnetic resonance (NMR) spectra of PECA and the degraded PECA in d-acetone are shown in Fig. 7(d) and (e), respectively. Non-existence of the unpolymerized monomer was also confirmed by the absence of the peaks from olefinic protons of the ECA monomer, which were expected to appear as two singlets at around d 6.5 and d 7.1 ppm [23,30]. The peaks at d 4.3 and d 1.3 ppm are due to protons of ethyl side group CH3 and CH2, respectively. On the other hand, broad peaks located at d 2.6–3.0 are from the backbone CH2. No prominent change between degraded and pristine samples was observed. 13C NMR spectra produced the similar results (data not shown). These FT-IR and NMR results allow us to conclude that the polymerization was successfully completed and there was not observable monomer in
M.G. Han et al. / Polymer Degradation and Stability 93 (2008) 1243–1251
Fig. 7. FT-IR spectra of ECA (a), PECA (b), and PECA after degradation in acetone for 2 weeks (c); NMR spectra of the PECA (d) and degraded PECA (e).
the degradation product. This may be due to instantaneous repolymerization of the monomer into daughter polymer after depolymerization in acetone. In addition, the data provided by both NMR and FT-IR analyses indicate that the constitution of the polymer after degradation remained unaltered. The degradation behavior of the PECA obtained by bulk phase polymerization was traced in several aspects and it was confirmed that the degradation of the polymer is predominantly an unzipping process. To further pursue the origin of the degradation behavior in solvent, the GPC curve was monitored in situ during the polymerization of the ECAs in solution phase. PECA was synthesized according to the same synthetic recipe as sample no. 3 in Table 1 except using acetone as a polymerization medium and reduced amount of each chemical. Polymerization was carried out at three different temperatures (5, 25, and 45 C) under nitrogen environment. The anionic/zwitterionic polymerization of ECA in acetone was successfully carried out; the reaction was still fast and the yields were over 93%. The average molecular weights of the polymers determined by GPC were smaller compared to those of bulkpolymerized samples. This is probably due to lower the local concentration of the monomer in the solution polymerization system. The weight average molecular weight (Mw) decreased with polymerization temperature, though the change was slight. The weight average molecular weight (Mw) of the prepared polymers at 45, 25, and 5 C are 485,000 (Mw/Mn ¼ 1.32), 409,000 (Mw/ Mn ¼ 1.30), and 381,000 (Mw/Mn ¼ 1.47), respectively. The degradation behavior was examined in situ through careful monitoring of the molecular weight change by taking samples periodically from the reaction vessel and transferring to GPC. This experiment also allows us to trace degradation behavior at different temperatures in solution. As was expected, similar degradation behavior was obtained and the degradation rate was very dependent on the temperature. The GPC curves of the polymers after 1 h of polymerization time at 5 and 25 C and those of the same samples after 2 weeks of aging at same temperatures are presented in Fig. 8A. Fig. 8B shows calculated GPC peak areas of the ‘‘daughter’’ polymer over the aging time. The degradation rate was extremely slow or unobservable at 5 C for the 2 weeks. On the other hand, the daughter polymer grows gradually at 25 C and is easily observable, but has a much slower growth rate than at 45 C (compare with Fig. 9). An interesting feature is that the molecular weight of the daughter polymer synthesized at 25 C was also smaller than that observed in the bulk-polymerized sample. In addition, the molecular weights of the ‘‘daughter’’ polymer degraded at 25 and 45 C are almost identical (compare Figs. 8 and 9). These results indicate that the average molecular weight of the repolymerized ‘‘daughter’’
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polymer is dependent on the environmental conditions such as residual initiator and the basicity of the solvent, which are almost similar in this case. Fig. 9(a) presents closer observation of the molecular weight change during thermal aging at 45 C. The depolymerization– repolymerization behavior can be better understood by quantitative analysis of the GPC areas, shown in Fig. 9(b). It was found that there is a more complicated process for the polymer degradation in solution. As the aging time increased, the ‘‘parent’’ polymer gradually disappears and the ‘‘daughter’’ polymer grows, similar to the case at 25 C, but with a much faster rate than at 25 C. It is noteworthy that another peak corresponding to an even smaller molecule also grows, with concomitant decrease of the molecular weight of the ‘‘daughter’’ polymer. On the other hand, the molecular weight of the ‘‘parent’’ polymer gradually increases in conjunction with the decrement of the population of the polymer chain. This may be due to a further redistribution process of both ‘‘parent’’ and ‘‘daughter’’ polymers in their average molecular weights, producing ‘‘grand daughter’’ polymer or oligomer [31] with extremely small molecular weight. The area of oligomer increases linearly without prominent change in average molecular weight. It is also worth mentioning that the polydispersity of the ‘‘parent polymer’’ is continuously decreasing with time, while the average weight molecular weight is increasing as is shown in Fig. 9(c). The reported ceiling temperature for the PECA polymerization was around 176 C [32] and that for the PBCA polymerization was over 150 C [13]. Therefore, it is difficult to consider that the depolymerization process originates from an equilibrium process for the polymerization. Consequently, the depolymerization–repolymerization process can be deduced to be a molecular weight rearrangement process induced by a thermodynamic equilibrium state occurring after completion of the polymerization. The living nature of the anionic/zwitterionic polymer chain end and the unzipping of the monomer from the chain end should be responsible for this process. Therefore, this reaction in solution leads the polymers to undergo a depolymerization and a simultaneous repolymerization process in response to the appropriate chemical stimuli or temperature, toward a thermodynamically more stable state. From these research results, another supposition was drawn that the degradation of the PECA and the corresponding depolymerization–repolymerization reaction in solvent might also be related to the solvent itself, as well as to some residual initiator or basic impurity effects. To this point, we have changed solvent to
Fig. 8. A. GPC curves of the solution polymerized PECA in acetone; (a) polymerized at 5 C, (b) sample (a) after 2 weeks in acetone, (c) polymerized at 25 C, and (d) sample (c) after 2 weeks in acetone. B. The calculated areas of the ‘‘daughter’’ polymer versus respective aging times (circle and diamond plots are the data obtained at 25 and 5 C, respectively).
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Fig. 9. (a) GPC traces of PECA synthesized and aged at 45 C, (b) the calculated area change obtained from GPC; normalized area of the ‘‘parent’’ polymer (square), the area of the ‘‘daughter’’ polymer (triangle), and the area of the oligomer (circle) (c) the calculated number average molecular weight (square) and the polydispersity index, Mw/Mn, (circle) obtained from multi-angle laser light scattering data during aging at 45 C.
nitromethane in order to investigate solvent effect because this solvent is more acidic but compatible with acetone. The observed degradation rate was much slower or even stopped in nitromethane as is shown in Fig.10(a). The theoretical solubility parameters calculated by the group contribution methods are reported to be 11.2, 12.7, and 9.9 (cal/cm3)1/2 for PECA, nitromethane, and acetone, respectively [33], which does not seem to affect the degradation behaviors. On the other hand, reported donor numbers of acetone and nitromethane are 17 and 2.7 kcal/mol, respectively. Therefore, higher Lewis basicity of the acetone may be responsible for the degradation of the polymer. Detailed solvent effect on the degradation behaviors of these polymer homologues will be discussed later in a separate paper. The degradation behavior of the PECA in acetone was also further examined by adding a small amount of acid (methanesulfonic acid) to investigate the effect of chain end capping or solvent acidity (Fig. 10(b)). It was found that the addition of the methanesulfonic acid in the polymer solution prevented the formation of ‘‘daughter’’ polymer. But the degradation behavior could be controlled depending on
the amount of acid used. For example, introduction of a small amount of acid only lead to the retardation of the degradation and caused the production of the monomer instead of producing ‘‘daughter’’ polymer, as is indicated by the arrow in Fig. 10(b). With increasing time, the monomer combined to form very low molecular weight oligomer product as is indicted by another arrow. This result is attributed to the failure of the formation of ‘‘daughter’’ polymer from the unzipped monomer, caused by the inhibition effect of the acid. As was known, adding basic species promoted molecular weight degradation. Based on the findings obtained from these experimental results, PECA is believed to undergo depolymerization and a simultaneous repolymerization process in solution phase with basic conditions. This process is a thermodynamic re-equilibrium process and the degradation–repolymerization of PECA could be controlled by using different solvents, changing temperatures, and introducing additives. An understanding of the degradation behavior and mechanism is important for possible applications of these polymer homologues in various fields; not only in biomedical applications but also in industrial
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References
Fig. 10. (a) GPC curves of PECA dissolved and preserved in nitromethane at 25 C and (b) GPC curves of PECA dissolved and preserved in acetone with the addition of methane sulfonic acid at 25 C. GPC curve of ECA monomer in acetone is included for comparison. The upper arrow indicated at retention time 43 min coincides with monomer evolution. The other peak indicated by an arrow would be a very low molecular weight oligomer.
applications such as adhesives, polymer composites, and plastic recycling.
4. 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 kcal/mol. The thermal degradation occurred predominantly through unzipping of the monomer with a concomitant process of chain end initiation and depolymerization. The polymer also underwent a degradation reaction in acetone through rate-determining chain-end initiation and a rapid depolymerization reaction. The unzipped monomer rapidly repolymerized to produce lower molecular weight ‘‘daughter’’ polymer and even much lower molecular weight ‘‘oligomer’’ depending on the environmental conditions. Based on the additional experimental observations of in situ monitoring of the degradation behavior, the degradation process in solvent was believed to be attributed to a thermodynamic re-equilibrium process leading to molecular rearrangement due to the living nature of the polymer chain end. Finally, the degradation profiles could be controlled by changing solvents, temperatures, and additives.
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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[7] Oowaki H, Matsuda S, Sakai N, Ohta T, Iwata H, Sadat A, et al. Non-adhesive cyanoacrylate as an embolic material for endovascular neurosurgery. Biomaterials 2000;21:1039–46. [8] Park KJ, Park JH, Lee SB, Lee DY, Kim KN, Kim KM. Bioactive cyanoacrylatebased filling material for bone defects in dental applications. Key Eng Mater 2005;284–286:933–6. [9] Vezin WR, Florence AT. In vitro heterogeneous degradation of poly(n-alkyl acyanoacrylates). J Biomed Mater Res 1980;14:93–106. [10] Hickey A, Leahy JJ, Birkinshaw C. End-group identity and its effect on the thermal degradation of poly(butyl cyanoacrylate). Macromol Rapid Commun 2001;22:1158–62. [11] Denchev ZZ, Kabaivanov VS. Thermal behavior and adhesive properties of some cyanoacrylate adhesives with increased heat resistance. J Appl Polym Sci 1993;47:1019–26. [12] Chrobadjiev KG, Novakov PC. Study on the thermal degradation of poly(alkyl a-cyanoacrylate)s. Eur Polym J 1991;27:1009–15. [13] Ryan B, McCann G. 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Hydrolysis of alkyl cyanoacrylate and ethoxycarbonylpropenoylphosphonate polymers. J Polym Sci Polym Chem 1989;27:1967–74. [20] Birkinshaw C, Pepper DC. The thermal degradation of polymers of n-butyl cyanoacrylate prepared using tertiary phosphine and amine initiators. Polym Degrad Stab 1986;16:241–59. [21] Johnston DS, Pepper DC. Polymerization via macrozwitterions, 3. Makromol Chem 1981;182:421–35. [22] Cronin JP, Pepper DC. Zwitterionic polymerization of butyl cyanoacrylate by triphenylphosphine and pyridine. Makromol Chem 1988;189:85–102. [23] Klemarczyk P. The isolation of a zwitterionic initiating species for ethyl cyanoacrylate (ECA) polymerization and the identification of the reaction products between 1, 2 , and 3 amines with ECA. Polymer 2001;42:2837–48. [24] Johnston DC, Pepper DC. Polymerisation via macrozwitterions, 1. Makromol Chem 1981;182:393–406. [25] Manring LE. Thermal degradation of saturated poly(methyl methacrylate). Macromolecules 1988;21:528–30. [26] Kawshiwagi T, Hirata T, Brown JE. Thermal and oxidative degradation of poly(methyl methacrylate): molecular weight. Macromolecules 1985;18: 131–8. [27] Portilla-Arias JA, Garcia-Alvarez M, De Liarduya AM, Holler E, Munoz-Guerra S. Thermal decomposition of fungal poly(b, L-malic acid) and poly(b, L-malate)s. Biomacromolecules 2006;7:3283–90. [28] Toinson SK, Ghita OR, Hooper RM, Evans KE. The use of near-infrared spectroscopy for the cure monitoring of an ethyl cyanoacrylate adhesive. Vib Spectrosc 2006;40:133–41. [29] Zhou Y, Bei F, Ji H, Yang X, Lu L, Wang X. Property and quantum chemical investigation of poly(ethyl a-cyanoacrylate). J Mol Struct 2005;737:117–23. [30] Katti D, Krishnamurti N. Anionic polymerization of alkyl cyanoacrylates: in vitro model studies for in vivo applications. J Appl Polym Sci 1999;74:336–44. [31] Denchev ZZ, Kabaivanov VS. On the synthesis of ethyl-2-cyanoacrylate from waste residue. J Appl Polym Sci 1992;44:1829–35. 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Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Synthesis and degradation behavior of poly(ethyl cyanoacrylate)q Moon Gyu Han*, Sanghoon Kim, Sean X. Liu Cereal Products and Food Science Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United Sates Department of Agriculture 1815 N. University Street, Peoria, IL 61604, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2008 Received in revised form 23 April 2008 Accepted 25 April 2008 Available online 2 May 2008
Poly(ethyl cyanoacrylate) was synthesized using N,N0 -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. Published by Elsevier Ltd.
Keywords: Degradation Poly(ethyl cyanoacrylate) Unzipping Gel permeation chromatography
1. 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 a-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
q Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. * Corresponding author. Tel.: þ1 309 681 6259; fax: þ1 309 681 6685. E-mail address: [email protected] (M.G. Han). 0141-3910/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2008.04.012
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 [10–12], 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 [15–17]. On the other hand, only limited works have been carried out on the degradation behavior of bulk phase PACAs [13,14,18–20]. The reason for this might be due to lack of a simple, general, and controllable polymer system for analyzing the properties.
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Fig. 1. Chemical structures and the schematic illustration of the zwitterionic polymerization process.
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,N0 -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 [21–23] as was proposed in Fig. 1.
were then dissolved in acetone to obtain molecular weight distribution data by gel permeation chromatography (GPC). A portion of the sample was precipitated in acidified methanol, washed twice with methanol and water, and dried under vacuum before conducting analysis by NMR and FT-IR. To evaluate the in situ degradation mechanism, polymerization was carried out in acetone at three different temperatures (5, 25, and 45 C) and the change in molecular weight was monitored in situ by GPC at fixed intervals during degradation at each temperature. The detailed procedure for solution polymerization is as follows: 1 mL of ECA was dissolved in 100 mL acetone in a 500 mL three-necked flask immersed in a water bath equipped with condenser. To the solution under nitrogen was introduced 0.2 mL of initiator DMPT dissolved in acetone (1/1000, v/v) with constant stirring. The polymerization of the monomer took place immediately after the introduction of the initiator, presumably in several seconds. While the reaction conditions were preserved, 1.5 mL of the reaction solutions were periodically collected and transferred to GPC in order to obtain molecular weight distribution curves of the synthesized polymer. A portion of the reaction solution was precipitated by pouring it into an excess amount of acidified MeOH solution. The precipitated polymer was filtered and repeatedly washed with MeOH and deionized water, then finally dried in vacuo. The yields of the produced polymers at all reaction temperatures were over 93%.
2. Experimental
2.3. Characterization
2.1. Materials
Reaction temperatures of the exothermic bulk polymerization were monitored by an IR thermometer (HIOKI HITESTER 3443) interfaced to a laptop computer. A program recorded the temperature changes every 2 s. Molecular weights of the polymers were measured with GPC using a Shimadzu model equipped with auto injector (SIL-10AD VP), column oven (TO-10AS VP), liquid chromatograph pump (LC-10AT VP), and GPC column (Showdex, GF-510 HQ) at 25 C using acetone as an eluent at a flow rate of 0.3 mL/min. Two detectors, refractive index (Optilab DSP Interferometric Refractometer, Wyatt Technology) and light scattering (DAWN EOS Enhanced Optical System, Wyatt Technology) were used to detect and record GPC output. Poly(methyl methacrylate) (PMMA) standards were used for molecular weight calibration. Specific refractive index increment (dn/dc) values were measured to be 0.134 in acetone at 690 nm by refractive index detector and this value was used to determine absolute molecular weights and molecular weight distributions of PECA. 13C and 1H NMR spectra were obtained on a Bruker Avance 500 MHz using a 5 mm BBI probe. All polymer samples for NMR experiments were dissolved in
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,N0 -dimethyl-p-toluidine (99%, Alfa Aesar), and acetone-d6 (99.8% D atom, Acros Organics) were also used without further purification. 2.2. Polymer synthesis Bulk polymerization of ECA monomer was carried out by introducing various amounts of initiator, N,N0 -dimethyl-p-toluidine (DMPT) diluted in acetone (1/1000, v/v), into 10 mL of monomer ECA, followed by rapid mixing for 10 s. Transparent, hard, and glassy solid polymer products were obtained after completion of the exothermic polymerization process. The polymerized samples
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acetone-d6 to make an approximately 5 wt% (w/v) solution. FT-IR spectra were collected using a Varian 3100 FT-IR spectrometer by dropping a small amount of sample solution onto a KBr plate and allowing the solvent to evaporate. Thermal degradation profiles and the resulting weight loss of the samples were obtained by thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) experiments were carried out under nitrogen purge from 20 to 800 C at a rate of 20 C/min with a TGA 2050 Thermogravimetric Analyzer (TA Instruments). Isothermal degradation experiments were also conducted by TGA at fixed temperatures of 150, 162.5, 175, 185, and 200 C. 3. 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 produce high molecular weight polymers [21]. Fig. 2(a) shows the exotherms of polymerization as a function of reaction time at six different synthetic recipes using different initiator amounts (monomer/initiator ratio ranging from 27,000 to 135,000). After introducing initiator into the reaction vessel, each sample exhibited a certain period of induction time followed by the exothermic polymerization. The induction period may arise from the presence of hydroquinone inhibitor in the commercial monomer. Therefore, polymerization is inhibited until enough anionic species are generated to neutralize the reaction mixture, whereupon rapid polymerization of the monomer commences. As is shown in the figure, both induction time and the time at maximum exothermic temperatures are shorter with increasing initiator amount. The relationship between reaction time at peak temperatures and monomer/initiator ratios is summarized in Fig. 2(b). The time at maximum temperature is linearly decreasing with increasing initiator amount. The number average molecular weights of the resulting bulk polymers were all high and were not significantly different with initiator amount, ranging from 851,700 to 111,900 g/mol as shown
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in Table 1. As an example, weight average molecular weight (Mw) of the polymer synthesized using 0.2 mL of DMPT and 10 mL of ECA was 1,030,000 g/mol and the molecular weight distribution was fairly narrow (Mw/Mn ¼ 1.09). This is due to the peculiar characteristics of the monomer ECA for the anionic/zwitterionic polymerization. Johnston and Pepper [21] reported that the reaction rates and polymer molecular weights of PECA were usually unaffected by the amount of initiator due to extremely high stability of the propagating cyanoacrylate anion. Klemarczyk [23] also reported that high molecular weight polymer could be obtained by using tertiary amines as initiators for the polymerization of ECA monomer. The growing species have been believed to be macrozwitterions when the ACA was polymerized by tertiary phosphates or amines [24], propagating anionically without termination (see Fig. 1). Therefore, the counter ions are attached to the termini of the growing polymer chains, not independently mobile as in the conventional anionic polymerization. Although it was difficult to tailor the molecular weight and molecular weight distribution due to the above-mentioned peculiar properties of the monomer and the limitations of the bulk polymerization, such as difficulty in controlling reaction exotherm, the polymerization of ECA using DMPT has been successfully performed in the wide range of polymer/initiator ratios. Fig. 3(a) presents representative thermal degradation and derivative thermogravimetry (DTG) curves of the bulk-polymerized poly(ethyl cyanoacrylate) (PECA) prepared using 0.2 mL of DMPT (sample no. 3 in Table 1). The TGA curves of the bulk-polymerized samples exhibited similar degradation profiles irrespective of the initiator amounts. The polymer starts losing weight at about 160 C, shows maximum degradation at around 265 C, and completely degraded at 300 C. The trend of degradation is similar to that observed by other researchers [20]. It has been suggested that the stability of the PACA depends on the nature of the initiator, polymerization method, and the length of alkyl side chains, and that the degradation occurs by unzipping of the polymer chain to produce monomer without significant molecular weight loss [12,20]. Later, the thermal degradation of the anionically polymerized poly(butyl cyanocrylate) (PBCA) was further identified to be initiated at the chain end containing polymerization initiator [10]. Isothermal TGA experiments were also conducted to investigate the degradation mechanism for our samples prepared using DMPT as an initiator
Fig. 2. (a) Exotherms of polymerization for bulk anionic/zwitterionic polymerization of 10 mL of ECA initiated by DMPT at different initiator amounts (mL). (b) Reaction time taken from the maximum temperature for the respective synthetic recipes from Fig. 2(a) as a function of monomer/initiator ratios.
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Table 1 GPC data of obtained PECA using various amounts of initiator in the polymerization of 10 mL of ECA Sample no.
Initiator amount (mL)
Monomer/initiator [M]/[I] 103
106 Mn
Mw/Mn
1 2 3 4 5 6
0.1 0.15 0.2 0.3 0.4 0.5
135 90 67.5 45 33.8 27
1.051 1.119 1.032 0.857 1.016 0.852
1.35 1.28 1.09 1.13 1.13 1.12
(Fig. 3(b)). The weight loss from PECA during isothermal heating was measured at five different temperatures between 150 and 200 C under nitrogen environment in order to observe the degradation trend more deeply and to determine the degradation kinetic constants. A faster degradation rate of the polymer was observed with increasing isothermal temperatures. Furthermore, the isothermal degradation phase is almost linear as a function of the time except when the isothermal heating at 200 C is employed and in the very late stage at 185 C (after 60 min). In order to determine the degradation kinetic constants based on weight loss, rates of weight loss were calculated from the data shown in Fig. 3(b). From the calculated rates of weight loss against the percentage weight loss, the initial degradation was determined to be a first-order reaction with respect to the sample weight loss [25]. Then, the degradation rate constants (k) based on a first-order reaction were calculated from the initial slopes for each temperature from Fig. 3(b). These values were used to calculate activation energy for thermal degradation using the Arrhenius equation, ln(k) ¼ ln(A) Ea/RT. Arrhenius plots of ln (k) versus 1/T fall on almost straight line fitting (R2 ¼ 0.95) in Fig. 4. From the slope of this figure, the calculated activation energy over the entire temperature range (150–200 C) of the polymer degradation was 37.4 kcal/mol. This value is lower than those obtained for poly(methyl methacrylate) (PMMA) [26] and most of the reported ones for other polymers, but is comparable to that observed for the poly(butyl cyanoacrylate) (PBCA) using various initiators [20] and PECA [12]. The thermal degradation mechanism of the PACAs has been considered to be a chain unzipping with a zip length greater than the degree of polymerization [20] and the activation energy for the degradation increased with increasing the size of the alkyl side chain [12]. Although the degradation behavior of the PACAs has been known to occur by an unzipping process, a question arises: what happens if the degraded product, in this case monomer, is not eliminated from the system? One of the possible solutions for this question is to carry out the degradation experiments in solution. Generally, liquid phase degradation occurs at lower temperature than bulk phase due to several factors including viscosity, thermal conductivity, heat capacity, density, or interaction between solvents and polymers. We have examined the degradation behavior with GPC by tracing the molecular weight change of PECA in acetone. For this experiment, polymers were dissolved in acetone, preserved at room temperature in a closed system, and sampled for GPC periodically. Fig. 5 shows the change of the GPC curve of a PECA sample during extended time of storage in dilute solution of acetone at room temperature. Interestingly, the intensity of the high molecular weight PECA peak gradually decreased, while a new peak corresponding to lower molecular weight appeared and evolved over the time period of 5 weeks. Ryan and McCann [13] and Robello et al. [14] have reported a similar phenomenon and they concluded that this phenomenon was due to rapid depolymerization of PACAs in solvents (by adding extra initiator in THF [13] and without adding extra initiator in acetonitrile [14]), and simultaneous formation of lower molecular weight polymers from the depolymerized monomer due to residual or added
initiators. In addition, they have coined the high molecular intrinsic polymer as ‘‘parent’’ polymer and the newly produced lower molecular weight polymer as ‘‘daughter’’ polymer. According to the terms they used, the GPC peak position of the originally synthesized ‘‘parent’’ PECA remains almost constant. However, the intensity of the GPC peak of original high molecular weight ‘‘parent’’ polymer gradually diminished, with simultaneous growing of the peak of ‘‘daughter’’ polymer. The observed weight average molecular weight (Mw) of the ‘‘daughter’’ polymer was ca. 68,000 g/mol (Mw/Mn ¼ 1.15). However, the degradation rate was much slower than the previous researchers’ results [13,14]. This is presumed to be related to the different environmental conditions in solution such as solvent, initiator, end group of polymer chain, and the chemical impurity. This experiment has been performed exactingly because trace amount of any chemical species, even in a glass vial, can affect the degradation behavior. To further characterize the degradation behavior, the changes of calculated normalized area of the ‘‘parent’’ polymer and the ‘‘daughter’’ polymer obtained from GPC data are plotted against aging time. The resulting curvature of normalized area of the growing ‘‘daughter’’ polymer and the diminishing ‘‘parent’’ polymer in Fig. 5(b) are almost opposite. This indicates that most of the degraded monomer from the ‘‘parent’’ polymer chain participated in creating new ‘‘daughter’’ polymer. The degradation process in solution has been believed to occur not by random scission mechanism along the chain but by unzipping mechanism from chain end because there should be a gradual
Fig. 3. (a) TG and DTG curves of poly(ethyl cyanoacrylate) synthesized in bulk phase under nitrogen obtained at a scanning rate of 20 C/min. (b) Isothermal TGA weight loss curves of PECA under nitrogen at five different temperatures (150, 165, 175, 185, and 200 C).
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Fig. 4. Arrhenius plot for the isothermal degradation of PECA. Circle symbol indicates data points, and solid line indicates linear fit of the data.
decrease of the molecular weight with the appearance of the polymers of intermediate size in the former case. But if the chain degradation occurs through unzipping from the terminus of the polymer chain and this process is a random concomitant process of chain-end initiation and unzipping, the molecular weight of the parent chain should be decreased and must be observable when the degradation is prominent. This fact lead us to suppose that this interesting degradation behavior may be due to the much faster rate of chain unzipping reaction compared to the chain-end activation; hence the chain-end activation process must be a ratedetermining step for the degradation as was discussed by Ryan and McCann [13]. There is another possibility that the degradation might have occurred by random chain scission of the polymer as a ratedetermining step followed by rapid unzipping depolymerization of the scission fraction and simultaneous formation of ‘‘daughter’’ polymer. That, however, is unlikely because there was no observable
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‘‘daughter’’ polymer when a small amount of methanesulfonic acid was added to the polymer solution to cap the chain end with methane sulfate, or at least to prevent repolymerization of the degraded monomers. In order to compare the degradation behavior in solution with thermal degradation, the molecular weight change after thermal degradation was studied. For this purpose, the molecular weights of the residual polymers were traced by GPC after isothermal degradation from TGA under nitrogen at 200 C for various durations (10, 20, 40 and 80 min). This experiment also eliminates the possibility of repolymerization of the monomer because the degradation products are volatilized and eliminated in the TGA chamber through nitrogen flow. In this case, the molecular weight of the polymer determined by GPC decreased with increasing isothermal degradation time as is shown in Fig. 6A. However, we found out that our result was not consistent with Birkinshaw and Pepper’s results on PBCA [20], where they mentioned that the molecular weights of the degraded samples were essentially unchanged after half of their weights were degraded in open pan under nonisothermal degradation. This may be due to differences in experimental methods and materials used. As was discussed earlier, however, the molecular weight of the degraded polymer should be changed when the degradation was prominent [26]. Otherwise, there must be an inhomogeneous thermal degradation process occurring such as preferential degradation of a lower molecular weight polymer or limitation of the diffusion of the degraded monomer through a polymer layer. To further understand the thermal degradation mechanism, the number average molecular weights of the residual polymers were compared with residual weight % remained after thermal degradation by TGA as a function of degradation time (Fig. 6B). Interestingly, the plots between molecular weight (Mn) change and the residual polymer weight % are almost fit. In addition, the normalized molecular weight, MN (¼ 100 Mt/M0) of the residual polymer is also plotted against weight loss % (in the inset of Fig. 6B), where Mt is number average molecular weight of the residual polymer after being heated at 200 C for time t, and M0 is the initial number average molecular weight of the polymer before thermal degradation. The relationship between normalized molecular
Fig. 5. (a) Change in the GPC curves of PECA during storage at room temperature in acetone. (b) Calculated area from GPC curves of ‘‘parent’’ polymer (square) and ‘‘daughter’’ polymer (triangle) as a function of storage time.
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Fig. 6. A. GPC curves after thermal degradation at 200 C for 0 min (a), 10 min (b), 20 min (c), 40 min (d) and 80 min (e). B. Number average molecular weight (Mn) change taken from GPC and the residual weight % obtained by TGA as a function of isothermal degradation time. Circle plot is the weight remaining and star plot is the number average molecular weight. Normalized number average molecular weight versus lost weight % is shown in the inset.
weight, MN, and the weight loss % of the polymer after thermal degradation is approximately linear with a slope close to one except for the sample degraded for 80 min. Based on these results, the thermal degradation process could be concluded to be a concomitant initiation over the entire polymer chain and the simultaneous unzipping of the monomer from chain end, which again is responsible for the strong correlation between residual weight % and number average molecular weight (Mn) of the isothermally degraded polymer. If the degradation was occurring predominantly by random scission process, the curve of weight residue recorded on TGA and that of the molecular weight changes observed by GPC would not match so closely. In addition, there was no intermediate state or bimodal distribution in the GPC output. This indicates that the degradation process is a very homogeneous unzipping process through all polymer chains and that the chain unzipping process is slow relative to chain-end activation. A similar phenomenon was reported by Munoz-Guerra and coworkers for their thermal decomposition analysis of poly(b, L-malic acid) [27]. In the GPC curves of samples (d) and (e) of Fig. 6A, ‘‘daughter polymer’’-like peaks were observed. It is not very clear about the origin of these peaks; it can be due to either random scission degradation in later stages of degradation or fast unzipping degradation after being dissolved in acetone with simultaneous production of the ‘‘daughter’’ polymer during the GPC experiment. The latter hypothesis is considered more plausible because a production and evolution of the ‘‘daughter’’ polymer together with the decrease of the peak intensity of the ‘‘parent’’ polymer were observed in every isothermally degraded sample during storage (data is not shown), a tendency similar to the pristine sample shown in Fig. 5. From these findings, it is concluded that the isothermal degradation of the polymer is a homogeneous chain-end initiated unzipping process. It should be again noted that the molecular weight decrease of the thermally degraded sample lies in the very fast chain end initiation process and the removal of the unzipped monomer. These differences compared to the degradation in solution resulted in the discrepancy between the two different experiments in the degradation behavior, although the degradation was carried out by unzipping processes in both cases. For example, the degree of degradation of the sample aged in solvent for 4 days (about 43% by calculated GPC curve area in Fig. 5) is similar to the sample thermally degraded for 20 min (ca. 50%, see Fig. 6(c)). But the average
molecular weight of the thermally degraded sample is about half that of the ‘‘parent’’ polymer of the solution degraded sample. FT-IR and NMR spectroscopy results are shown in Fig. 7 for studying the possible structural change of the polymer before and after degradation in solvents. FT-IR spectra of the monomer ECA, bulk-polymerized sample using 0.2 mL of DMPT, and the same sample after 2 weeks of degradation in d-acetone are presented in Fig. 7(a)–(c), respectively. The observed FT-IR spectra of ECA and PECA are similar to those of the reported typical ECA and PECA [28,29]. Our particular interests are in the peaks of C]C bond, because this functional group is only observed in the monomer and disappear in the polymer due to the formation of polymer chain as suggested in Fig. 1. The peak at 1616 cm1 in Fig. 7(a) is ascribed to C]C stretching vibration of the monomer ECA and disappears in the polymer samples (Fig. 7(b) and (c)). The peak at 3131.0 cm1 is due to ]C–H stretching vibrations of vinyl structures (]CH2, ]CH–) and is also only observed in Fig. 7(a). An interesting feature is that the peaks due to the existence of monomer do not appear in Fig. 7(c). In addition, the FT-IR spectra of (b) and (c) were almost identical, which means that there was no isolated monomer when the polymer was degraded in solvent. The peaks at around 3080– 2800 cm1 are from symmetric and asymmetric C–H stretching vibrations of –CH2 and CH3 groups in the ethyl substituent, which are internal standards that are not changed after polymerization. The CN stretching vibration peak shifts from 2239.4 to 2248.8 cm1 along with the diminution in peak intensity when the monomer is polymerized. This might be due to the conjugation effects between –CN and C]O/C]C bonds after polymerization [28]. The strong C]O peak at 1735.8 cm1 shifted to 1743.7 cm1 after polymerization. 1H nuclear magnetic resonance (NMR) spectra of PECA and the degraded PECA in d-acetone are shown in Fig. 7(d) and (e), respectively. Non-existence of the unpolymerized monomer was also confirmed by the absence of the peaks from olefinic protons of the ECA monomer, which were expected to appear as two singlets at around d 6.5 and d 7.1 ppm [23,30]. The peaks at d 4.3 and d 1.3 ppm are due to protons of ethyl side group CH3 and CH2, respectively. On the other hand, broad peaks located at d 2.6–3.0 are from the backbone CH2. No prominent change between degraded and pristine samples was observed. 13C NMR spectra produced the similar results (data not shown). These FT-IR and NMR results allow us to conclude that the polymerization was successfully completed and there was not observable monomer in
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Fig. 7. FT-IR spectra of ECA (a), PECA (b), and PECA after degradation in acetone for 2 weeks (c); NMR spectra of the PECA (d) and degraded PECA (e).
the degradation product. This may be due to instantaneous repolymerization of the monomer into daughter polymer after depolymerization in acetone. In addition, the data provided by both NMR and FT-IR analyses indicate that the constitution of the polymer after degradation remained unaltered. The degradation behavior of the PECA obtained by bulk phase polymerization was traced in several aspects and it was confirmed that the degradation of the polymer is predominantly an unzipping process. To further pursue the origin of the degradation behavior in solvent, the GPC curve was monitored in situ during the polymerization of the ECAs in solution phase. PECA was synthesized according to the same synthetic recipe as sample no. 3 in Table 1 except using acetone as a polymerization medium and reduced amount of each chemical. Polymerization was carried out at three different temperatures (5, 25, and 45 C) under nitrogen environment. The anionic/zwitterionic polymerization of ECA in acetone was successfully carried out; the reaction was still fast and the yields were over 93%. The average molecular weights of the polymers determined by GPC were smaller compared to those of bulkpolymerized samples. This is probably due to lower the local concentration of the monomer in the solution polymerization system. The weight average molecular weight (Mw) decreased with polymerization temperature, though the change was slight. The weight average molecular weight (Mw) of the prepared polymers at 45, 25, and 5 C are 485,000 (Mw/Mn ¼ 1.32), 409,000 (Mw/ Mn ¼ 1.30), and 381,000 (Mw/Mn ¼ 1.47), respectively. The degradation behavior was examined in situ through careful monitoring of the molecular weight change by taking samples periodically from the reaction vessel and transferring to GPC. This experiment also allows us to trace degradation behavior at different temperatures in solution. As was expected, similar degradation behavior was obtained and the degradation rate was very dependent on the temperature. The GPC curves of the polymers after 1 h of polymerization time at 5 and 25 C and those of the same samples after 2 weeks of aging at same temperatures are presented in Fig. 8A. Fig. 8B shows calculated GPC peak areas of the ‘‘daughter’’ polymer over the aging time. The degradation rate was extremely slow or unobservable at 5 C for the 2 weeks. On the other hand, the daughter polymer grows gradually at 25 C and is easily observable, but has a much slower growth rate than at 45 C (compare with Fig. 9). An interesting feature is that the molecular weight of the daughter polymer synthesized at 25 C was also smaller than that observed in the bulk-polymerized sample. In addition, the molecular weights of the ‘‘daughter’’ polymer degraded at 25 and 45 C are almost identical (compare Figs. 8 and 9). These results indicate that the average molecular weight of the repolymerized ‘‘daughter’’
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polymer is dependent on the environmental conditions such as residual initiator and the basicity of the solvent, which are almost similar in this case. Fig. 9(a) presents closer observation of the molecular weight change during thermal aging at 45 C. The depolymerization– repolymerization behavior can be better understood by quantitative analysis of the GPC areas, shown in Fig. 9(b). It was found that there is a more complicated process for the polymer degradation in solution. As the aging time increased, the ‘‘parent’’ polymer gradually disappears and the ‘‘daughter’’ polymer grows, similar to the case at 25 C, but with a much faster rate than at 25 C. It is noteworthy that another peak corresponding to an even smaller molecule also grows, with concomitant decrease of the molecular weight of the ‘‘daughter’’ polymer. On the other hand, the molecular weight of the ‘‘parent’’ polymer gradually increases in conjunction with the decrement of the population of the polymer chain. This may be due to a further redistribution process of both ‘‘parent’’ and ‘‘daughter’’ polymers in their average molecular weights, producing ‘‘grand daughter’’ polymer or oligomer [31] with extremely small molecular weight. The area of oligomer increases linearly without prominent change in average molecular weight. It is also worth mentioning that the polydispersity of the ‘‘parent polymer’’ is continuously decreasing with time, while the average weight molecular weight is increasing as is shown in Fig. 9(c). The reported ceiling temperature for the PECA polymerization was around 176 C [32] and that for the PBCA polymerization was over 150 C [13]. Therefore, it is difficult to consider that the depolymerization process originates from an equilibrium process for the polymerization. Consequently, the depolymerization–repolymerization process can be deduced to be a molecular weight rearrangement process induced by a thermodynamic equilibrium state occurring after completion of the polymerization. The living nature of the anionic/zwitterionic polymer chain end and the unzipping of the monomer from the chain end should be responsible for this process. Therefore, this reaction in solution leads the polymers to undergo a depolymerization and a simultaneous repolymerization process in response to the appropriate chemical stimuli or temperature, toward a thermodynamically more stable state. From these research results, another supposition was drawn that the degradation of the PECA and the corresponding depolymerization–repolymerization reaction in solvent might also be related to the solvent itself, as well as to some residual initiator or basic impurity effects. To this point, we have changed solvent to
Fig. 8. A. GPC curves of the solution polymerized PECA in acetone; (a) polymerized at 5 C, (b) sample (a) after 2 weeks in acetone, (c) polymerized at 25 C, and (d) sample (c) after 2 weeks in acetone. B. The calculated areas of the ‘‘daughter’’ polymer versus respective aging times (circle and diamond plots are the data obtained at 25 and 5 C, respectively).
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Fig. 9. (a) GPC traces of PECA synthesized and aged at 45 C, (b) the calculated area change obtained from GPC; normalized area of the ‘‘parent’’ polymer (square), the area of the ‘‘daughter’’ polymer (triangle), and the area of the oligomer (circle) (c) the calculated number average molecular weight (square) and the polydispersity index, Mw/Mn, (circle) obtained from multi-angle laser light scattering data during aging at 45 C.
nitromethane in order to investigate solvent effect because this solvent is more acidic but compatible with acetone. The observed degradation rate was much slower or even stopped in nitromethane as is shown in Fig.10(a). The theoretical solubility parameters calculated by the group contribution methods are reported to be 11.2, 12.7, and 9.9 (cal/cm3)1/2 for PECA, nitromethane, and acetone, respectively [33], which does not seem to affect the degradation behaviors. On the other hand, reported donor numbers of acetone and nitromethane are 17 and 2.7 kcal/mol, respectively. Therefore, higher Lewis basicity of the acetone may be responsible for the degradation of the polymer. Detailed solvent effect on the degradation behaviors of these polymer homologues will be discussed later in a separate paper. The degradation behavior of the PECA in acetone was also further examined by adding a small amount of acid (methanesulfonic acid) to investigate the effect of chain end capping or solvent acidity (Fig. 10(b)). It was found that the addition of the methanesulfonic acid in the polymer solution prevented the formation of ‘‘daughter’’ polymer. But the degradation behavior could be controlled depending on
the amount of acid used. For example, introduction of a small amount of acid only lead to the retardation of the degradation and caused the production of the monomer instead of producing ‘‘daughter’’ polymer, as is indicated by the arrow in Fig. 10(b). With increasing time, the monomer combined to form very low molecular weight oligomer product as is indicted by another arrow. This result is attributed to the failure of the formation of ‘‘daughter’’ polymer from the unzipped monomer, caused by the inhibition effect of the acid. As was known, adding basic species promoted molecular weight degradation. Based on the findings obtained from these experimental results, PECA is believed to undergo depolymerization and a simultaneous repolymerization process in solution phase with basic conditions. This process is a thermodynamic re-equilibrium process and the degradation–repolymerization of PECA could be controlled by using different solvents, changing temperatures, and introducing additives. An understanding of the degradation behavior and mechanism is important for possible applications of these polymer homologues in various fields; not only in biomedical applications but also in industrial
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References
Fig. 10. (a) GPC curves of PECA dissolved and preserved in nitromethane at 25 C and (b) GPC curves of PECA dissolved and preserved in acetone with the addition of methane sulfonic acid at 25 C. GPC curve of ECA monomer in acetone is included for comparison. The upper arrow indicated at retention time 43 min coincides with monomer evolution. The other peak indicated by an arrow would be a very low molecular weight oligomer.
applications such as adhesives, polymer composites, and plastic recycling.
4. 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 kcal/mol. The thermal degradation occurred predominantly through unzipping of the monomer with a concomitant process of chain end initiation and depolymerization. The polymer also underwent a degradation reaction in acetone through rate-determining chain-end initiation and a rapid depolymerization reaction. The unzipped monomer rapidly repolymerized to produce lower molecular weight ‘‘daughter’’ polymer and even much lower molecular weight ‘‘oligomer’’ depending on the environmental conditions. Based on the additional experimental observations of in situ monitoring of the degradation behavior, the degradation process in solvent was believed to be attributed to a thermodynamic re-equilibrium process leading to molecular rearrangement due to the living nature of the polymer chain end. Finally, the degradation profiles could be controlled by changing solvents, temperatures, and additives.
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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