Photoinduced Copolymerization of APMP-MMA: The Role of Reactive Hindered Amine APMP
4-Acryloyl-1,2,2,6,6-pentamethyl-piperidinol (APMP) is a reactive hindered amine that prolongs the service life of the polymeric materials and exhibits high stability, good resistance to extraction, and low toxicity. In this paper, a photoinduced free radical copolymerization of APMP and methyl methacrylate (MMA) is accomplished at ambient temperature in solution. APMP plays a key role in the copolymerization, owing to the nitroxides generated in situ from the moiety of 1,2,2,6,6-pentamethyl-piperidine under UV irradiation, and mediates the copolymerization, which is confirmed by the linear kinetics. With the increment of initial monomer feed ratios of APMP/MMA, both the copolymerization rate and the average molecular weight increase. According to the reactivity ratios from the extended Kelen-Tüdos (KT) method at high conversion by 1H NMR spectroscopy, a nonlinear model is established and the sequential distribution in the copolymers is also investigated. The dispersion of APMP units is regulated by the feed ratios and reactivity ratios.
Comparing with thermal process, the photoinduced polymerization is a generally economical and convenient technology that offers a number of advantages, including ambient temperature processing, fast curing, and spatial and temporal control over the polymerization . It has been used in extensive application, such as coating industry, paints or printing inks, adhesives, composite materials, and dental restorative formulations . In a photoinduced polymerization, the monomers determine the polymers with the desired physical and chemical properties, which will be in evaluation of the specific final applications.
Acrylates are often used as monomers in photoinduced polymerization for their high activity. The acrylate monomer with functional groups can provide plenty of excellent characteristics. For example, the acrylates containing fluorine provide the polymers with unique surface properties in high thermal stability, good chemical inertness and oxidative stability, superior weatherability, oil and water repellency, low flammability, and good biocompatibility as well as low refractive index [3, 4]. The acrylates containing silicon enhanced the adhesion to the surface and extraction resistance in coating . The acrylates containing azobenzene group exhibit excellent photoelectric property, which is considered as a light-responsible smart monomer . Acrylates are versatile when modified by the functional groups. Herein, an acrylate monomer containing a hindered amine group 1,2,2,6,6-pentamethyl-piperidine, called 4-acryloyl-1,2,2,6,6-pentamethyl-piperidinol (APMP), is introduced.
The oxidation of APMP in a photoinduced polymerization has been shown in Scheme 1, where the product is 4-acryloyl-2,2,6,6-tetramethyl-piperidinyl-1-oxyl (A-TEMPO). Since 2,2,6,6-tetramethyl-piperidinyl-1-oxyl (TEMPO) is well known for the hindered stable radical, excellent capacity in trapping active radicals , and fast electron transfer, the molecule structures with TEMPO moiety are used as antioxidants [8, 9], hindered amine light stabilizers (HALS) , molecular probes , spin labels , mediators for the controlled/living polymerization , and reactive redox mediators in organic radical battery [14, 15].
As a reactive hindered amine, APMP can incorporate into the polymer by photoinduced radical polymerization to show higher stability, more resistance to extraction, and lower toxicity [16–20]. It has been proved that the UV-curing coating which copolymerized APMP exhibits higher photostability than those with the commercial light stabilizer Tinuvin292 . When APMP is copolymerized with other monomers, the reactivity ratios of the system become important for that they are the quantitative values to predict the copolymer composition for any starting feed to understand the kinetic and mechanistic aspects of copolymerization . It has attracted certain interest in the living radical copolymerization . The determination of monomer reactivity ratio is carried out by the methods of Mayo-Lewis , Fineman-Ross , Kelen-Tüdos, and extended Kelen-Tüdos [26–28].
In this paper, the reaction kinetics in the photoinduced copolymerization of APMP with methyl methacrylate (MMA) have been investigated. By the analysis of 1H NMR spectra, the reactivity ratios of the two monomers at high monomer conversions and the sequential distribution of poly(APMP-co-MMA) have been obtained . The study of this photoinduced copolymerization could be the guidance for the application of APMP derivatives as the reactive light stabilizers.
2. Experimental Section
4-Acryloyl-1,2,2,6,6-pentamethyl-piperidinol (APMP) was prepared according to the literature [30, 31]. Darocur1173 (2,2-dimethyl-2-hydroxy-acetophenone, BASF) was used as a photoinitiator without further purification. Methyl methacrylate (MMA) and benzene were distilled before application.
2.2. APMP-MMA Copolymerization
APMP and Darocur1173 (2.5 wt.% of the total monomer dosage) were accurately weighed and dissolved in benzene. The feeding molar ratios of APMP/MMA (/) for the copolymerization system were as follows: 10/90, 30/70, 50/50, 70/30, and 90/10. The solution (3 mL) with 50 wt.% monomers was charged through a syringe into a polyethylene bag, which was quickly sealed up with a plastic sealer after the air inside had been driven out by lightly pressing.
The sample bag was irradiated at ambient temperature under the light intensity of 2.1 mW·cm−2 measured by a UV radiometer (Photoelectric Instrument Factory of Beijing Normal University, China), which was at a distance of 20 cm from the UV radiation photoreactor (Mejiro Genossen CHG-200). After irradiation, the polymer was purified from the mixture using two or three times the reprecipitation with water/acetone and dried for 12 hrs in vacuum at 50°C subsequently. Percentage of monomer conversions was obtained gravimetrically.
2.3. Characterization of Poly(APMP-co-MMA) Copolymers
The structures and compositions of poly(APMP-co-MMA) had been characterized using 1H NMR spectroscopy, recorded at 400 MHz on a Varian Unity 400 NMR from acetone-d6 or chloroform-d6 solution at ambient temperature. Gel permeation chromatography (GPC) was performed using a Waters 515 instrument equipped with a 510 Waters pump and a RI-410 refractometer. A polystyrene gel column was used with THF as the eluent at 40°C.
APMP monomer: 1H NMR (400 MHz, acetone-d6,δ, ppm): 1.15–0.89 (m, 12H, 4 -CH3), 1.56–1.37 (m, 2H, -CH2-), 1.95–1.66 (m, 2H, -CH2-), 2.18 (d, = 4.65 Hz, 3H, >N-CH3), 5.20–4.90 (m, 1H, >CH-), 5.73 (d, = 10.43 Hz, 1H, =CH-), 6.03 (dd, = 17.34, 10.40 Hz, 1H, =CH-), 6.31 (d, = 17.29 Hz, 1H, =CH-).
3. Results and Discussion
3.1. APMP-MMA Copolymerization
The APMP-MMA copolymerization was performed under the light irradiation at ambient temperature. The total monomer conversion changed with irradiation time and the kinetics was shown in Figure 1. From Figure 1(a), it revealed that the total monomer conversion was 0.605 in 60 min, where the feed ratio was 10/90 of APMP/MMA. When the feed ratio was 90/10, the conversion reached 0.609 only in 8 min. It demonstrated that, with the higher feed ratio of APMP, the conversion increased in a shorter time. Figure 1(b) exhibited that the reaction kinetics of the APMP-MMA copolymerization were linear, owing to the nitroxides that oxidized by the dissolved oxygen in solution under irradiation from APMP monomers and APMP units in the copolymers. The nitroxides exhibited a good performance to trap the radicals and mediated the polymerization .
However, the molecular weight distribution (MWD) of the copolymers was not the same as those in other nitroxides mediated polymerization . The MWD of the copolymers from the GPC had a wide distribution that was shown in Table 1. With the increment of the feed ratios of APMP/MMA, the average molecular weight increased dramatically but with different MWD. It was inferred that the nitroxides were generated from both the APMP monomer and APMP units in the copolymers. When the APMP units in the copolymer were generated into nitroxides, they could trap the propagating radicals to form branched dormant chains. The concentration of branched dormant chains increased with the concentration of AMPM in the copolymerization. Therefore, the copolymer had a high average molecular weight when the feed ratios of APMP/MMA increased. The assumption was shown in Scheme 2.
3.2. 1H NMR Spectra of Poly(APMP-co-MMA)
The 1H NMR spectra of the poly(APMP-co-MMA) where total monomer conversion was about 0.4 were shown in Figure 2. It was shown that, with the decrement of APMP feed ratio, the peaks at 0.85 ppm which indicated the trials of MMA units  increased dramatically. When the feed ratio of APMP/MMA was no more than 50/50, MMA tended to form syndiotactic units. Meanwhile, the minor peaks at 0.9~1.0 ppm suggested that some MMA formed random trials. The implication for isotatic trials of MMA units at 1.2~1.3 ppm was little [33, 34].
In Figure 3, the 1H NMR spectra in the range of 0.8~1.2 ppm at different total monomer conversion for each APMP-MMA copolymerization system were shown. The peaks at 0.85 ppm enlarged evidently with the increment of total monomer conversion from Figures 3(a) and 3(b) and kept nearly constant in Figure 3(c); no peaks were found at 0.85 ppm in Figures 3(d) and 3(e). It could be deduced that when the feed ratio of APMP/MMA was less than 50/50, most of the APMP monomer is exhausted at relatively lower total monomer conversion and the rest of the MMA monomer had to form homopolymerized segments in the chain propagation. When the feed ratio of APMP/MMA equaled 50/50, the feed ratio approached the azeotropic copolymerization. While the feed ratios were larger than 50/50, the MMA monomer copolymerized with APMP, with single MMA unit dispersing in the copolymer backbone.
3.3. Determination of Reactivity Ratios
The absorbing band of -OCH3 () was selected as a quantitative standard of methacrylate component due to the fact that there were no interfere bands in the nearby region, while the absorbing bands of all protons () except for -OCH3 were as a standard of APMP component. Assuming the peak area of each proton was and each -OCH3 in MMA was in the copolymer, it could be concluded that equaled 3a for there were 3 -H atoms in -OCH3 moiety. In other words, the amount of MMA units that were in the copolymer was . When the other peak area of the absorbing bands of all protons except for -OCH3 was , it could be inferred that the APMP units in the copolymer were for there were 23 -H atoms in a APMP unit and 8 -H atoms in a MMA unit. Therefore, the molar ratio of APMP units and MMA units in the copolymers was shown asFrom the results of (1), the composition of (APMP units) and (MMA units) in the copolymers was obtained by (2). The composition data of poly(APMP-co-MMA) estimated from 1H NMR were listed in Table 2:
In Table 2, it was shown that (APMP units) decreased with the increment of total monomer conversion when the initial feed ratios of APMP/MMA were 10/90 and 30/70. Meanwhile, the trends of (MMA units) versus conversion were opposite to versus conversion. However, and changed a little while the conversion increased when the initial feed ratios were no less than 50/50.
The composition of APMP in the copolymers could be obtained according to the initial feed ratio of APMP (), which was shown in Figure 4. When the initial feed ratio of APMP was 0.57, it would be an azeotropic copolymerization of APMP with MMA.
The extended Kelen-Tüdos (K-T) methods (see (3)) are applicable for the manipulation of high conversion. In (3), is the feed ratio and is the molar ratio of monomers and contained in the copolymers; is the conversion of the copolymerization, which is estimated by gravimetric measurement; stand for the molecular weight ratio of monomers, in which and are the molecular weights of monomers and , respectively. and are the highest and lowest values calculated from the experimental data:
The corresponding versus plots were shown in Figure 5. From the values of the slope and intercept of the fitting line in Figure 5, it was obtained that and . The results showed that both APMP and MMA had a tendency toward alternating copolymerization in the copolymerization system.
3.4. A Nonlinear Model for APMP-MMA Copolymerization
Skeist  developed a relation to describe the drift in monomer feed ratio in respect of the course of a copolymerization, which Meyer and Lowry  later derived to where and represented the initial monomer feed composition and the feed composition of at overall monomer conversion , respectively. The equation described the drift of monomer feed composition as each monomer was consumed at different rates during the copolymerization. The cumulative copolymer composition of APMP () depended on the overall conversation , which was shown in
The cumulative copolymer composition versus total monomer conversion for the photoinduced APMP-MMA copolymerization was shown in Figure 6, where the plots were obtained from the experimental results of the copolymerization and the lines obtained from the reactivity ratios (, ) by (4) and (5). The constant composition of the APMP in the copolymers would be 0.57 when the azeotropic copolymerization would occur with the initial feed ratio of APMP of 0.57. In Figure 6, it was illustrated that the plots of versus were dispersed on the lines, which indicated that the nonlinear model from (4) and (5) was fit for the photoinduced copolymerization of APMP-MMA. In other words, the reactivity ratios that were from the extended KT method were confirmed in the copolymerization.
3.5. Sequential Distribution of Poly(APMP-co-MMA)
When was the sequential number of and , the probabilities to have the and in the copolymer were presented by the molar ratios and , which were obtained bywhere , , , and . and denoted the probabilities of and unit in the copolymers, respectively. The sequential distribution of APMP units was greatly affected by the feed ratios and the reactivity ratios. When and , the number of the sequence lengths (APMP units) and (MMA units) was illustrated in Figure 7. For , it was about 98.8% of unitary APMP contained in the backbone of poly(APMP-co-MMA). The results showed that unitary APMP units were dispersed in the MMA chains. For , the chance to have unitary APMP was 88.6% and unitary MMA was 62.7% in poly(APMP-co-MMA) for . For , the probability of unitary APMP or MMA was 73.0% and 82.8%, respectively. It was obtained that the alternating tendency prevailed. For , the probability of unitary MMA was 93.1%, which indicated that the APMP units were separated by the single MMA unit. While was further up to 0.9, the probability of unitary APMP was 12.3% and of APMP diads was 10.8%. The sequence of 10 units of APMP still had the probability of 3.8% existing in the backbone, where the sequential distribution was nearly opposite to the results from . It could be judged that the sequence distribution was affected by the feed ratios of APMP/MMA. When the APMP units dispersed in the copolymer chains equally, the copolymer could exhibit high ability to trap the radicals to protect the materials. The sequential distribution could be the guidance for the application of APMP derivatives as the light stabilizers.
The photoinduced copolymerization of APMP with MMA at ambient temperature was investigated. Both the rate of the photoinduced copolymerization and the average molecular weight increased with the increment of initial monomer feed ratio of APMP/MMA. The reaction kinetics of the copolymerization were linear. However, the WMD was irregular for nitroxide that was generated from APMP trapping the propagating radicals to form branched dormant. The reactivity ratios of the copolymerization had been estimated by the data from 1H NMR spectra, using the extended KT method at high conversions. The reactivity ratio for APMP and MMA was 0.353 and 0.168, respectively, which revealed that the resulting copolymer had a tendency toward alternation. Using the obtained reactivity ratios, a nonlinear model of the photoinduced copolymerization was established. The sequential distribution in poly(APMP-co-MMA) was also investigated, which was regulated by the feed ratios and reactivity ratios.
The authors declare that they have no competing interests.
The project was financially supported by the National Natural Science Foundation of China (Grant no. 51403041).
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