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Microstructure determination of 2-hydroxy ethyl methacrylate and methyl acrylate copolymers by NMR spectroscopy
Journal of Molecular Structure 828 (2007) 25–37 www.elsevier.com/locate/molstruc
Microstructure determination of 2-hydroxy ethyl methacrylate and methyl acrylate copolymers by NMR spectroscopy A.S. Brar *, Sunita Hooda 1, Ashok Kumar Goyal Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India Received 6 April 2006; received in revised form 3 May 2006; accepted 6 May 2006 Available online 30 June 2006
Abstract Copolymers of 2-Hydroxy ethyl methacrylate and methyl acrylate (H/M) of different compositions were synthesized by free radical bulk polymerization using azobisisobutyronitrile (AIBN) as an initiator under nitrogen atmosphere. The copolymers compositions were calculated from 1H NMR spectra. The reactivity ratios for H/M copolymers obtained from a linear Kelen–Tudos method (KT) and nonlinear error-in-variables method (EVM) are rH = 3.31 ± 0.08, rM = 0.23 ± 0.00 and rH = 3.32, rM = 0.23, respectively. The complete spectral assignment of methine, methylene, methyl and carbonyl carbon regions in terms of compositional and configurational sequences of H/M copolymers was done with the help of 13C{1H} NMR, distortionless enhancement by polarization transfer (DEPT), two-dimensional heteronuclear single quantum coherence (HSQC) along with total correlated spectroscopy (TOCSY). Further, the assignments of carbonyl region were made with the help of heteronuclear multiple bond coherence (HMBC) spectrum. Ó 2006 Elsevier B.V. All rights reserved. Keywords: 2-Hydroxy ethyl methacrylate; Methyl acrylate; Copolymerization; Microstructure; NMR; Configuration
1. Introduction The high resolution NMR spectroscopy [1–4] is the most versatile, reliable and generally acceptable technique for the determination of microstructure of polymers. Copolymers based on 2-hydroxy ethyl methacrylate have found wide application in contact lenses, surgery and clinical medicine because of their ability to form biocompatible hydrogels with excellent tolerance and good stability [5–10]. Copolymerization of 2-hydroxy ethyl methacrylate with alkyl acrylate may be of practical interest considering that linear and cross linked copolymers based on 2-hydroxy ethyl methacrylate are widely utilized in ophthalmic industry, as a controlled drug release matrix and as non-thrombogenic materials and surgical prostheses [11–14], etc. Various coworkers *
Corresponding author. Tel.: +91 11 26591377; fax: +91 116 581 102. E-mail address: [email protected] (A.S. Brar). 1 Present address: Department of Chemistry, Acharya Narendra Dev College, Govindpuri, Kalkaji, New Delhi-110019, India. 0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.05.031
[15–17] have reported the controlled polymerization of poly (2-hydroxy ethyl methacrylate) or PHEMA by atom transfer radical polymerization. The sequence distribution in methyl methacrylate and methyl acrylate copolymers has been well reported earlier [18–20]. Copolymerization of 2-hydroxy ethyl methacrylate with methacrylic monomers have also been well reported [21–23]. To the best of our knowledge, the microstructure of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) copolymers has not been reported, but the reactivity ratios and copolymerization of 2-hydroxy ethyl methacrylate with alkyl acrylate have been reported by various coworkers [24–26]. In this manuscript, we report the microstructure of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) copolymers. The reactivity ratios of the comonomers calculated using a linear Kelen–Tudos method [27] and nonlinear least square error-in-variables method [28]. The complete 1H and 13C{1H} NMR spectral assignments of 2-hydroxy ethyl methacrylate and methyl acrylate copolymers were done with the help of DEPT and 2D (HSQC, TOCSY and HMBC) NMR experiments.
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2. Experimental
FH ¼
2-Hydroxy ethyl methacrylate and methyl acrylate (H/M) monomers were distilled under reduced pressure and stored below 5 °C. A series of H/M copolymers of different composition were prepared by bulk polymerization using AIBN as an initiator at 80 °C under nitrogen atmosphere. The percent conversion was kept below 10% by precipitating the copolymers in hexane. The copolymers were further purified using hexane/methanol system. The 1D and 2D NMR spectra were recorded on Bruker DPX-300 spectrometer in CD3OD. 1H and 13C measurements were made at frequencies of 300.13 and 75.5 MHz, respectively, and calibrated with respect to the solvent signal. Gradient HSQC and HMBC experiments were recorded using the pulse sequence invigpt and inv4gplplrnd of the Bruker software, respectively. The spectra were acquired with 512 increments in F1 dimension and 2048 data points in F2 dimension. TOCSY experiment was performed using standard pulse sequence. Thirty-two scans were accumulated for 512 experiments with 2 s delay time [29].
IðOCH2 ÞH =2 ½IfðOCH2 ÞH þ ðOCH3 ÞM g IðOCH2 ÞH M =3 þ IðOCH2 ÞH =2
where FH represents the composition of H-unit in the copolymer. Table 1 shows the copolymer composition data of H/M copolymer. According to Kelen–Tudos (KT) method, the terminal model reactivity ratios were calculated using the copolymer composition data. The reactivity ratios from error in variable method (EVM) were calculated using the reactivity ratio values obtained from KT method along with copolymer composition data. The values of reactivity ratios obtained from Kelen–Tudos (KT) [27] and nonlinear error in variable methods (EVM) [28] are rH = 3.31 ± 0.08, rM = 0.23 ± 0.00 and rH = 3.32, rM = 0.23, respectively. The reported values [25,26] of reactivity ratios for H/M copolymers are rH = 7.14, rM = 0.012, the difference in two values is due to different experimental conditions. The reactivity ratios of 2-hydroxy ethyl methacrylate and t-butyl acrylate copolymers [11] are rH = 1.792, rTBA = 0.510, these values are close to our values for H/M copolymer.
3. Results and discussion 3.1. Reactivity ratios determination The composition of H/M copolymers was determined from completely assigned one dimensional 1H NMR spectrum (Fig. 1). The intensity of –OCH2 protons of H-unit [I(OCH2)H] and intensity of combined signal of –CH2O– of H- and –OCH3 of M-unit were used for determination of copolymers composition as given in the following equation:
Table 1 Copolymer composition data of H/M copolymers S. No.
Sample No.
fH
FH
1 2 3 4 5
HM1 HM2 HM3 HM4 HM5
0.12 0.18 0.23 0.33 0.55
0.35 0.46 0.53 0.64 0.81
fH is the mole fraction of H comonomer in feed and FH is the mole fraction of H comonomer in copolymer.
Fig. 1. The 1H NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
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3.2.
13
C{1H} NMR studies
The 13C{1H} NMR spectrum of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) (FH = 0.53) copolymer in methanol at room temperature (25 °C) is shown in Fig. 2. The signals around d 17.0–24.0 ppm are due to a-CH3 carbon resonances of H-unit in the copolymer. The region around d 35.0–56.0 ppm is assigned to the overlap of methylene carbon of both H- and M-units, methine carbon of M-unit and quaternary carbon of H-unit in H/M
Fig. 2. The
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copolymer. The carbonyl carbon signals of H- and M-units of H/M copolymer are overlapped and assigned around d 176.1–180.2 ppm, whereas the signals around d 52.59, d 60.75 and d 67.77 ppm are assigned to (–OCH3)M, (–CH2O)H and (–OCH2)H carbon, respectively. The overlap of b-methylene and methine carbon resonance signals around d 35.0–56.0 ppm can be resolved by DEPT-135 NMR spectrum (Fig. 3). In DEPT-135 NMR spectrum, methine and methyl carbon appeared as a positive signal while methylene carbon appeared as a negative
13
C{1H} NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
Fig. 3. The DEPT-135 NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
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signal. The expanded b-methylene carbon regions of H- and M-units of H/M copolymers are sensitive to the distribution of dyad as well as tetrad compositional and configurational sequences. The resonance signal around d 37.5–43.5 ppm, which appeared as a positive signal, was assigned to –CH carbon of M-unit, while the negative signal around d 35.0–56.0 ppm was assigned to –CH2 carbon, which spread over 21 ppm range and because of its symmetry it is sensitive to dyad, tetrad and higher compositional and configurational placements. The expanded a-methyl carbon region of H-unit of H/M copolymers along with poly (2-hydroxy ethyl methacrylate) is shown in Fig. 4(a–d). Although the splitting pattern of a-CH3 carbon resonances seems to be well separated but spread over a wide range of chemical shifts due to tacticity effects in the spectrum of poly (2-hydroxy ethyl methacrylate). The multiplet in the methyl carbon region of H-unit of the copolymer is assigned to both the compositional and configurational sequences. The assignments to various signals have been carried out with the help of spectrum of poly (2-hydroxy ethyl methacrylate) and by observing
change in intensity of signals with change in composition of copolymers. The resonance signal 1 is assigned to HrHrH triad on comparison with poly (2-hydroxy ethyl methacrylate). The additional signal 2 that appears in copolymer spectrum is assigned to HrHrM triad. On comparison with spectrum of poly (2-hydroxy ethyl methacrylate), the signal 3 is assigned to HrHmH triad. The signals at 4 and 5 appeared on increasing the concentration of
Table 2 Assignment of a-CH3 carbon resonances of H/M copolymers from 13 C{1H} NMR and 2D HSQC spectra Peak No.
Peak assignments
Peak position (13C{1H}NMR; ppm)
Peak position (2D HSQC; 13C/1H; ppm)
1 2 3 4 5 6 7
HrHrH HrHrM HrHmH HrHmM MrHrM HmHmH HmHmM
17.75 18.80 19.80 21.05 21.50 22.50 23.15
17.76/1.15 18.80/1.20 19.82/1.31 21.05/1.28 21.55/1.38 22.53/1.45 23.20/1.30
Fig. 4. The expanded a-methyl region in 13C{1H} NMR spectrum of (a) PHEMA and H/M copolymers with compositions (FH=): (b) 0.81, (c) 0.53 and (d) 0.35 in CD3OD at 25 °C.
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M-unit and its intensity increases with increase in concentration of M-unit. So these signals are assigned to HrHmM and MrHrM triads, respectively. The resonance signals 6 and 7 are assigned to HmHmH and HmHmM triads, respectively. All the assignments are given in Table 2. The expanded DEPT-90 spectrum of H/M copolymers along with poly (methyl acrylate) is shown in Fig. 5. DEPT-90 spectra were used to distinguish the methine signals for 1D NMR analysis, as they were overlapping with
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the methylene resonances. On the basis of variation in intensity of signals with copolymer composition, the various resonance signals can be assigned up to triad compositional sequences. The resonance signal I is assigned to MMM triad while the resonance signals II and III are assigned to MMH, and HMH triads, respectively. The overlapped carbonyl carbon resonance signal, of H- and M-units in copolymer of different composition along with poly (2-hydroxy ethyl methacrylate) and poly
Fig. 5. The expanded DEPT-90 spectra of (a) PMA and H/M copolymers with compositions (FH=): (b) 0.35, (c) 0.53 and (d) 0.81 in CD3OD at 25 °C.
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(methyl acrylate) are shown in Fig. 6. The overlapped resonance signals in carbonyl carbon region have been assigned on the basis of variation in intensity of signals with concentration. On comparison with spectrum of poly (2-hydroxy ethyl methacrylate) and poly (methyl acrylate) [18], the signals 1 and 2 were assigned to HrHrH and HmHrH. On the basis of variation in intensity of signals with copolymer composition, the signal at 2 was also assigned to HHM and signal 3 to MHM triad of H monomer. The signals at 4, 5 and 6 were assigned to MMH + HMH, MMH and MMM triads of M-units, respectively. All the assignments are listed in Table 3.
Table 3 Assignment of carbonyl carbon resonances of H/M copolymers from 13 C{1H} NMR spectra S. No.
Peak position (13C{1H}NMR; ppm)
Assignment
1 2 3 4 5 6
179.75 178.72 178.28 177.82 177.30 176.70
HrHrH HmHrH + HHM MHM MMH + HMH MMH MMM
Fig. 6. The expanded carbonyl carbon regions of (a) PHEMA and H/M copolymers with compositions (FH=): (b) 0.81, (c) 0.53, (d) 0.35 and (e) PMA in CD3OD at 25 °C.
A.S. Brar et al. / Journal of Molecular Structure 828 (2007) 25–37
3.3. 2D HSQC and TOCSY NMR studies 3.3.1. Methylene carbon resonances 2D TOCSY spectra were used to confirm the 1, 2 bond geminal couplings between nonequivalent protons of the same methylene group. Methylene proton resonance signals which were overlapped and could not be assigned by 1 H NMR spectral analysis only were assigned from one-to-one correlation between carbon and proton signals in 2D HSQC spectra.The protons in the racemic methylene of MrM and HrH centered tetrads are in the same environment (as depicted in Scheme 1), resulting in a single cross-peak in 2D HSQC spectra. The two nonequivalent methylene meso protons, Ha and Hb, of HH and MM centered tetrads as shown in Scheme 1 result in two crosspeaks in the 2D HSQC spectra and a cross-correlation peak in 2D TOCSY spectra (Ha proton was attributed to the proton having high chemical shift and Hb having lower chemical shift). Thus, 2D TOCSY spectra enabled us to differentiate between the meso and racemic protons and confirm the 2D HSQC assignments.
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The expanded b-methylene region in 2D HSQC spectra is given in Fig. 7(a–c) and the assignments are listed in Table 4. The cross-peaks 1 and 3 were assigned to HHmHH(Ha) and HHmHH(Hb) tetrads, respectively. HHrHH tetrad was assigned to the cross-peak 2. Ha and Hb protons of HHmHH tetrads, being nonequivalent, resulted in cross-correlation peak 1 0 in the 2D TOCSY spectrum in Fig. 8(a–c). MHHH and MHHM tetrads of copolymer were assigned on observing the change in intensities of the cross-peaks with change in copolymer composition. MHmHH(Ha) and MHmHH(Hb) gave rise to two cross-peaks 4 and 6 while MHrHH tetrad was assigned to cross-peak 5. The Ha and Hb protons of MHmHH tetrad coupled to give cross-correlation peak 2 0 in 2D TOCSY spectrum. On the basis of variation in intensity of signals with copolymer composition the cross-peaks 7, 9 and 8 were assigned to MHmHM(Ha), MHmHM(Hb) and MHrHM, respectively, and can be observed at higher composition of M-unit, thus enabling us to differentiate between MHHH and MHHM. Ha and Hb protons of
Scheme 1. The structures of HH, HM and MM dyads.
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Fig. 7. The expanded b-methylene carbon regions of 2D HSQC spectra of H/M copolymers with composition (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
MHmHM coupled to give cross-correlation peak 3 0 in TOCSY spectrum (Fig. 8b). The methylene protons Ha and Hb of both HmM and HrM centered tetrads are in different magnetic environment as shown in Scheme 1, thus results in two cross-peaks by coupling with the methylene carbon in 2D HSQC spectra. Ha and Hb of HmM and HrM being nonequivalent also give cross correlation in 2D TOCSY spectra, enabling us to differentiate between cross-peaks of both HmM and HrM in 2D HSQC spectra. HHMH tetrad concentration decreases with increase in composition of M-unit, on this basis HHMH tetrad was
assigned. HHmMH(Ha) and HHmMH(Hb) tetrads were assigned to the cross-peaks 10 and 13, respectively, as marked in Fig. 7. HHrMH(Ha) and HHrMH(Hb) tetrads were assigned to cross-peaks 11 and 12. MHmMH/ HHmMM(Ha), MHmMH/HHmMM(Hb), MHrMH/ HHrMM(Ha) and MHrMH/HHrMM(Hb) were attributed to the cross-peaks 14, 17, 15 and 16, respectively. Similarly MHmMM(Ha), MHmMM(Hb), MHrMM(Ha) and MHrMM(Hb) were assigned to the cross-peaks 18, 21, 19 and 20, respectively. Ha and Hb protons of HHmMH, MHmMH/HHmMM, MHmMM and Ha and Hb protons of HHrMH, MHrMH/HHrMM, MHrMM tetrads
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Table 4 Assignment of b-methylene carbon resonance of H/M copolymers from 2D HSQC spectra Cross-peak No.
Cross-peak assignment
Peak position (2D HSQC;
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
HHmHH(Ha) HHrHH HHmHH(Hb) MHmHH(Ha) MHrHH MHmHH(Hb) MHmHM(Ha) MHrHM MHmHM(Hb) HHmMH(Ha) HHrMH(Ha) HHrMH(Hb) HHmMH(Hb) MHmMH/HHmMM(Ha) MHrMH/HHrMM(Ha) MHrMH/HHrMM(Hb) MHmMH/HHmMM(Hb) MHmMM(Ha) MHrMM(Ha) MHrMM(Hb) MHmMM(Hb) HMmMH(Ha) HMrMH HMmMH(Hb) HMmMM(Ha) HMrMM HMmMM(Hb) MMmMM(Ha) MMrMM MMmMM(Hb)
55.25/2.37 55.25/2.25 55.25/1.86 53.18/2.31 53.18/2.05 53.18/1.93 51.74/2.35 51.74/2.13 51.74/2.00 48.30/2.27 48.30/2.06 48.30/1.87 48.30/1.63 45.65/2.32 45.65/2.05 45.65/1.92 45.65/1.63 43.45/2.32 43.45/2.05 43.45/1.90 43.45/1.64 39.18/2.04 39.80/1.88 39.18/1.65 37.80/2.03 37.80/1.85 37.80/1.65 36.05/2.07 36.05/1.91 36.05/1.76
coupled to give cross-correlation peaks 4 0 and 5 0 , respectively, in 2D TOCSY Spectrum (Table 5). MM centered tetrads also showed compositional and configurational sensitivity. HMmMH(Ha), HMrMH and HMmMH(Hb) tetrads have cross-peaks 22, 23 and 24, respectively. HMmMM(Ha), HMrMM and HMmMM(Hb) have cross-peaks 25, 26 and 27, respectively, while MMMM tetrad was assigned by comparing with 2D HSQC and 2D TOCSY spectra of poly (methyl acrylate). MMmMM(Ha) was assigned to the cross-peak 28, MMmMM(Hb) to cross-peak 30, and cross-peak 29 was assigned to MMrMM tetrad. Geminal coupling between Ha and Hb protons of HMmMH and HMmMM tetrads was assigned to the cross-correlation peak 6 0 in 2D TOCSY spectrum. While Ha and Hb protons of MMmMM tetrad coupled to give cross-correlation peak at 7 0 . 3.3.2. Methine carbon resonances Methine group of methyl acrylate was assigned up to triad level of compositional sensitivity in the copolymer, based on the 2D HSQC assignments. 2D TOCSY studies were used to ascertain these assignments by assigning 1, 3 bond order couplings between methylene protons and methine protons of M monomer in MM and MH centered methylene.
13
C/1H; ppm)
The cross-peak I is assigned to MMM triad while the cross-peaks II and III are assigned to MMH, HMH triads, respectively (Fig. 7). The chemical shifts of methine group on proton axis were in a very narrow range (2.42– 2.75 ppm). The cross-correlation peaks 8 0 , 9 0 , 1 0 and 10 0 , as shown in Fig. 8, were assigned to the 1, 3 bond order couplings of –CH with –CH2 of MmH(Hb), MmM/ MrH(Ha), MrH(Hb)/MrM and MmH(Ha), respectively. 3.3.3. Methyl resonances The expanded a-CH3 region of 2D HSQC NMR spectra of H/M copolymer is shown in Fig. 9 [(a) FH = 0.35, (b) FH = 0.53, and (c) FH = 0.81]. The a-CH3 region of H-monomer in the copolymer shows compositional and configurational sensitivity. The cross-peaks 1, 2 and 3 are assigned to HrHrH, HrHrM and HrHmH triads, respectively. While the cross-peaks at 4 and 5 are assigned to HrHmM and MrHrM triads, respectively. HmHmH and HmHmM triads are assigned to crosspeaks 6 and 7, respectively. All the assignments are given in Table 2. 3.4. 2D HMBC studies The configurational and compositional sensitivity within carbonyl carbon region was investigated through
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Fig. 8. The 2D TOCSY spectrum of H/M copolymers with composition (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
Table 5 1 H-1H cross correlation between nonequivalent geminal protons in H/M copolymers observed from 2D TOCSY spectra Correlation peak No.
1
0
20 30 40 50 60 70 80 90 10 0
Peak position (2D TOCSY; 1H/1H; ppm)
Coupled protons Proton I
Proton II
CH of : M CH2 of : HHmHH(Ha) CH2 of : MHmHH(Ha) CH2 of : MHmHM(Ha) CH2 of : HmM(Ha) CH2 of : HrM(Ha) CH2 of : HMmMM(Ha) and HMmMH(Ha) CH2 of : MMmMM(Ha) CH of : M CH of : M CH of : M
CH2 of : MrH(Hb)/MrM CH2 of : HHmHH(Hb) CH2 of : MHmHH(Hb) CH2 of : MHmHM(Hb) CH2 of : HmM(Hb) CH2 of : HrM(Hb) CH2 of : HMmMM(Hb) and HMmMH(Hb) CH2 of : MMmMM(Hb) CH2 of : MmH(Hb) CH2 of : MrH(Ha)/MmM(Ha) CH2 of : MmH(Ha)
2.32/1.85 2.33/1.93 2.35/2.00 2.30/1.65 2.05/1.90 2.04/1.65 2.06/1.78 2.55/1.68 2.55/2.05 2.55/2.33
A.S. Brar et al. / Journal of Molecular Structure 828 (2007) 25–37
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Fig. 9. The expanded a-methyl carbon regions of 2D HSQC spectra of H/M copolymers with compositions (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
the analysis of long-range couplings between carbon and hydrogens with 2D HMBC NMR spectra, which has proven to be highly informative for the microstructural characterization of polymers. The completely assigned 1 H NMR spectra, performed with the help of 2D HSQC in conjunction with TOCSY, were used to assign the 2D HMBC NMR spectra. Fig. 10 shows the HMBC spectra of H/M copolymers, showing long-range coupling between carbonyl carbon with methyl and methylene protons of H- and M-units. The cross-peaks 1, 2 and 3 are assigned to the coupling of carbonyl carbon of MMH triad with methyl protons in MmMmH, MmMrH and MrMrH triads, respectively.
The cross-peaks 4, 5 and 6 are assigned to the coupling of >C=O carbon in MHM triads with methyl protons in MrHmM, MrHrM and MmHmM triads, respectively while the cross-peaks 7 and 8 are assigned to the coupling of carbonyl carbon in MHH + HmHrH and HrHrH triads with methyl protons in MHH + HmHrH and HrHrH triads, respectively. The cross-peaks for the coupling of carbonyl carbon with methylene protons are more predominant in the copolymer with higher compositions of M-unit, so most of the cross-peaks are assigned to the coupling of M-centered carbonyl carbon triad. The cross-peaks 9, 10 and 11 are assigned to the coupling of carbonyl carbon in MMM triads with methylene
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Fig. 10. The 2D HMBC spectra of H/M copolymers showing coupling of carbonyl carbon with methyl, methine and methylene protons of compositions (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
protons in MmM(Hb), MmM(Hb) diads and MMrMM tetrads, respectively. The cross-peaks 12 and 14 are assigned to the coupling of carbonyl carbon in MMH + HMH and MMH triads with Hb proton of HrM and Ha proton of MmM diads, respectively. While the carbonyl carbon of MMM triad gives a cross-peak 13 by coupling with Ha proton of MmM diad. The coupling of carbonyl carbon in MHM and MMH triads together with HrM(Ha) and HmM(Ha) diads is assigned to 15 and 16 cross-peaks, respectively. The cross-peak 17 is assigned to the coupling of carbonyl carbon in MMM
triad with methine proton of MMM triad. All the assignments are given in Table 6. 4. Conclusions The reactivity ratios of comonomers in H/M copolymers are rH = 3.32 and rM = 0.23. The complex and overlapped 1H and 13C{1H} NMR spectra of the copolymers were resolved with the help of DEPT and 2D HSQC spectra. The carbonyl carbons of H- and M-units were assigned up to triad compositional and
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Table 6 Coupling of carbonyl carbon with methyl, methine and methylene protons of H/M copolymers based on 2D HMBC spectra S. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Peak position (2D HMBC;
13
C/1H; ppm)
177.30/1.35 177.30/1.27 177.30/1.20 178.25/1.36 178.28/1.22 178.28/1.18 178.70/1.36 179.73/1.18 176.70/1.69 176.60/1.71 176.70/1.93 177.85/1.87 176.69/2.09 177.30/2.10 178.35/2.25 177.30/2.34 176.70/2.54
configurational sequences in 13C{1H} NMR spectrum. The methyl carbon signals were assigned upto triad compositional and configurational sequences, whereas bmethylene carbon resonances were assigned upto tetrad compositional sequences. The methine carbon of M-unit was assigned upto triad compositional sequences. Acknowledgement One of the authors, Sunita Hooda, wants to thank Department of Science and Technology (DST), New Delhi, India, for providing financial support. References [1] F.A. Bovey, L. Jelinski, P.A. Mirau, NMR of polymers, Academic Press, New York, 1996. [2] H.N. Cheng, J. Appl. Polym. Sci. Polym. Symp. 43 (1989) 129. [3] J.C. Randall, Polymer Sequence Determination: carbon-13 NMR Method, Academic Press, New York, 1977. [4] K. Matsuzaki, T. Uryu, T. Asakura, NMR spectroscopy and stereoregularity of polymers, Japan Sci. Soc. press, Tokyo, 1996. [5] S. Wen, X. Yin, W.T.K. Stevenson, J. Appl. Polym. Sci. 43 (1991) 205. [6] J. Gallar do, J.S. Roman, Polymer 34 (1993) 567. [7] L. Barannon-Peppas, N.A. Peppas, Biomaterials 11 (1990) 635. [8] J.P. Montheard, M. Chatzopoulos, D. Chappard, J. Macro. Sci.-Rev. Macro. Chem. Phys. C32 (1992) 1. [9] J. Singh, K.K. Agrawal, J. Macro. Sci.-Rev. Macro. Chem. Phys. C32 (1992) 521.
Type of carbon
Coupled to proton of
CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO
CH3 of MmMmH CH3 of MmMrH CH3 of MrMrH CH3 of MrHmM CH3 of MrHrM CH3 of MmHmM CH3 MHH + HmHrH CH3 of HrHrH CH2 of MmM(Hb) CH2 of MmM(Hb) CH2 of MMrMM CH2 of HrM(Hb) CH2 of MmM(Ha) CH2 of MmM(Ha) CH2 of HrM(Ha) CH2 of HmM(Ha) CH of MMM
of of of of of of of of of of of of of of of of of
MMH MMH MMH MrHmM MrHrM MmHmM MHH + HmHrH HrHrH MMM MMM MMM MMH + HMH MMM MMH MHM MMH MMM
[10] H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, Encyclopedia Polym. Sci. and Eng. 1 (1985) 234. [11] G. Martinez, M. Sanchez Chaves, E.L. Madruga, C. FerrandezMonreal, Polymer 41 (2000) 6021. [12] R. Jeyanthi, K. Pandurang Rao, Biomaterials 11 (1990) 238. [13] B.D. Ratner, J. Biomed. Mater. Res. 27 (1993) 283. [14] D.F. Williams, Concise Encyclopedia of Medical and Dental Materials, Pergamon Press, Oxford, 1990. [15] K.L. Beers, S. Boo, S.G. Gaynor, K. Matyjaszewski, Macromolecules 32 (1999) 5772. [16] S. Coca, C.B. Jasieczek, K.L. Beers, K. Matyjaszewski, J. Poly. Sci. Part A: Poly. Chem. 36 (1998) 1417. [17] K.L. Robinson, M.A. Khan, M.V. Paz Benez, X.S. de Wang, S.P. Armes, Macromolecules 34 (2002) 3155. [18] Y. Kim, H.J. Harwood, Polymer 43 (2002) 3229. [19] A.S. Brar, G. Singh, R. Shankar, J. Appl. Polym. Sci. 99 (2006) 2174. [20] A.S. Brar, G. Singh, R. Shankar, J. Appl. Polym. Sci. 99 (2006) 1437. [21] P. Tiemblo, M.F. Laguna, F. Garcia, J.M. Garcia, E. Riande, J. Guzman, Macromolecules 37 (2004) 4156. [22] D.J.T. Hill, J.H. O’Donnell, P.J. Pomery, Polym. Gels Netw. 3 (1995) 85. [23] N.G. Moss, P.J. Pomery, A.K. Whittaker, Polymer 41 (1999) 1287. [24] D. Mohan, G. Radhakrishnan, S. Rajadurai, K.T. Joseph, J. Polym. Sci. Part C: Polym. Lett. 28 (1990) 307. [25] I.K. Varma, S. Patnaik, Eur. Polym. J. 12 (1976) 259. [26] I.K. Varma, S. Patnaik, Eur. Polym. J. 13 (1977) 175. [27] T. Kelen, F. Tudos, J. Macro. Sci. Chem. A I (1975) 1. [28] M. Dubey, R.A. Sanyel, A. Penlidis, K.F. Driscoll, P.M. Reilly, J. Polym. Sci. Part A: Polym. Chem. 29 (1991) 703. [29] A.S. Brar, G. Singh, R. Shanker, J. Mol. Struct. 703 (2004) 69.
Microstructure determination of 2-hydroxy ethyl methacrylate and methyl acrylate copolymers by NMR spectroscopy A.S. Brar *, Sunita Hooda 1, Ashok Kumar Goyal Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India Received 6 April 2006; received in revised form 3 May 2006; accepted 6 May 2006 Available online 30 June 2006
Abstract Copolymers of 2-Hydroxy ethyl methacrylate and methyl acrylate (H/M) of different compositions were synthesized by free radical bulk polymerization using azobisisobutyronitrile (AIBN) as an initiator under nitrogen atmosphere. The copolymers compositions were calculated from 1H NMR spectra. The reactivity ratios for H/M copolymers obtained from a linear Kelen–Tudos method (KT) and nonlinear error-in-variables method (EVM) are rH = 3.31 ± 0.08, rM = 0.23 ± 0.00 and rH = 3.32, rM = 0.23, respectively. The complete spectral assignment of methine, methylene, methyl and carbonyl carbon regions in terms of compositional and configurational sequences of H/M copolymers was done with the help of 13C{1H} NMR, distortionless enhancement by polarization transfer (DEPT), two-dimensional heteronuclear single quantum coherence (HSQC) along with total correlated spectroscopy (TOCSY). Further, the assignments of carbonyl region were made with the help of heteronuclear multiple bond coherence (HMBC) spectrum. Ó 2006 Elsevier B.V. All rights reserved. Keywords: 2-Hydroxy ethyl methacrylate; Methyl acrylate; Copolymerization; Microstructure; NMR; Configuration
1. Introduction The high resolution NMR spectroscopy [1–4] is the most versatile, reliable and generally acceptable technique for the determination of microstructure of polymers. Copolymers based on 2-hydroxy ethyl methacrylate have found wide application in contact lenses, surgery and clinical medicine because of their ability to form biocompatible hydrogels with excellent tolerance and good stability [5–10]. Copolymerization of 2-hydroxy ethyl methacrylate with alkyl acrylate may be of practical interest considering that linear and cross linked copolymers based on 2-hydroxy ethyl methacrylate are widely utilized in ophthalmic industry, as a controlled drug release matrix and as non-thrombogenic materials and surgical prostheses [11–14], etc. Various coworkers *
Corresponding author. Tel.: +91 11 26591377; fax: +91 116 581 102. E-mail address: [email protected] (A.S. Brar). 1 Present address: Department of Chemistry, Acharya Narendra Dev College, Govindpuri, Kalkaji, New Delhi-110019, India. 0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.05.031
[15–17] have reported the controlled polymerization of poly (2-hydroxy ethyl methacrylate) or PHEMA by atom transfer radical polymerization. The sequence distribution in methyl methacrylate and methyl acrylate copolymers has been well reported earlier [18–20]. Copolymerization of 2-hydroxy ethyl methacrylate with methacrylic monomers have also been well reported [21–23]. To the best of our knowledge, the microstructure of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) copolymers has not been reported, but the reactivity ratios and copolymerization of 2-hydroxy ethyl methacrylate with alkyl acrylate have been reported by various coworkers [24–26]. In this manuscript, we report the microstructure of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) copolymers. The reactivity ratios of the comonomers calculated using a linear Kelen–Tudos method [27] and nonlinear least square error-in-variables method [28]. The complete 1H and 13C{1H} NMR spectral assignments of 2-hydroxy ethyl methacrylate and methyl acrylate copolymers were done with the help of DEPT and 2D (HSQC, TOCSY and HMBC) NMR experiments.
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2. Experimental
FH ¼
2-Hydroxy ethyl methacrylate and methyl acrylate (H/M) monomers were distilled under reduced pressure and stored below 5 °C. A series of H/M copolymers of different composition were prepared by bulk polymerization using AIBN as an initiator at 80 °C under nitrogen atmosphere. The percent conversion was kept below 10% by precipitating the copolymers in hexane. The copolymers were further purified using hexane/methanol system. The 1D and 2D NMR spectra were recorded on Bruker DPX-300 spectrometer in CD3OD. 1H and 13C measurements were made at frequencies of 300.13 and 75.5 MHz, respectively, and calibrated with respect to the solvent signal. Gradient HSQC and HMBC experiments were recorded using the pulse sequence invigpt and inv4gplplrnd of the Bruker software, respectively. The spectra were acquired with 512 increments in F1 dimension and 2048 data points in F2 dimension. TOCSY experiment was performed using standard pulse sequence. Thirty-two scans were accumulated for 512 experiments with 2 s delay time [29].
IðOCH2 ÞH =2 ½IfðOCH2 ÞH þ ðOCH3 ÞM g IðOCH2 ÞH M =3 þ IðOCH2 ÞH =2
where FH represents the composition of H-unit in the copolymer. Table 1 shows the copolymer composition data of H/M copolymer. According to Kelen–Tudos (KT) method, the terminal model reactivity ratios were calculated using the copolymer composition data. The reactivity ratios from error in variable method (EVM) were calculated using the reactivity ratio values obtained from KT method along with copolymer composition data. The values of reactivity ratios obtained from Kelen–Tudos (KT) [27] and nonlinear error in variable methods (EVM) [28] are rH = 3.31 ± 0.08, rM = 0.23 ± 0.00 and rH = 3.32, rM = 0.23, respectively. The reported values [25,26] of reactivity ratios for H/M copolymers are rH = 7.14, rM = 0.012, the difference in two values is due to different experimental conditions. The reactivity ratios of 2-hydroxy ethyl methacrylate and t-butyl acrylate copolymers [11] are rH = 1.792, rTBA = 0.510, these values are close to our values for H/M copolymer.
3. Results and discussion 3.1. Reactivity ratios determination The composition of H/M copolymers was determined from completely assigned one dimensional 1H NMR spectrum (Fig. 1). The intensity of –OCH2 protons of H-unit [I(OCH2)H] and intensity of combined signal of –CH2O– of H- and –OCH3 of M-unit were used for determination of copolymers composition as given in the following equation:
Table 1 Copolymer composition data of H/M copolymers S. No.
Sample No.
fH
FH
1 2 3 4 5
HM1 HM2 HM3 HM4 HM5
0.12 0.18 0.23 0.33 0.55
0.35 0.46 0.53 0.64 0.81
fH is the mole fraction of H comonomer in feed and FH is the mole fraction of H comonomer in copolymer.
Fig. 1. The 1H NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
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3.2.
13
C{1H} NMR studies
The 13C{1H} NMR spectrum of 2-hydroxy ethyl methacrylate and methyl acrylate (H/M) (FH = 0.53) copolymer in methanol at room temperature (25 °C) is shown in Fig. 2. The signals around d 17.0–24.0 ppm are due to a-CH3 carbon resonances of H-unit in the copolymer. The region around d 35.0–56.0 ppm is assigned to the overlap of methylene carbon of both H- and M-units, methine carbon of M-unit and quaternary carbon of H-unit in H/M
Fig. 2. The
27
copolymer. The carbonyl carbon signals of H- and M-units of H/M copolymer are overlapped and assigned around d 176.1–180.2 ppm, whereas the signals around d 52.59, d 60.75 and d 67.77 ppm are assigned to (–OCH3)M, (–CH2O)H and (–OCH2)H carbon, respectively. The overlap of b-methylene and methine carbon resonance signals around d 35.0–56.0 ppm can be resolved by DEPT-135 NMR spectrum (Fig. 3). In DEPT-135 NMR spectrum, methine and methyl carbon appeared as a positive signal while methylene carbon appeared as a negative
13
C{1H} NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
Fig. 3. The DEPT-135 NMR spectrum of H/M copolymer (FH = 0.53) in CD3OD at 25 °C.
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signal. The expanded b-methylene carbon regions of H- and M-units of H/M copolymers are sensitive to the distribution of dyad as well as tetrad compositional and configurational sequences. The resonance signal around d 37.5–43.5 ppm, which appeared as a positive signal, was assigned to –CH carbon of M-unit, while the negative signal around d 35.0–56.0 ppm was assigned to –CH2 carbon, which spread over 21 ppm range and because of its symmetry it is sensitive to dyad, tetrad and higher compositional and configurational placements. The expanded a-methyl carbon region of H-unit of H/M copolymers along with poly (2-hydroxy ethyl methacrylate) is shown in Fig. 4(a–d). Although the splitting pattern of a-CH3 carbon resonances seems to be well separated but spread over a wide range of chemical shifts due to tacticity effects in the spectrum of poly (2-hydroxy ethyl methacrylate). The multiplet in the methyl carbon region of H-unit of the copolymer is assigned to both the compositional and configurational sequences. The assignments to various signals have been carried out with the help of spectrum of poly (2-hydroxy ethyl methacrylate) and by observing
change in intensity of signals with change in composition of copolymers. The resonance signal 1 is assigned to HrHrH triad on comparison with poly (2-hydroxy ethyl methacrylate). The additional signal 2 that appears in copolymer spectrum is assigned to HrHrM triad. On comparison with spectrum of poly (2-hydroxy ethyl methacrylate), the signal 3 is assigned to HrHmH triad. The signals at 4 and 5 appeared on increasing the concentration of
Table 2 Assignment of a-CH3 carbon resonances of H/M copolymers from 13 C{1H} NMR and 2D HSQC spectra Peak No.
Peak assignments
Peak position (13C{1H}NMR; ppm)
Peak position (2D HSQC; 13C/1H; ppm)
1 2 3 4 5 6 7
HrHrH HrHrM HrHmH HrHmM MrHrM HmHmH HmHmM
17.75 18.80 19.80 21.05 21.50 22.50 23.15
17.76/1.15 18.80/1.20 19.82/1.31 21.05/1.28 21.55/1.38 22.53/1.45 23.20/1.30
Fig. 4. The expanded a-methyl region in 13C{1H} NMR spectrum of (a) PHEMA and H/M copolymers with compositions (FH=): (b) 0.81, (c) 0.53 and (d) 0.35 in CD3OD at 25 °C.
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M-unit and its intensity increases with increase in concentration of M-unit. So these signals are assigned to HrHmM and MrHrM triads, respectively. The resonance signals 6 and 7 are assigned to HmHmH and HmHmM triads, respectively. All the assignments are given in Table 2. The expanded DEPT-90 spectrum of H/M copolymers along with poly (methyl acrylate) is shown in Fig. 5. DEPT-90 spectra were used to distinguish the methine signals for 1D NMR analysis, as they were overlapping with
29
the methylene resonances. On the basis of variation in intensity of signals with copolymer composition, the various resonance signals can be assigned up to triad compositional sequences. The resonance signal I is assigned to MMM triad while the resonance signals II and III are assigned to MMH, and HMH triads, respectively. The overlapped carbonyl carbon resonance signal, of H- and M-units in copolymer of different composition along with poly (2-hydroxy ethyl methacrylate) and poly
Fig. 5. The expanded DEPT-90 spectra of (a) PMA and H/M copolymers with compositions (FH=): (b) 0.35, (c) 0.53 and (d) 0.81 in CD3OD at 25 °C.
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(methyl acrylate) are shown in Fig. 6. The overlapped resonance signals in carbonyl carbon region have been assigned on the basis of variation in intensity of signals with concentration. On comparison with spectrum of poly (2-hydroxy ethyl methacrylate) and poly (methyl acrylate) [18], the signals 1 and 2 were assigned to HrHrH and HmHrH. On the basis of variation in intensity of signals with copolymer composition, the signal at 2 was also assigned to HHM and signal 3 to MHM triad of H monomer. The signals at 4, 5 and 6 were assigned to MMH + HMH, MMH and MMM triads of M-units, respectively. All the assignments are listed in Table 3.
Table 3 Assignment of carbonyl carbon resonances of H/M copolymers from 13 C{1H} NMR spectra S. No.
Peak position (13C{1H}NMR; ppm)
Assignment
1 2 3 4 5 6
179.75 178.72 178.28 177.82 177.30 176.70
HrHrH HmHrH + HHM MHM MMH + HMH MMH MMM
Fig. 6. The expanded carbonyl carbon regions of (a) PHEMA and H/M copolymers with compositions (FH=): (b) 0.81, (c) 0.53, (d) 0.35 and (e) PMA in CD3OD at 25 °C.
A.S. Brar et al. / Journal of Molecular Structure 828 (2007) 25–37
3.3. 2D HSQC and TOCSY NMR studies 3.3.1. Methylene carbon resonances 2D TOCSY spectra were used to confirm the 1, 2 bond geminal couplings between nonequivalent protons of the same methylene group. Methylene proton resonance signals which were overlapped and could not be assigned by 1 H NMR spectral analysis only were assigned from one-to-one correlation between carbon and proton signals in 2D HSQC spectra.The protons in the racemic methylene of MrM and HrH centered tetrads are in the same environment (as depicted in Scheme 1), resulting in a single cross-peak in 2D HSQC spectra. The two nonequivalent methylene meso protons, Ha and Hb, of HH and MM centered tetrads as shown in Scheme 1 result in two crosspeaks in the 2D HSQC spectra and a cross-correlation peak in 2D TOCSY spectra (Ha proton was attributed to the proton having high chemical shift and Hb having lower chemical shift). Thus, 2D TOCSY spectra enabled us to differentiate between the meso and racemic protons and confirm the 2D HSQC assignments.
31
The expanded b-methylene region in 2D HSQC spectra is given in Fig. 7(a–c) and the assignments are listed in Table 4. The cross-peaks 1 and 3 were assigned to HHmHH(Ha) and HHmHH(Hb) tetrads, respectively. HHrHH tetrad was assigned to the cross-peak 2. Ha and Hb protons of HHmHH tetrads, being nonequivalent, resulted in cross-correlation peak 1 0 in the 2D TOCSY spectrum in Fig. 8(a–c). MHHH and MHHM tetrads of copolymer were assigned on observing the change in intensities of the cross-peaks with change in copolymer composition. MHmHH(Ha) and MHmHH(Hb) gave rise to two cross-peaks 4 and 6 while MHrHH tetrad was assigned to cross-peak 5. The Ha and Hb protons of MHmHH tetrad coupled to give cross-correlation peak 2 0 in 2D TOCSY spectrum. On the basis of variation in intensity of signals with copolymer composition the cross-peaks 7, 9 and 8 were assigned to MHmHM(Ha), MHmHM(Hb) and MHrHM, respectively, and can be observed at higher composition of M-unit, thus enabling us to differentiate between MHHH and MHHM. Ha and Hb protons of
Scheme 1. The structures of HH, HM and MM dyads.
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Fig. 7. The expanded b-methylene carbon regions of 2D HSQC spectra of H/M copolymers with composition (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
MHmHM coupled to give cross-correlation peak 3 0 in TOCSY spectrum (Fig. 8b). The methylene protons Ha and Hb of both HmM and HrM centered tetrads are in different magnetic environment as shown in Scheme 1, thus results in two cross-peaks by coupling with the methylene carbon in 2D HSQC spectra. Ha and Hb of HmM and HrM being nonequivalent also give cross correlation in 2D TOCSY spectra, enabling us to differentiate between cross-peaks of both HmM and HrM in 2D HSQC spectra. HHMH tetrad concentration decreases with increase in composition of M-unit, on this basis HHMH tetrad was
assigned. HHmMH(Ha) and HHmMH(Hb) tetrads were assigned to the cross-peaks 10 and 13, respectively, as marked in Fig. 7. HHrMH(Ha) and HHrMH(Hb) tetrads were assigned to cross-peaks 11 and 12. MHmMH/ HHmMM(Ha), MHmMH/HHmMM(Hb), MHrMH/ HHrMM(Ha) and MHrMH/HHrMM(Hb) were attributed to the cross-peaks 14, 17, 15 and 16, respectively. Similarly MHmMM(Ha), MHmMM(Hb), MHrMM(Ha) and MHrMM(Hb) were assigned to the cross-peaks 18, 21, 19 and 20, respectively. Ha and Hb protons of HHmMH, MHmMH/HHmMM, MHmMM and Ha and Hb protons of HHrMH, MHrMH/HHrMM, MHrMM tetrads
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Table 4 Assignment of b-methylene carbon resonance of H/M copolymers from 2D HSQC spectra Cross-peak No.
Cross-peak assignment
Peak position (2D HSQC;
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
HHmHH(Ha) HHrHH HHmHH(Hb) MHmHH(Ha) MHrHH MHmHH(Hb) MHmHM(Ha) MHrHM MHmHM(Hb) HHmMH(Ha) HHrMH(Ha) HHrMH(Hb) HHmMH(Hb) MHmMH/HHmMM(Ha) MHrMH/HHrMM(Ha) MHrMH/HHrMM(Hb) MHmMH/HHmMM(Hb) MHmMM(Ha) MHrMM(Ha) MHrMM(Hb) MHmMM(Hb) HMmMH(Ha) HMrMH HMmMH(Hb) HMmMM(Ha) HMrMM HMmMM(Hb) MMmMM(Ha) MMrMM MMmMM(Hb)
55.25/2.37 55.25/2.25 55.25/1.86 53.18/2.31 53.18/2.05 53.18/1.93 51.74/2.35 51.74/2.13 51.74/2.00 48.30/2.27 48.30/2.06 48.30/1.87 48.30/1.63 45.65/2.32 45.65/2.05 45.65/1.92 45.65/1.63 43.45/2.32 43.45/2.05 43.45/1.90 43.45/1.64 39.18/2.04 39.80/1.88 39.18/1.65 37.80/2.03 37.80/1.85 37.80/1.65 36.05/2.07 36.05/1.91 36.05/1.76
coupled to give cross-correlation peaks 4 0 and 5 0 , respectively, in 2D TOCSY Spectrum (Table 5). MM centered tetrads also showed compositional and configurational sensitivity. HMmMH(Ha), HMrMH and HMmMH(Hb) tetrads have cross-peaks 22, 23 and 24, respectively. HMmMM(Ha), HMrMM and HMmMM(Hb) have cross-peaks 25, 26 and 27, respectively, while MMMM tetrad was assigned by comparing with 2D HSQC and 2D TOCSY spectra of poly (methyl acrylate). MMmMM(Ha) was assigned to the cross-peak 28, MMmMM(Hb) to cross-peak 30, and cross-peak 29 was assigned to MMrMM tetrad. Geminal coupling between Ha and Hb protons of HMmMH and HMmMM tetrads was assigned to the cross-correlation peak 6 0 in 2D TOCSY spectrum. While Ha and Hb protons of MMmMM tetrad coupled to give cross-correlation peak at 7 0 . 3.3.2. Methine carbon resonances Methine group of methyl acrylate was assigned up to triad level of compositional sensitivity in the copolymer, based on the 2D HSQC assignments. 2D TOCSY studies were used to ascertain these assignments by assigning 1, 3 bond order couplings between methylene protons and methine protons of M monomer in MM and MH centered methylene.
13
C/1H; ppm)
The cross-peak I is assigned to MMM triad while the cross-peaks II and III are assigned to MMH, HMH triads, respectively (Fig. 7). The chemical shifts of methine group on proton axis were in a very narrow range (2.42– 2.75 ppm). The cross-correlation peaks 8 0 , 9 0 , 1 0 and 10 0 , as shown in Fig. 8, were assigned to the 1, 3 bond order couplings of –CH with –CH2 of MmH(Hb), MmM/ MrH(Ha), MrH(Hb)/MrM and MmH(Ha), respectively. 3.3.3. Methyl resonances The expanded a-CH3 region of 2D HSQC NMR spectra of H/M copolymer is shown in Fig. 9 [(a) FH = 0.35, (b) FH = 0.53, and (c) FH = 0.81]. The a-CH3 region of H-monomer in the copolymer shows compositional and configurational sensitivity. The cross-peaks 1, 2 and 3 are assigned to HrHrH, HrHrM and HrHmH triads, respectively. While the cross-peaks at 4 and 5 are assigned to HrHmM and MrHrM triads, respectively. HmHmH and HmHmM triads are assigned to crosspeaks 6 and 7, respectively. All the assignments are given in Table 2. 3.4. 2D HMBC studies The configurational and compositional sensitivity within carbonyl carbon region was investigated through
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Fig. 8. The 2D TOCSY spectrum of H/M copolymers with composition (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
Table 5 1 H-1H cross correlation between nonequivalent geminal protons in H/M copolymers observed from 2D TOCSY spectra Correlation peak No.
1
0
20 30 40 50 60 70 80 90 10 0
Peak position (2D TOCSY; 1H/1H; ppm)
Coupled protons Proton I
Proton II
CH of : M CH2 of : HHmHH(Ha) CH2 of : MHmHH(Ha) CH2 of : MHmHM(Ha) CH2 of : HmM(Ha) CH2 of : HrM(Ha) CH2 of : HMmMM(Ha) and HMmMH(Ha) CH2 of : MMmMM(Ha) CH of : M CH of : M CH of : M
CH2 of : MrH(Hb)/MrM CH2 of : HHmHH(Hb) CH2 of : MHmHH(Hb) CH2 of : MHmHM(Hb) CH2 of : HmM(Hb) CH2 of : HrM(Hb) CH2 of : HMmMM(Hb) and HMmMH(Hb) CH2 of : MMmMM(Hb) CH2 of : MmH(Hb) CH2 of : MrH(Ha)/MmM(Ha) CH2 of : MmH(Ha)
2.32/1.85 2.33/1.93 2.35/2.00 2.30/1.65 2.05/1.90 2.04/1.65 2.06/1.78 2.55/1.68 2.55/2.05 2.55/2.33
A.S. Brar et al. / Journal of Molecular Structure 828 (2007) 25–37
35
Fig. 9. The expanded a-methyl carbon regions of 2D HSQC spectra of H/M copolymers with compositions (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
the analysis of long-range couplings between carbon and hydrogens with 2D HMBC NMR spectra, which has proven to be highly informative for the microstructural characterization of polymers. The completely assigned 1 H NMR spectra, performed with the help of 2D HSQC in conjunction with TOCSY, were used to assign the 2D HMBC NMR spectra. Fig. 10 shows the HMBC spectra of H/M copolymers, showing long-range coupling between carbonyl carbon with methyl and methylene protons of H- and M-units. The cross-peaks 1, 2 and 3 are assigned to the coupling of carbonyl carbon of MMH triad with methyl protons in MmMmH, MmMrH and MrMrH triads, respectively.
The cross-peaks 4, 5 and 6 are assigned to the coupling of >C=O carbon in MHM triads with methyl protons in MrHmM, MrHrM and MmHmM triads, respectively while the cross-peaks 7 and 8 are assigned to the coupling of carbonyl carbon in MHH + HmHrH and HrHrH triads with methyl protons in MHH + HmHrH and HrHrH triads, respectively. The cross-peaks for the coupling of carbonyl carbon with methylene protons are more predominant in the copolymer with higher compositions of M-unit, so most of the cross-peaks are assigned to the coupling of M-centered carbonyl carbon triad. The cross-peaks 9, 10 and 11 are assigned to the coupling of carbonyl carbon in MMM triads with methylene
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Fig. 10. The 2D HMBC spectra of H/M copolymers showing coupling of carbonyl carbon with methyl, methine and methylene protons of compositions (FH=): (a) 0.35, (b) 0.53 and (c) 0.81 in CD3OD at 25 °C.
protons in MmM(Hb), MmM(Hb) diads and MMrMM tetrads, respectively. The cross-peaks 12 and 14 are assigned to the coupling of carbonyl carbon in MMH + HMH and MMH triads with Hb proton of HrM and Ha proton of MmM diads, respectively. While the carbonyl carbon of MMM triad gives a cross-peak 13 by coupling with Ha proton of MmM diad. The coupling of carbonyl carbon in MHM and MMH triads together with HrM(Ha) and HmM(Ha) diads is assigned to 15 and 16 cross-peaks, respectively. The cross-peak 17 is assigned to the coupling of carbonyl carbon in MMM
triad with methine proton of MMM triad. All the assignments are given in Table 6. 4. Conclusions The reactivity ratios of comonomers in H/M copolymers are rH = 3.32 and rM = 0.23. The complex and overlapped 1H and 13C{1H} NMR spectra of the copolymers were resolved with the help of DEPT and 2D HSQC spectra. The carbonyl carbons of H- and M-units were assigned up to triad compositional and
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Table 6 Coupling of carbonyl carbon with methyl, methine and methylene protons of H/M copolymers based on 2D HMBC spectra S. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Peak position (2D HMBC;
13
C/1H; ppm)
177.30/1.35 177.30/1.27 177.30/1.20 178.25/1.36 178.28/1.22 178.28/1.18 178.70/1.36 179.73/1.18 176.70/1.69 176.60/1.71 176.70/1.93 177.85/1.87 176.69/2.09 177.30/2.10 178.35/2.25 177.30/2.34 176.70/2.54
configurational sequences in 13C{1H} NMR spectrum. The methyl carbon signals were assigned upto triad compositional and configurational sequences, whereas bmethylene carbon resonances were assigned upto tetrad compositional sequences. The methine carbon of M-unit was assigned upto triad compositional sequences. Acknowledgement One of the authors, Sunita Hooda, wants to thank Department of Science and Technology (DST), New Delhi, India, for providing financial support. References [1] F.A. Bovey, L. Jelinski, P.A. Mirau, NMR of polymers, Academic Press, New York, 1996. [2] H.N. Cheng, J. Appl. Polym. Sci. Polym. Symp. 43 (1989) 129. [3] J.C. Randall, Polymer Sequence Determination: carbon-13 NMR Method, Academic Press, New York, 1977. [4] K. Matsuzaki, T. Uryu, T. Asakura, NMR spectroscopy and stereoregularity of polymers, Japan Sci. Soc. press, Tokyo, 1996. [5] S. Wen, X. Yin, W.T.K. Stevenson, J. Appl. Polym. Sci. 43 (1991) 205. [6] J. Gallar do, J.S. Roman, Polymer 34 (1993) 567. [7] L. Barannon-Peppas, N.A. Peppas, Biomaterials 11 (1990) 635. [8] J.P. Montheard, M. Chatzopoulos, D. Chappard, J. Macro. Sci.-Rev. Macro. Chem. Phys. C32 (1992) 1. [9] J. Singh, K.K. Agrawal, J. Macro. Sci.-Rev. Macro. Chem. Phys. C32 (1992) 521.
Type of carbon
Coupled to proton of
CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO
CH3 of MmMmH CH3 of MmMrH CH3 of MrMrH CH3 of MrHmM CH3 of MrHrM CH3 of MmHmM CH3 MHH + HmHrH CH3 of HrHrH CH2 of MmM(Hb) CH2 of MmM(Hb) CH2 of MMrMM CH2 of HrM(Hb) CH2 of MmM(Ha) CH2 of MmM(Ha) CH2 of HrM(Ha) CH2 of HmM(Ha) CH of MMM
of of of of of of of of of of of of of of of of of
MMH MMH MMH MrHmM MrHrM MmHmM MHH + HmHrH HrHrH MMM MMM MMM MMH + HMH MMM MMH MHM MMH MMM
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