Indian Journal of Chemistry Vol. 41A, October 2002, pp. 2008-2016

Investigation of microstructure of glycidyl methacrylate/methyl acrylate copolymers by ID and 2D- NMR spectroscopy

AS Brar*t & Anil Yadav

Department of Chemistry, Indian Institute of Technology, De lhi, Hauz Khas, New Delhi 110016, India

Received 7 May 2002; revised 16 July 2002

The copolymers of glycidyl methacrylate/methyl acrylate (G/M) have been sy nthesized by solution polymerisati on using benzene as solvent and benzoylperoxide as an initiator. The IH-NMR spectroscopy has been used to determi ne the copolymer compositions. The comonomer reactivity ratios have been determined by Kelen- Tudos (KT) and non-linear least sq uares error in variable method (EVM) and are found to be rG = 4.03 ± 0.7, rM = 0.45 ± 0.07 and rG=4.6, rM = 0.49, respective ly. The Di stortionless Enhancement by Polarization Transfer (DEPT), DC_1H heteronuclear single quantum coherence (HSQC) and Total Correlation Spectroscopy (TOCSY) have been used fo r the complete spectral assignment of DC anclIH-NMR spectra.

The high-resolution ID and 2D-NMR spectroscopy has proved to be one of most informative techniques for the investigation of microstructure of the polymersl. DEPT2 and uC-'H inverse HETCOR NMR techniques have been used for the compositional3-5

and configurational6-8 assignments of the polymers.

In our earlier publications, the microstructure determination of glycidyl methacrylate / acrylonitrile, glycidyl methacrylate /vinyl acetate and glycidyl methacrylate /methacrylonitrile copolymers have been reported9

-11 . In continuation of our earlier work, in

this paper we report complete compositional and configurational assignments of G/M copolymers using the UC{ IH }, DEPT and 2D NMR (HSQC and TOCSY) spectroscopy. Co-monomers reactivity ratios were calculated using the Kelen-Tudos (KT)1 2 and non-linear error in variable (EYM)1 3 methods, using compositional data obtained from the IH -NMR spectra.

Materials and Methods Glycidyl methacrylate (G) (Merck) and methyl

acrylate (M) (Merck) were purified by distillation under reduced pressure and both the monomers were then stored below 5°C. A series of G/M copolymers containing different mole fraction of G in feed were prepared by solution polymerisation in benzene at 60°C using benzoyl peroxide as an initiator. The conversIOn was kept below 10% by timely precipitating the copolymer III methanol. The copolymers were further purified using

tTc l.: 91-11-6591377; Fax: 91 - 11-6581102

chloroform/methanol system. All the NMR spectra were recorded in CDCl3 as solvent at 25°C as reported in our earlier publications9

-11 .

Results and Discussion

1 D studies, copolymer composition and reactivity ratios determination

The IH NMR spectrum (FG= 0.51) along with complete assignments is shown in Fig. 1. The assignments in IH NMR spectrum are done by comparing the assignments done in poly(glycidyl methacrylate) and poly(methyl acrylate) IH NMR spectra l4 . The copolymer composition was determined using the formula

I (Hu+Hv+Hy+Hz)/4 FG=---------------------------------

I (Hu+Hv+Hy+ Hz)/4 + [I {(OCH1)M +Hx)

- 1 (Hu+Hv+Hy+Hz)/4] /3

The mole fraction of the monomers in the feed and copolymer are as shown in Table I . The copolymer composition data were used to determine the reactivity ratios by terminal model (KT) method and by non-linear error in variable method (EYM). The value of reactivity ratios as obtained from Kelen­Tudos (KT)12 and non-linear error in variable method

(EYM)13 are rG = 4.03 ± 0.7, rM = 0.45 ± 0.07 and rG= 4.6 rM = 0.49 respectively, which are in agreement with each other. .

The 13C{ 'HI NMR spectrum of G/M copolymer (FG= 0 .51) in CDCh is shown in Fig. 2 along with the complete signal assignments. The ex-methyl group of

BRAR et at.: MICROSTRUCTURE OF ACRYLATE COPOLYMERS BY I D & 2D-NMR 2009

CH) 1 + CH 2-C _ .J, ( CH2 - CH-+--

I Hx Hz H I ~C" I I / U ,f'C "-

, " 0 O--CH) o O-C-C-C

I 'o.l"-H Hy v

z

x

y

Fi g, I- The IH_ NMR spectrum of G/M copol ymer (FG= 0.51 ) in CDCI, .

Table I- The feed in mol fraction and copolymer composition data of G/M copolymers

Sample Feed in Copolymer M 10-1 Mw* 10-1 Po lydispersity n*

mol fractio n composition g/mole g/mole fG FG

GMI 0.10 0.21 2.1 3.5 1.67

GM2 0.15 0.33 2.3 3.8 1.62

GM3 0 .20 0.43 2 . 1 3.6 1.72

GM4 0.25 0.51 2.3 3.9 1.65

GM5 0.30 0.57 1:8 3.1 1.73

GM6 0.50 0.77 1.8 3.0 1.67

fG is the mole fracti ons of G-como no me r in feed. FG is the mole fractions o f G-comonomer in the copolymer. Mn and M" are number average and weight average molecular weight of copolymers calcul ated using Gel permeation chromatography

tec hnique

G-unit is assigned around 816.2-21.4 ppm. The spectral region around 833.0-66.0 ppm is very complex and overlapped and can be assigned to aliphatic carbons in the main and side chain of the copolymer. The extent of overlap of epoxy methylene, ~-methylene and epoxy methine carbon signals cannot be ascertained from J3C{ IH } NMR spectra alone and so DEPT-135 spectrum (Fig. 3) was used to ass ign the spectral region. The epoxy methylene,

epoxy methine and -OCH2 carbon signals are assigned at 8 44.5, 8 48.8 and 8 66.0 ppm respectively, by comparing with the assignment '> in 13C{ I H} NMR spectrum of poly(glycidyl methacrylate)1 5. The quaternary carbon of glycidyl methacrylate in the copolymer is assigned around 849.0 ppm. The ~­methylene carbon of both methyl-acrylate and GMA unit resonate around 833.5-56.0 ppm. The -OCH} carbon signals are assigned at 8 51.7 ppm by

2010 INDIAN J CHEM, SEC A, OCTOBER 2002

::: u o u

1.9

N :r: u o ..'...

N :r: u

(>=OlM+G

~~~~~~~~~ 1

ppm 1

160 I i I

140 1

120 1

100 1

80 1

60 I I ,

40 1

20

Fig. 2-The 1.1C{ I H} NMR spectrum of G/M copolymer (FG =0.5 I) in CDCI ,1

~~~, ,I ~~~'~'TI ~~~""~' ~, ~~~''''~' ~~~I~~~~' ~, "I ,~,~~,~,,,, I ~~~~l'""l- r - '

pom 80 70 60 50 40 30 20 10

Fig. 3-The DEPT-135 spectrum of G/M copolymer (FG =0.51) in CDCI ,1

comparing with 13CeH} NMR spectrum of poly(methyl acrylate) 14. The extent of overlap of methine carbon signals with methylene carbon signals still cannot be ascertained. Hence, DEPT 90 spectrum was recorded to resolve them. The methine carbon resonances are assigned around 835.7-42.0 ppm. The carbonyl carbon resonances of M- and G-centered units are overlapped and assigned around 8 174.7-175.8 ppm and 8175.2-178.2 ppm respectively.

The a-methyl region in IDeH and 13C{ IH}) NMR spectrum is quite complex and overlapped. The 13C{ IH} resonances of the a-methyl groups for poly(glycidyl methacrylate)15 and copolymer samples of different feed composition are shown in Fig. 4(a­d). The assignments to various signals are done by comparing with the assignments done in homopolymer spectrum and by observing change in signal intensity with change in the copolymer

BRAR et ai. : MICROSTRUCTURE OF ACRYLATE COPOLYMERS BY 10 & 20-NMR 2011

MGM

II --~-"--~--'I --~--TI--~--T'--~---"--~--~I --~~I--~--'--'---,~~--

ppm 28 26 24 22 20 18 16 14 12

Fi g. 4- Expanded ' :lC{ I H ) spectrum showing ex-methyl carbon s ignal in (a) poly(glycidy lmethacrylate), copoly mer w ith composition FG: (b) 0.77 , (c) 0.51 , and (d) 0.21.

composition. The signals at 8 16.9 ppm and 8 18.9 ppm are assigned to GrGrG, GrGmG respectively, by comparing with the assignments done in \3C{ I H }_ NMR spectrum of poly(glycidyl methacrylate) 15. The signals around 8 17.4-19.4 ppm, which first show increase and then decrease in intensity with decrease in G-content in copolymers is assigned to GGM triad sequence. The broad signal around 8 19.4-22.0 ppm, whose intensity increases with increase in M-content in copolymers, is assigned to MGM triad sequence. The various CX-CH3 assignments made in 13C{ I H} NMR are further confirmed by 2D-HSQC NMR assignments.

Figure 5(a-d) shows the expanded methine region in the DEPT 90 spectrum of poly(methyl acrylate)(a) and G/M copolymers of various composition (FG = 0.21(b), 0.51(c) and O.77(d)). The splitting within the

methine carbon resonance signals can be assigned to triad compositional signals GMG, MMG and MMM in the region 8 36.35-38.65, 8 38.65-40.65 and 8 40.65-41.9 ppm respectively, by observing the change in intensity of signals · with change in the copolymer composition. Further splitting in GMG and MMG triad sequences are assigned to configurational sequences on the basis of the observation that these signals do not show relative change in intensities with change in the copolymer composition. In GMG triad region, the signals around 8 36.35-37.9 ppm are assigned to GmMrG and GmMmG, while signals around 8 37.9 -38.65 ppm are assigned to GrMrG configurational sequence. In MMG triad sequence, the signals around 8 38 .65-39.7 ppm are assigned to GmMrM and GmMmM, while signals around 8 39.7-40.65 ppm are assigned to GrMrM configurational

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ppm 42 40 I

38 I

36 I

34

Fig. 5- Expanded DEPT 90 spec trum showing methine carbon signals in (a) poly(methyl acrylate) and copolymer with compositi on Fc; : (b) 0.21, (c) 0.51, (d) 0.77.

sequence. The MMM triad is assigned around 840.65-41.9 ppm by comparing with DEPT 90 spectrum of poly(methyl acrylate).

2D-NMR studies HSQC studies-The expanded 2D-HSQC NMR

spectra of a-methyl group of G- unit is as shown in Fig. 6(a, b and c) (FG =0.21, 0.51 and 0.77) . The a-methyl group shows both compositional and configurational sensitivity. The a-methyl group carbon resonance is assigned to triad compositional sequences GGG, GGM and MGM on the basis of change in intensity with change in copolymer composition . The cross peaks at 8 16.8/0.95 and 8 18 .8/1.1 ppm are assigned to GrGrG and GrGmG respectively, on basis of the assignments made in poly(glycidyl methacrylate) HSQC NMR spectrum. Both GGM and MGM triads further show splitting which IS assigned to their sensitivity to

configurational sequences on the basis of observation that they do not show appreciable relative change in intensities with change in the copolymer composition. In GGM region, the cross peaks at 8 18.110.96 and 18.311.02 ppm are assigned to GrGrM and GrGmM respectively. Similarly, in MGM region, the cross peaks at 8 20.02/1.09, 20.0/1.08 and 20.6/1.15 ppm are assigned to MrGrM, MrGmM and MmGmM configurational sequences respectively .

The ~-CH2 region, due to its symmetry is sensitive to dyad, tetrad etc. This region is divided into MM, MG and GG dyads on the basis of change in intensity of the signals with change in the copolymer composition and by comparing with the 20 HSQC NMR spectra of poly(methyl acrylate) and poly(glycidyl methacrylate). These three dyads are assigned around 8 35.6/1.35-2.0 , 8 41.0-48.4/1.22-2.15 and 848.4-56.011 .76-2 .13 ppm respectively . In the MM dyad, the three cross peaks along proton axis

BRAR et al.: MICROSTRUCTURE OF ACRYLATE COPOLYMERS BY I D & 2D-NMR 20 13

(0)

20

MrGmM MmGm M

m r 30 30

~ 40

50

ppo , , ppo 2.5 2.0 1.5 1.0

40

50

ppo ; I

ppo 2.5 2.0 1.5 1.0

30

40

ppo

ppo 2.5 1.5 1.0

Fig. 6-The expanded methyl, (:I-methylene region and <l-methine in 2D-HSQC NMR spectrum of GIM copolymers of composition FG: (a) 0.21, (b) 0.51 and (c) 0.77.

are assigned due to configurational sequences. The meso-configuration gives two peaks because the two­methylene protons lie in different environments and the racemic give one cross peak between two cross peaks due to meso configurations. The two cross peaks at 8 35 .6/1.5 and 8 35.6/1.92 ppm are due to meso configuration and are assigned to MmM and that at 8 35 .6/l.7 ppm is due to racemic and assigned to MrM configurational sequence.

In MG dyad, further splitting along the carbon axis are assigned to tetrad compositional sequences on the basis of change in intensity of the signals with change in copolymer composition. The MG dyad besides showing sensitivity to tetrad compositional sequences also shows further splitting of each tetrad into four cross peaks along the proton· axis due to configurational sequences. The meso- configuration gives two peaks because the two methylene protons lie in different environments and racemic also give two cross peaks in between two cross peaks cOlTesponding to meso configurations. Here, two

cross peaks are observed for racemic configuration, because the two protons are not equivalent, owing to the different environments of methyl acrylate and G side chain groups. The two cross peaks at 8 42.5/1 .37 (Ha) and 8 42.5/2.13 (Hb) ppm are due to meso configuration and are assigned to MMmGM and those at 842.511.68(H;) and 842.5/1.86 (Hb') ppm are due to racemic and are assigned to MMrGM configurational sequence. Similarly, the cross peaks at 8 44.5/1.30 ppm and 8 44.5/2.1 ppm are assigned to MMmGG (GMmGM) and those at 8 47.0/1.32 and 8 47.012.04 ppm are assigned to meso configuration in GMmGG tetrad for Ha and Hb respectively. The cross peaks at 844.5/1.65 and 8 44.5/1.8 ppm are assigned to MMrGG (GMrGM) and those at 8 47.0/1.5 ppm and 847.0/1.82 ppm are assigned to GMrGG for Ha' and Hb' respectively. The GG dyad region shows sensitivity to both compositional and configurational sequences. The cross peak around 8 48.5/1 .99 ppm is assigned to MGGM tetrad sequence. The crosspeak around 8 51.012.16 ppm is assigned to MGGG

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20

30

40

50

I~~I {- OCH2 )G

j

I

60

-~---~----.-~ .. .-- .. - _ ____ .J ..ppm

r--.--r--r-r..,-~I-.I"·--,--l · -.----.---r-r-~--r--r-r-r-r-r-r~.-r ·-o-r---._,__ r - ,­

ppm 4 3 2 1,

Fig. 7-The complete 2D-HSQC NMR spectrum ofG/M copolymers (Fo= 0.51)

~. ! (a)

1.0

1.5

2.0

, ppo 2.5 2.0 1.5 1. 0 ppo 2.5 2.0 1.5 1.0

Fig. 8- The 2D-TOCSY NMR spectrum of G/M copolymer of composition Fa = (a) 0.21 , and (b) 0.51 .

sequence. The crosspeak around 8 54.412.08 ppm is assigned to GGmGG sequence, while the crosspeak at 8 52.45/1.98 ppm is assigned to GGrGG sequence on the basis of assignments done in poly(glycidyl methacrylate) 2D HSQC NMR spectrum. All these assignment are shown in the 2D-HSQC NMR spectra (FG=0.21, 0.51 and 0.77 in copolymer), Fig 6(a-c).

In Fig. 7, the cross peak at 849.0/3.28 ppm is assigned to epoxy methine group of the copolymer. The -OCH2 methylene protons that are adjacent to the chiral center in G/M copolymer show diastereomerism and give two crosspeaks at 8 66.0/3.82 and 8 66.0/4.32 ppm. Similarly, the epoxy methylene proton signals give two cross peaks at

BRAR ef ai.: MIC ROST RUCTURE OF ACRY LATE COPOLYMERS BY 10 & 20 -NMR 20 15

Table 2- TOCSY IH_IH shi ft correlations

Peak no

1

2

3

4

5

6

7

8

9

10

II

12

13

14

15

Proton (ppm)

(a-CH)M (2 .3)

(a-C H)M (2.3)

(a-CH)M (2.3)

H"of ~-CH2 in MmM dyad (1 .44)

a -CH (M) (2.02-2.5)

a-CH (M) (2.02-2. 5)

a-C H (M) (2.02-2.5)

a -C H (M) (2.02-2.5)

Ha' of ~-CH2 in MrG ( 1.5- 1.68)

(C H2)cU (2.60)

(C H2)cU (2.60)

(C H2)c v (2.80)

(C H)e(3· 18)

(C H)c (3. 18)

(OCH2), (3.78)

Q ,

Coupled to proton (ppm)

H" of ~-CH 2 in MmM dyad ( 1.44)

Hb of ~-CH2 in M mM dyad ( 1.88)

~-CH 2 in M rM dyad ( 1.65)

Hb of ~-CH2 in M mM dyad ( 1.88) (gemi nal)

H"of ~-CH2 in MmG ( 1.28- 1.4)

H.,' of ~-CH2 in MrG ( 1.5- 1.68)

Hb' o f ~-CH2 in M rG ( 1.78 -1.92)

Hh of ~-CH 2 in MmG (2.05-2.1 8)

Hh' of ~-CHl in MrG ( 1.78 -1.92)

(C H2)/ (2.8)

(C H)e (3. 1 8)

(C H)c (3 . 1 8)

(OCH2)' (3.79)

(OCH2) Y (4.28)

(OCH2) Y (4.28)

" J-n 12

ppm

Fi g. 9- The complete 20- TOCSY NMR spectrum of G/M copolymers (FG = 0.5\ )

8 44.5/2.68 and 8 44.5/2.87 ppm. The ester methyl group of the M unit resonates at 8 52.0/3.65 ppm.

TOCSY spectra studies-Once 13C{ 'HJ-NMR spectrum is assigned completely, the various overlapped resonance signals in 'H-NMR spectrum

are ass igned by one to one con'elation between carbon and proton with the help of 2D-HSQC NMR spectra. The proton spectrum along with complete assignments is shown in Fig. 1 (FG=0,51 ). In order to understand the connectivity and confirm the various couplings in the polymer chain, the TOCSY spectrum

2016 INDIAN J CHEM, SEC A, OCTOBER 2002

was recorded. Three bond coupling between the protons of different directly coupled groups in G/M copolymer can be clearly seen in TOCSY experiments in low mixing time (4.1ms) as shown in Fig. 8 (FG = (a) 0.21, and (b) 0.51) (Table 2).

The ex-methine protons are coupled to ~-methylene protons in various dyads and tetrads. The crosspeaks at 8 2.311.44(1) , 8 2.3/1.88(2) and 8 2.3/1.65(3) ppm are assigned to coupling of methine proton with Ha of ~-methylene protons in MmM, Hb of ~-methylene protons in MmM and two equivalent ~-methylene protons in MrM dyads respectively. The two non­equivalent protons (Ha and Hb) in MmM dyad are further coupled, giving cross peak at 8 1.4411 .88(4). These assignments are done by comparing with TOCSY (4.1 ms) spectrum of poly(methyl acrylate) . The methine proton can also couple with ~-methylene protons in MG centered tetrads. The crosspeaks at 8 2.06-2 .3511.28-1.4 (5), 8 2.13-2.3511 .5-1.68 (6), 8 2.18-2.3611.78-1.92 (7) and 8 2.4212.05-2.18 (8) are assigned to coupling of methine proton with Ha of ~­methylene protons in MmG, H; of ~-methylene

protons in MrG, Hb' of ~-methylene protons in MrG and Hb of ~-methylene protons in MmG, respectively. The two non-equivalent protons (Ha and Hb) in MmG dyad and (Ha' and Hb") in MrG dyad are further coupled. The crosspeak resulting from coupling of non equivalent protons in MmG dyad overlaps with crosspeak 5, while non equivalent protons in MrG couples to give cross peaks at 81.56-1.6711 .78-1.92 ppm (9) respectively. The vicinal coupling between side chain group of G-unit can be clearly seen in TOCSY (4.1 ms) in Fig. 9 . The cross peaks at 8 2.60/2.8 (10) and 82.6/3.18 (11) ppm are assigned to geminal coupling between diastereomeric protons 'u' with 'v' and to vicinal coupling of proton 'u' with epoxymethine proton respectively. Similarly, the cross peak at 8 2.8/3 .18(12) ppm is assigned to vicinal coupling of proton 'v' with epoxymethine proton . The cross peaks at 8 3.18/3.79 (13) and 3.18/4.28 (14) ppm are due to coupling of epoxymethine proton with proton 'x' and epoxymethine proton with proton 'y', respectively . The cross peak at 8 3.78/4.28(15) ppm is

due to coupling between diastereomeric protons 'x' and 'y'.

Conclusions The reactivity ratios of G/M copolymer system are

found to be rG= 4 .6 and rm= 0.49. The various 10 (DEPT) and 2D(HSQC, TOCSY) NMR spectroscopic techniques are used to resolve the broad and overlapped signals in IH and I3C{ IH } NMR spectra. The ex-methyl carbon resonances are assigned to triad compositional and configurational sequences and the ~-methylene carbon resonances are assigned to dyad to tetrad compositional sequences with the help of 2D-HSQC NMR spectrum. The ex-methine in the M­unit of G/M copolymer IS assigned to triad compositional sequences.

Acknowledgement The authors wi sh to thank the CSIR, India for

providing the financial support to carry OLlt thi s work.

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