Indian Journal of Chcmi st ry Vo l. 41A, April 2002, pp. 723-729

Reactivity ratio determination and complete spectral assignment of 4-vinyl pyridine-ethyl acrylate copolymer by NMR spectroscopy

Sunita Hooda

Department of Chcmistry , Acharya Narcndra Dev College, University of Delhi , Kalkaji , New Del hi 11 0019

Received 16 Jun e 200 1,' revised 19 Seprernber 2001

4-Vinylpyridine-ethyl acrylate (VIE) copolymers of different compositions have been preparcd by free radical bulk polymeri za ti on. The copolymcr compositi on has been calculated from I H-NMR spectra. The comonomer react ivity rati os of VIE copolymer have been ca lcu lated from linea r Kelcn-Tudos (KT) and non-linear error-in -vari ables method (EV M). The reacti vity rati os obtained from KT and EV M are rv = 0.34 ± 0.07 , rE = 0.23 ± 0.07 and rv =0. 36 ± 0.08, rE = 0.25 ± 0.04 respecti ve ly. The theoretical tri ads concent rat ion has been determ ined from stati stica l model using terminal model reacti vity rat ios. The complete spect ral ass ignment of these copolymers has been done with the hclp of di storti on less enhancement by polarization transfer (DEPT) and two dimensional heteronuclear single quantum cohere nce (HSQC) spectra. The va ri ous type of I H-I H couplings in copolymer ha vc been ass igned with the help of 2D- lOtal correlated spectroscopy (TOCSY).

High reso lution NMR spectroscopy has been particul arl y effect ive in the determin ati on of the intramolecular chain structure of the polymersl . The informati on about the microstructure of the polymer is essential for clari fy ing the polymeri za ti on mechanism 2-~ . The 2D-NMR spectroscopy has become an important technique for the determination of microstructure of copolymers5

-7

. DEPT is used extensively for the analys is of the overl appi ng carbon resonances in 13C_NM R spectra. 4-V iny lpyridine polymers are industri all y very important due to the presence of weak ly basic nitrogen atom in the ring, they can be used in electrical fi eld, polymer reagents, pol ymer supports in reacti on cata lys is such as ox idati on-reduction reactions and in polyelectro ly te8

.

The microstructure of 4-v iny lpyridine-methy l acry late copol ymer is reported by Natonsohn el 0 1.9 and by us but to the best of my knowledge, the reactivity ratios and microstructure of 4-v inyl pyridine-ethy l acry late (V IE) copolymer has not been reported so far. In thi s paper, we report the reacti vity ratios and microstructure of V IE copolymer, prepared by free radica l bulk pol ymen zation . The copolymer composition W:lS calcul ated from IH-NMR spectroscopy. The reacti vity ratios were ca lculated using linear Kelen-Tudos (KT) method lO and non­linear leas t square error-in-vari abl es method using RREVM program 1 I. The theoreti ca l triads sequence distri bution In terms of V- and E-units were calcul ated from stati sti ca l model using reacti vity ratios obtained from RREVM program. The overl apped and complex IH_ and 13C{I H}-NMR

spectra of these copolymers have been ass igned completely with the help of DEPT-l3S , DEPT -90, HSQC and TOCSY (low mi xing time) experiments. The variou s resonance signals were found to be insensiti ve to compositional and configurational sequences. 2D TOCSY spectra of V IE copolymers show splitting in aliphatic and aromatic region which indica te their sensiti vity toward vari ous compos itiona l and con figurational sequences.

Materials and Methods 4-V iny lpyrid ine (Merck) and ethyl acryl ate (GSC)

monomers were first treated with NaOH and distill ed water and then purified by vacuum distillation. A seri es of 4-viny Ipyridine-ethy l ac ry late (V IE) copolymers of different compos itions was prepared by free rad ica l bulk polymerization using benzoy l peroxide as an initiator (0.5 % (w/w) of monomers concentration) at 60°C. The detail s about polymerization time and monomer concentrat ions are given in Table l. The per cent conversion was kept below 10 by precIpitating the copol ymers in petroleum ether. It is well known that if the per cen t conversion is less than 10%, the formati on of homopolymer is negligible so there is no need for remova l of homopolymer. The copolymers were f urther puri fied by chloroform/petroleum ether system. NMR experiments were pelformed in CDCI1

on a Bruker DPX-300 spectrometer at a frequency of 300. 13 and 75.7 MHz for IH and 13C{I H }-NM R spectra, respecti ve ly. The detail s of recording of variou s I D-NMR spectra i .e. IH, 13C{ IH }-NMR,

724 INDIAN J CHEM , SEC A, APRIL 2002

DEPT- 135, DEPT-90 and 2D-NMR spectra i.e. HSQC, TOCS Y using different pulse sequences have been described elsewhere l 2

.

Results and Discussion Determination of copolymer cOl11position and reacti vity ratios

The copolymer compos ition was calculated from the relati ve in tensity of -OCH2 protons of E-unit and orrho protons (2, 6) of heterocycli c ring of V-unit using the fo llowing equ ati on:

Tab lc 1- Thc mole fraction of monomers in fccd and in VIE copo lymcrs

S.No. Sample fv fE Fv FE Polym time (mi n)

VE2 0.80 0.20 0.70 0.30 15

2 VE3 0.70 0.30 0.6 1 0.39 25

3 VE4 0.60 0.40 0.56 0.44 35

4 VE6 0.40 0.60 0.50 0.50 45

5 VE7 0.30 0.70 0.42 0.58 50

6 VE8 0.20 0.80 0.35 0.65 55

fv and f[ are the molc fraction of V- and E-unit in feed. Fv and FE are the mole frac tion of V-and E-unit in copolymer.

I (-OCH2 ) E 12 Fr = -----=------=...:.--"-- - --

: 1 ( -OCH 2 ) 10 I 2 + 1(2.6 prolons 01 Y I .I 2

where 1(-OCH2) E and ] (2.6 prolOns 01 y) are the relati ve areas under the resonance signal of -OCH2 protons of E-unit and 2, 6 protons of V-unit respec ti vely. FE is the mole fraction of E-unit in copolymers. Table I shows the comonomer mole frac ti ons in feed and in VIE copolymer. The copolymer composition data were used to calculate the terminal model reactivity rati os using linear Kelen-Tudos method. The reacti vity ratios obtained from Kelen -Tudos method are ry = 0.34 ± 0.07 , rIO = 0.23 ± 0.07. The reacti vity rati os obtained from KT method along with feed in composition served as the initial est imate fo r the determination of reactivity rati os by non- linear elTor­in-vari ables method (EVM). The reacti vity ratios obtained from EVM are ry = 0.36 ± 0.08 and rIO = 0.25 ±0.04.

I H-NMR studies The IH-NMR spectrum of 4-vinylpyridine-ethyl

acrylate (VIE) copolymer (FE = 0.44 mole fraction in

+ CHZ- l.H .' CHZ - CH +-5~3 ,) 1 z

~II'I '

9

-CH(V) + CH (E )

6lNJlz 0 ' 0- CH Z- CH3 - CHz(V ) + CHz(V)

1 r--... V E

i ' l . i'i i i l., lil,il ,....~·, f , ' i . , I 'I . ~i,'. I' I " j f •• i l.i , 1 1 " " 1 ' li, i ' l ,l l

8 7 6 5

8(ppm)

4 3 2

Fig. I - IH-NMR spectrum of 4-v iny lpyridinc-cthy l acry late (VIE) copolymer (FE = 0.44 mole frac tion in copo lymer) in CDC!, at 24°C.

HOODA: SPECTRAL STUDY OF 4-VINYLPYRIDIN E-ETHYL ACRYLATE COPOLYMER 725

copolymer) along with ass ignment of various resonance signals is shown in Fig. I. In the case of V IE copolymer, the spectral region around 8 0.30-2.00 ppm is quite complex and can be ass igned to aliphatic protons (-CH, -CH2 and -CH3) present in the copolymer. The resonance signal around 8 0.30-0.90 ppm is assigned to 2CH3 protons of E- unit while the broad signal around 8 0.90-2.00 ppm is ass igned to -CH and -CH2 protons of V-and E-unit. The resonance signal around 8 3.49 ppm is ass igned to -OCH2 protons of E-unit. The heterocyclic protons appeared as a set of two multiplet, one centered around 86.25 ppm and another centered around 8 8.07 ppm. The multiplet centered around 8 6.25 ppm is ass igned to 3, 5 protons of heterocyc lic ring while another around 8 8.07 ppm is ass igned to 2, 6 protons of heterocyc lic ring. The complicated 'H-NMR spectrum can be simplified with the help of DEPT and 2D-HSQC spectra.

/3C{'H/-NMR studies The 13C{ 'H }-NMR spectrum of VIE copolymer (FE

= 0.44 mole fraction in copolymer) along with assign ment of various resonance signals is shown in Fig. 2. Various resonance signals in 13C{ 'H }-NMR

C-2,C- 6 {V}

r:---

C-3 , C- 5 (V)

r:---

' ;'001, " JL __ L

•• , I If i

180 160

spectrum of VIE copolymer have been ass igned completely by comparing the spectrum of copolymer with those of homopolymers i.e. poly (4-vinyl pyridine)'3 and poly (ethyl acry late). The spectral region around 8 34.30-44.50 ppm is very complex and contain overlapping signal s. This can be ass igned to aliphatic carbons in the backbone i.e. -CH and -CH2 group of V-and E-unit. The extent of overlapping of various carbon signals cannot be ascertained from 13C{'H}-NMR spectrum alone. The overl apped carbon regions can be reso lved using DEPT-135 spectrum (Fig. 3). In thi s experiment, the methylene carbon appeared as a negative phase while methine carbon appeared as a pos iti ve phase, which is based upon the pulse sequence given in DEPT experiment '4 .

On compari son with 13C{ 'H} -NMR spectra of homo polymers, the broad signal from 8 33 .60-39.50 ppm is assigned to -CH2 group of E-unit while the signal around 8 42.00-44.90 ppm is ass igned to -CH2 group of V- unit. The resonance signal around 8 39.50-42 .00 ppm is ass igned to -CH group of both V-and E-un it. The -CH region in VIE copolymer is confirmed by DEPT-90 spectrum also. The multiplet around 8 39.45-42.15 ppm is ass igned to -CH group of both V­and E-unit. In HSQC spectrum as shown in Fig. 4, the

+ CH2-C H - CH2-CH + S)Z3 ci

S (ppm}

~.J .f'" 1 2 6 N 2 0 O- CH2-CH3 1

V E

- CH(V} +

- CH (E) - OCH2 ( E) r:---~

\\-CH2(E}

Fig. 2_· I.1C ( I H l-NMR spectrum of V IE copolymer (FE = 0.44 mole fraction in copolymer) in CDCI ]

726 INDIAN J CHEM. SEC A. APRIL 2002

C-2 ,C-6 (V)

C-3 ,C-5 (V)

~

+CH2-CH - CH2- CH + 5~3 t 6lNJ 2 ~ 'O-CHr CH3

1

V E

\.'...I - 0 CH2 (E)

-CH (V)

+ - CH (E)

f"""T" I'TTTTTr""Tr:-rr"T"TITTTTTTTTTTT1'~rTTn"T'Tf"'"T\'TTTTrTTTTTTTTlTr1n·n ,.'TTrnrTTTr p-TTTrnTTTTTT1TTTT"TTTrrrTTTTTTTTI ",, , 'I 0 Ii •• "f HTTn""TJ rnTITTTTTrl'TTl'TT'I'T'f""'"fTl'Tn '·rn ....... fTT'j

180 160 140 120 100 00 60 ~O 20

6(ppm)

Fig. 3~DEPT- 1 35 NMR spcctrum of VIE copolymcr (FE = 0.44 mole fraction in copolymer) in CDCl l .

cross peak centered around 8 34.50-45.00 I 0.75-l.85 ppm is ass igned to -CH and -C H1 group of V-and E­unit respectively.

The resonance signa l around 8 60.10 ppm is ass igned to -OCH 1 group of E-unit. In DEPT - J 35

spectrum (Fig. 3), the inverse peak around 8 60.50 ppm also confirms the -OCHl region of E-unit. The

cross peak centered around 8 3.451 59.50 ppm is ass igned to -OC Hl group in HSQC spectrum (Fig. 4).

The resonance signal around 8 13.80 ppm is ass igned to ~C H 3 group of side chain. In HSQC spectrum also

the cross peak around 80.70/13.75 ppm is ass igned to l CH\ group of side chain of E-unit. The heterocyc li c carbons appea red as a set of three signals. The signal

around 8 15 1. 83 ppm is ass igned to quaternary carbon (C-4) of V- unit whi le the signal around 8 149.74 and

8 122.49 ppm are ass igned to C-2, C-6 and C-3 , C-5 carbons of heterocyc li c ring of V-unit respectivel y. In IISQC spectrum of VIE copo lymer as shown in

Fig . 4, the cross peaks around 8 149.5017.98 and 8 122 .50/6 .25 ppm are ass igned to C-2, C-6 and C-3,

C-5 carbons of heterocyc li c ring of V-unit. The presence of these single cross peaks confirm their insensItIvIty toward compos itional and configurati onal sequences . The carbony l carbon of E­

unit appeared around 8 174. 18 ppm. None of the resonance signals in UC{ IH }-N MR

spectrum of VIE copolymer appeared as a multiplet, indicat ing their insensiti v ity to various compos itiona l and conf igurat ional sequences. So the experimental concentrati on of various triad and tetrad sequences cannot be obta ined by curve fitting of the resonance signals of V- and E-unit in the copolymer. But the theoreti ca l concentrat ion of triads can be obtained using termin al model reacti vity rati os i.e. rv = 0.36 and rE = 0.25 and feed in composi tion in Harwood's program l 5

. Figure 5 (a, b) shows the plot of the normal ized V-and E-centered tri ads concentration versus the mole fraction of 4-vinyl py ridine and ethyl ac ry late monomers in feed, respecti vely . The increase in the concentrat ion of V- unit in the copolymer increases the frac tion of VVV triad whi le it decreases the fraction o f EVE triad. T he fracti on of VVE triad

U c 0 u

" 0 .. ....

HOODA : SPECTRAL STUDY OF 4-VINYLPYRIDINE-ETHYL ACRYLATE COPOLYMER

C-3,C-S(V)

<a

C-2,C- 6( V)

0>

cOl - OCH2 (E)

- CH ( E),-CH(V)

~ - CH2 (V),-CH2(E)

,i iii iii I il" iii" Iii, ii, i, i f i I I i . r rn r' iii ill" I i It" i' Iii i i II iii iii 'I i, r i, I iii I iii iii rTll PPII B 6 4 2

Fig. 4-20- HSQC spectrum of VIE copolymer (FE = 0.44 mole fraction in copolymer) in CDCll .

'·0 0 (a ) (b)

VEV

0 ·80 EVE

* 0·60

0.i.0

0 .20

o .00O:--O.---'~=---.JL-----1----L---lI

0.00 0.20 0 ·i.0 0·60 0. 80 1-000.00 0·60 0 ·80 0 ·20 0 ·i.0

fv

r i

50

150

ppm

'·00

727

ri g. (Sa.b)- The vari ation of V- and E-centcrcd triad fractions versus the feed in mole fraction of V- and E-ll1onomers respecti vely.

728 INDIAN J CHEM, SEC A, APRIL 2002

&5

7·0

7·5

B'O

B'S

(b) ppm

, , , , , ppm ppm B·S B'O 7·5 7·0 6·5

Fi g. 6- 20 TOCSY speclrum of V IE copolymer (FE = 0.44 mole fraclion in copolymer) in CDCI, (a) the expanded speclrum of aliphalic region (b) the expanded speclrum of aromalic reg ion.

first increases with increase in the concentration of V­unit, passes through the maximum value and then starts decreasing. The maximum fraction of VVE triad is observed at 0.73 mole fraction of V-unit. Similar type of variation is observed for E-centered triad and maximum frac tion of EEV triad is observed at 0.80 mole fraction of E-unit.

2D TOCSY sludies In order to understand the connectivity and confirm

the various coupling in the copolymer chain , the TOCSY spectrum was recorded. The three bond coupling between the protons of directly coupled groups in VIE copolymers can be clearly seen in TOCSY spectrum (Fig. 6) . The cross peak at 8 1.5011 .70 ppm (l) is due to the coupling between the protons of meth ylene group of V-and E-unit of the main chain . The cross peak around 81.65/2.20 ppm (2) is due to the coupling between -CH 2 (E) and -CH(V) uni t while the cross peak around 8 1.85/2.05 ppm (3) is assigned to the coupling of -CH of V- unit with -CH of another V- unit. The va ri ous geminal coupling between the protons of -CH2 (V), -CH2 (E), 2CH3 (E) and -OCH2 (E) are also possible which could not be di stingui shed due to the low va lue of the co upling constants. The coupling between the protons of -OCH1(E) with 2CH3 (E) is also possib le. It appeared as a set of three cross peaks as shown in Fig. 6 (a). On the basi s of va ri ation in intensi ty of these cross peaks with copol ymer compos iti on, the cross peaks around 8 3.82 / 1.08 (4), 8 3.6 1 /0.97 (5) and 8

Table 2-TOCSY IH_ IH shift correlalion For VIE copolymer

Peak Type of prOlan Coupled lo number (ppm) (ppm)

2

3

4

5

6

CH2 (V) ( 1 50)

CH (V) ( I .85)

-OCH2 (E) in

EEE (3.82)

EEV (3.6 1)

VEV (3.55)

CH (E) (2.20)

CH (V) (2.05)

lCHJ (0.97)

lCH1 (0.89)

Helerocycl ic 2H & 6H Helerocyclic .1H & in sH in

7 VVV (8 .1 2)

8 VVE (8.26)

9 EVE (8.38)

VVV (6.28) VVE (6.48)

EVE (6 .55)

3.55 / 0.89 (6) ppm are ass igned to the coupling of -OCH2 (E) in EEE, EEV and VEV triads with 1CH3 (E) protons respectively. The coupling within the heterocyc li c protons ( l H_3H and 5H_6 H ) of V- unit is also poss ible rFig. 6 (b)). It appeared as a multipl et around 8 8. 10-8.45/6.22-6.72 ppm. This multipl et may be assigned to the coupling of heterocyc li c protons in different compos iti onal sequences. The

HOODA: SPECTRAL STUDY OF 4-V INYLPYRlDINE-ETHYL ACRYLATE COPOLYMER 729

assignment of various coupling in thi s region IS

carried ou t on the basis o f va ri ation in intensity of these signal s with copolymer composition . The cross

peak centered around 8 8. 12 I 6.28 (7) ppm is assigned to the coupling of heterocyclic protons in

VVV triad. The cross peak centered around 8 8.26 I 6.48 (8) ppm and 8 8.38 16.55 (9) ppm are assigned to the coupling in VVE and EVE tri ads respect ive ly . These all poss ible couplings are show n in Table 2. Further four and five bond coup lings are also possible in VIE copolymer whi ch can be clearly seen in 80 ms TOCSY spectra. Due to these higher bond couplings TOCSY spectra at 80 ms is quite complex and overlapped.

The reactivity ratios of VIE copolymer calcul ated

from KT and RREYM method are rv = 0.34 ± 0.07 , rE

= 0.23 ± 0.07 and rv = 0.36 ± 0.08 , rE = 0.25 ± 0.04 respectively. The overlapped and complex proton and carbon -13 NMR spectra are well resolved with the help of DEPT-135 and 2D-HSQC spectra. The various types of couplings between different groups in VIE copolymer are assigned by TOCSY experiment.

Acknowledgement The author is thankful to Prof. A S Brar, lIT Delhi ,

for his valuable guidance for the completion of this

manuscript. The author is also thankful to the UGC, New Delhi , Indi a, for providing financial assistance.

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