Configurational double bond selectivity in the epoxidation of hydroxy-terminated polybutadiene with...

Macromol. Chem. Phys. 195,3937-3948 (1994) 3937

Configurational double bond selectivity in the epoxidation of hydroxy-terminated polybutadiene with rn-chloroperbenzoic acid

Marcel0 Aguiar a)

Laboratbrio de Materiais PolimCricos - Escola de Quimica da Universidade Federal do Rio de Janeiro - Ilha do FundHo, Rio de Janeiro CEP 21949-900 Brasil

Sonia Cabral de Menezes

Centro de Pesquisas da Petrobras (CENPES), Rio de Janeiro

Leni Akcelrud* a)

Laboratbrio de Materiais PolimCricos - Escola de Quimica da Universidade Federal do Rio de Janeiro - Ilha do FundHo, Rio de Janeiro CEP 21949-900 Brad

(Received November 13, 1993; revised manuscript of February 21, 1994)

SUMMARY The selectivity of the three different double bonds (CIS, TRANS and VINYL) of hydroxy-

terminated polybutadiene regarding epoxidation was evaluated, using m-chloroperbenzoic acid in toluene below room temperature (- 10 "C, - 5 "C, 0 "C and 5 "C). To determine the kinetic constants for the three configuratons (kl,cis = (2,59-0,6) x lo-' L . mol-' . min-'; kz,mns = (5,35-0,82) x L * mol-' . min-'), 'H and 13C NMR was used along with a system of three parallel equations. The activation energy values were also evaluated (E,, cis = 53,9 kJ . mol - ' ; E,, = 70,7 kJ . mol - ' and E,, vinyl = 261,l kJ . mol-') for the three double bond types.

L 1 mol-' min-'; k3,vinyl = (0,97-0,Ol) x

Introduction

Since 1963, when rn-chloroperbenzoic acid (MCPBA) became commercially available, the epoxidation of several olefins, not feasible until then, began to be explored l). The epoxidation reaction with organic peracids follows a second-order kinetics, for the majority of olefins and solvents employed 's2) . Hydrogen-bonding solvents decrease and high-dielectric-constant solvents increase the reaction rate, due to the high polarity of the non-ionic intermediate, which maintains its internal hydrogen bond in the transition state ').

The structural effects found for the epoxidation reaction are typical of unsaturated compounds (nucleophiles) reacting with electrophiles. Therefore electron donor substituents must raise the reaction rate by increasing the nucleophilic character of the olefin'-4). Indeed, it has been shown by a number of workers that, under similar

a) Present address: University of Massachusetts at Amherst, Polymer Science and Engineering, Amherst MA 01003. U. S. A.

0 1994, Hiithig & Wepf Verlag, Zug CCC 1022-1352/95/$10.00

3938 M. Aguiar, S. Cabral de Menezes, L. Akcelrud

conditions, the reaction rate is governed by the nature and number the substituents in the double bond'). The substitution of one hydrogen atom by one alkyl group (electron donor) increases the rate by a factor of 25, without any influence of the branch length, whereas two alkyl substituents increase the rate by a factor of 500-600 in relation to ethylene l). For aryl substituents, steric hindrance makes the reaction slower when compared with their alkyl analogs.

The effects due to structural changes in the electrophile (peracid) and in the nucleophile (olefin) had been studied by Lynch and Pausacker5). It was shown by these authors that electron acceptors linked to the ring of aromatic peracids increase the electrophilic character of these compouds, accelerating the reaction. On the other hand, the introduction of electron acceptors in the olefin, such as carboxylic groups, were shown to slow down the rates). The CIS-TRANS isomerism had also been studied '), but still there are doubts concerning to which configuration is more reactive and why.

In this paper the epoxidation of hydroxy-terminated polybutadiene (HTPB) with MCPBA is reported. The three configurations present in the chain (CIS, TRANS and VINYL) produce two types of oxirane rings: one with two alkyl substituents (CIS and TRANS) and other with only one (VINYL). We aimed to determine the selectivity of these three forms with regard to MCPBA when reacting simultaneously.

Experimental part

Equipment

- Vapor pressure osmometer Wescan 232 A, using toluene as solvent and benzyl standard for

- VXR 300 NMR, using tetramethylsilane (TMS) as internal reference. calibration.

Materials

- Hydroxy-terminated polybutadiene (HTPB) - Liquiflex H, from Petroflex, Brazil. Dried under reduced pressure (1 1 mmHg) for 3 h at 100°C. Number-average molecular weight M, = 2300 as determined by vapor pressure osmometry (VPO), and corrected for antioxidant (2,6-di- tert-butyl-4-methylphenol) by means of ultraviolet spectroscopy as described in the literature6). - rn-Chloroperbenzoic acid (MCPBA) - Aldrich (aOolo), used without further purification.

The exact concentration in peracid was determined by back titration with iodine as described in the literature'). - Toluene Merc:, dried by refluxing over metallic sodium for 2 h and distilled, received over

molecular sieves 4 A. - Methanol, Grupo Quimica purified by distillation.

Epoxidation reactions

The epoxidation reactions were carried out in a three-necked roud-bottomed reactor, using [C=C], = 0,4 mole L-' and [MCPBA], = 0,3 mole L-', in toluene medium. The reaction system was kept under constant magnetic stirring in dry nitrogen atmosphere, in the dark.

The temperature range studied was - 10 "C to + 5 "C, and cold methanol (5 "C) was used to interrupt the reaction. The precipitated polymer was separated and dried in a vacuum oven at room temperature, in the dark, for 16 h. The reported results represent an average of at least three runs. An error of 3% was found.

Configurational double bond selectivity in the epoxidation of hydroxy-terminated . . . 3939

The conversion in epoxy was maintained below 77%. It was found that higher yields rendered the polymer insoluble in CDCI,.

Nuclear magnetic resonance (NMR) spectroscopy

NMR was used to analyse the original microstructure of HTPB and epoxy products under the following conditons:

1) Nucleus: 'H-frequency = 300 MHz, 5% (w/v) solution in CDCl, 2) Nucleus: "C-frequency = 75 mHz, 30% (w/v) solution in CDCI,, proton decoupled.

Results and discussion

Microstructure of the original hydroxy-terminated polybutadiene (HTPB)

In this work we aimed to analyse the selectivity of the double bonds in HTPB, towards epoxidation with m-chloroperbenzoic acid (MCPBA), namely CIS, TRANS (1,4-enchainment) and VINYL (1 ,Zenchainment), when reacting simultaneously. The reactivity of these isomeric forms had already been evaluated when reacting alone, using the epoxidation reaction to study polybutadiene crystallinity ') or for determina- tions of the global kinetic constants for 1,4 and 1,2 polybutadjenes in epoxidation reactions 9).

The first step was obviously to determine the microstructure of the original HTPB. In Figs. 1 and 2 the 'H and I3C NMR spectra of HTPB are shown. The correspond-

ing assignments, shown in Tabs. 1 and 2, were based on previously reported data for similar HTPB (R 45M-ARCO) 10-14). Integration values lead to the composition:

Vo CIS: 23,2

To TRANS: 54,O

Vo VINYL: 22.8

From these results and the initial amount of total insaturation it was possible to know the initial concentration of each type of double bond for each reaction.

It should be noted that 13C NMR was used only to analyse the original HTPB samples, and qualitative spectra were recorded. However, due to the similarity in

Fig. 1. 'H NMR spectrum of hydroxy- terminated polybutadiene (Liquiflex H; 0% conversion). Cvo, C,, and C , stand for VINYL, CIS and TRANS double bond configurations

6 5 1 3 2 1 6 in ppm


Fig. 2. 13C NMR spectrum of hydroxy- terminated polybutadiene (Liquiflex H; 0% conversion). V, C and T stand for VINYL, CIS and TRANS double bond configurations

110 120 100 80 60 LO 20 6 in pprn

Tab. 1. Assignments of chemical shifts 6 from 'H NMR spectra of hydroxy-terminated polybutadiene (Liquiflex H) a)

6 in ppm Hydrogen

5,7-5,l -CH=(1,2) + -CH=CH- (1,4) 594 -CH=CH- (1,4) 5,l-4,s =CH, (1,2) 4 2 -C&-OH (GoH) 4,i -C_Hz-OH 3,7-3,4 -c&-oH(Vo~) 2,5-1,7 1,7-1 -CH2- (1,2)

-CH2- (1,4) + -CH- (1,2)

a) GOH: HO-CH,-CH=h-; HOH: HO-CH2-CH=CH-CH2-; VoH: HO-CH,-CH(CH=CH,).

I

chemical environment, the ratio of -CH,-CIS (27,3 ppm)/TRANS (32,6 ppm) found for the 1,4-enchainment was the same as that of the 'H NMR spectrum, within experimental error.

Conversion in epoxy

The assignment of CIS and TRANS epoxy groups was based on data reported by Gemmert5) and Zuchowska'). The former assigned 13C NMR spectra of partly epoxidized cis- and trans-polybutadiene and the latter determined a global kinetic constant for 1,2- and 1,4-polybutadiene on epoxidation reactions.

Comparing the integration values for epoxy hdrogens absorbing at 2,7 and 3,O ppm (Fig. 3) with those relative to CIS and TRANS epoxy carbons absorbing at 56,5 and 58,5 ppm (Fig. 4), respectively (as assignment by Gemmer Is)) we concluded that the 'H 2,7 ppm absorption was related to the TRANS epoxy hydrogens, whereas the one at 'H 3,O ppm was related to the CIS epoxy hydrogens. This conclusion is not in


>c-'w .' 2- 2: N

Fig. 3. 'H NMR spectrum of epoxidized hydroxy-terminated polybutadiene (26,7% conversion). C,,, C,, and C,, stand for VINYL, CIS and

Tab. 2. Assignments of chemical shifts 6 from I3C NMR spectra of hydroxy-terminated polybutadiene (Liquiflex H) a)

TRANS eqoxy

6 in ppm Carbon

142,5 -CH= (V isolated)

115 =CH2 (V isolated) 134-128 -CH=CH- (1,4)

67 --CH,-OH (VoH) 64 --CH,-OH (HOH) 58,4 --CH,-OH (GOH) 47 -CH- (V) 44 -CH- (V isolated) 39 -CH2- (V isolated) 35 -CHZ- (V) 33 -CH2- (1,4 T) 31 -CH2- (1,4) (VT) 28 -CH2- (1,4 C) 25 -CH2- (1,4) (VC)

HOH: HO-CH2-CH=CH-CH2; VoH: HO-CH,--CH(CH=CH& GOH: HO-CH2-CH=C-; T TRANS, C: CIS, W VINYL.

Fig. 4. "C NMR spectrum of epoxidized hydroxy-terminated polybutadiene (26,7% conversion)

1LO 120 100 80 60 40 20 b i n ppm


agreement with Zuchowskag), who assigned 2,7 ppm for CIS epoxy hydrogens and 3,O ppm for TRANS epoxy hydrogens.

For the assignment of VINYL epoxy hydrogens, diglycidyl ether of bisphenol A (DGEBA) was used as a model compound. It was observed that the 'H NMR absorptions of H(I), H(2) and H(3) hydrogens of this molecule overlap with the CIS and TRANS epoxy absorptions already assigned for epoxidized HTPB (Fig. 5).

6(H3]:3pprn and 6(H'.H2)=2.3pprn

0 / \ -CH-C-C-H'

I I H3 H2

Fig. 5 . 'H NMR spectrum of diglycidyl ether of bisphenol A (DGEBA). (TMS = tetramethylsilane)

7 6 5 L 3 2 1 0 d i n pprn

According to this assignment, from the NMR data it was possible to calculate the conversion in epoxy (x) for each type of double bond (xc, x, and xv) and consequently the total epoxidation (x, + + J as a function of time for each temperature (- 10 "C, - 5 "C, 0°C and 5°C). For any condition, we have:

xc = Aepc/(Ac + At + Av + Aepc + Aept + Aepv)

Xt = A e p t l M c + At + Av + Aepc + Aept + Aepv)

Xc + t + v = (Aepc + Aept + A e p v Y ( 4 + At + Av + Aepc + Aept + Aepv)

where Aepc, Aept, A,, A,, A, and A, stand for absorptions of cis, trans, and vinyl epoxy and cis, trans and vinyl double bonds, respectively.

Due to steric hindrance, the 1,Zenchainment (VINYL) is less reactive than 1,4-enchainment. In fact, it was found that for short periods of time (20 min) the


1,2-VINYL content of HTPB remained unchanged. In that case there is no overlap between the epoxy peaks.

As the VINYL epoxy absorptions were overlapped with the CIS and TRANS epoxy absorptions, as stated above, the real CIS and TRANS epoxy content (Aepc (real) and A,, (real)) had to be calculated from the total absorptions (AeF and A,,,) of the corresponding peaks (Fig. 3), as follows:

Figs. 6, 7 and 8 describe how the concentration of CIS, TRANS, and total epoxy varied with the time, for each temperature.

Observing the characteristic terminal -OH region at both 13C (50-70 ppm) and 'H (3 -4 ppm) NMR spectra, it was verified that there were no changes on those sites at any experimental conditions. Thus, the HTPB -0H's were preserved in the epoxidations.

Fig. 6. Conversion of CIS epoxy xcIs with time (5"C(0);0°C(.); - 5 T ( + ) ; -1O0C(O))

Fig. I. Conversion of TRANS epoxy x,,,, with time (5 "C ( 0 ) ; 0 ° C (U); -5OC(+); - 1 o o c (0))

25 s

10

5

0 0 5 10 20 30 LO 60

Time in min

, 50 ......................................................................

c .- LO

F 30 z 4

x 20

10

n

0 .......................................................... d.: ........................

.............................................. 0. ........................................................ /: _. /---

................ ....,, .............................. rc;. ................................................. 6,' /----

+ ,<vm /-

...n ................................................................

v -

0 5 10 20 30 LO 60 Time in min


._ C + 60

,O

,/'- ............................................................................ dr.0 ..........................................

,,

Determination of the kinetic constants

The data treatment for determination of the distinct kinetic constants for each configuration (kc, k, and k,) was based on a system of three parallel equations, in which a first-order kinetic was assumed for each reacting species.

A + C k l E c

A + T & Et

A + V L E V

Where A stands for peracid; C, T and V for CIS, TRANS and VINYL double bonds; E,, E, and E, for CIS, TRANS and VINYL epoxy groups and k , , k, and k3 for corresponding kinetic constants.

The reaction rates relative to peracid (A) and each configuration (C, T and V) can be represented by, upper-case italic C denoting amount concentration:

From mass balance, we have:


Now, if we make = CAE,, @C = CC/C,, @T = CT/C,, @v = CV/CAO* @EC

= CEc/C,, @ET = CET/CAo, GEV = CEv/C, and T = kl t, we will have from Eqs. (1) to (4):

where a = k,/kl and /l = k , /k , . From Eqs. (5 ) to (8) we will have:

@A + @EC + @ET + @EV = 1 (13)

@C @EC = (14)

@T + @ET = N (1 5 )

@V + @EV = (16)

where M = Cco/C,, N = C,/C,, W = Cvo/C,.

T = 0, Gc0 = M and @To = N, @vo = W, we will have: Dividing Eqs. (2) over (3) and (2) over (4) and taking into account that when

@ET = N - @T = N - N/[MO(@c)"l

and Eq. (18) into Eq. (16), we have:


Integrating

(22)

(23)

Which can be expressed as

Now, we can determine the a = k,/k, and /3 = k, /k , parameters, replacing Eq. (17) into Eq. (15) and Eq. (18) into Eq. (16):

From NMR integration data for the epoxy HTPB and numerical integration, the F(GC) values were calculated for the different reaction times and temperatures. Values of k, were graphically obtained from the slope of the straight lines C , * T = F(&). The rate constants k,, relative to 1,4 CIS configuration are shown in Tab. 3. For the determination of k, and k, we used a = k,/k, and /3 = k3/k, . The rate constants k, (lp-TRANS) and k, (1.2-VINYL) values are shown in ribs. 4 and 5, respectively.

Activation energies for each configuration

The dependence of reaction rate in relation to temperature is expressed by the well known Arrhenius equation:

Where k, is a constant related to the number of collisions between molecules per unit of time and E, is the so-called activation energy, which represents the minimum energy required for the reaction to occur. The influence exerted by temperature over the reaction rate depends on the activation energy. The Arrhenius equation in its logarithmic form represents a straight line, that is 16):

In k = In k, - E,/(RT)

For high activation energy values, small temperature variations will cause large variations in k, due to the high slope (EJR) .


Temperature

Slope in min-' Ch/(mol . L-') k, , cis/(L . mol - I . min - I )

5 "C 0 "C -5°C -10°C

0,0078 0,0072 0,0062 0,0018 0,2993 0,2993 0,2993 0,2993 0,0259 0,0241 0,0205 O,O060

Tab. 4. Rate constants k,, relative to 1,4-TRANS configuration

Temperature I 5°C 0 "C -5°C - 10 "C

a I 2,0602 1,9492 1,665 1 1,3687 k2, ,,,,,/(L * mol - ' . min - ') 0,0535 0,0469 0,0342 0,0082

Thb. 5. Rate constants k,, relative to 1,2-VINYL configuration

Temperature I 5°C 0 "C - 5 o c

P 0,3758 0,0972 0,0057 k,,Vinyl/(L. mol-I . min-') 0,0097 0,0023 0,Oool

The activation energy for each double bond configuration in HTPB towards epoxidation with MCPBA was graphically evaluated and the values are summarized in Tab. 6. Comparing these values with those of Tables 3,4 and 5 (k,, k, and k, values), it can be observed that for TRANS configuration, which reacts faster, E, is greater than for CIS configuration, which reacts slower. This apparent contradiction can be explained in terms of the smaller value found for the temperature dependence of constant k, for CIS configuration (dk,/dT) than the corresponding k, for TRANS, (dk,/dT).

dk/dT = kE,/(RT2)

Conclusions

The expoxidation of hydroxy-terminated polybutadiene (HTPB) was carried out with m-chloroperbenzoic acid (MCPBA), with yields up to 77%. Maintaining IC=C], = 0,3957 mol L - I and [MCPBA], = 0,2292 mol - L-I, temperature in the range (- 10 "C to + 5 "C) and using toluene as solvent, it was observed that the TRANS configuration reacted 1,s times faster than the CIS. The 1 ,Zenchainment (VINYL) was much less reactive as expected, due to the number of substituents.

The activation energy value for TRANS configuration, which reacts faster, was slightly higher than that for CIS, and this was explained in terms of the high slope


Thb. 6. Activation energy for each double bond configuration in hydroxy-terminated polybutadiene towards epoxidation with m-chloroperbenzoic acid (gas constant R = 8,3 14 J . mol -’ . K - l )

Double bond type I 1,4-CIS 1,4TRANS 1,2-VINYL

Ea/(kJ. mol-’) 1 5399 70,7 261,1

Ea/R in the Arrhenius plot In k = In k, - E,/(RT) in which small temperature variations cause large variations in reaction rate, and the dk,/dT (CIS) value was smaller than the corresponding dk,/dT (TRANS).

The initial hydroxy concentration was not reduced by the epoxidation. The reactivity of olefinic linkages in HTPB towards epoxidation with MCPBA

depends on the microstructure, and in the conditions employed:

1,4-TRANS > 1,4-CIS > 1,2-VINYL

The authors are very grateful to Prof. Quang Tho Pham, from CNRS (Centre National de la Recherche Scientifique), who provided the DGEBA model compoud and whose helpful comments improved the final form of the manuscript.

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