Metathesis copolymerization of norbornene with tert-Butylacetylene initiated by a tungsten carbene complex

Eur. Polym. J. Vol. 32, No. 2, pp. 215-221, 1996 Copyright 8 19% Elsevier Science Ltd Printed in Great Britain. All rights maewed OOM-30571% $15.00 + 0.00

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METATHESIS COPOLYMERIZATION OF NORBORNENE WITH tert -BUTYLACETYLENE INITIATED BY A TUNGSTEN CARBENE COMPLEX DER-JANG

LIAW,‘*

HAO-HSIEN

CHIANG,’

BING-HUNG

JIN’ and

EN-TANG

KANG* ‘Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei 106, Taiwan and 2Department of Chemical Engineering, National University of Singapore, Kent Ridge, Singapore 0511 (Received 6 September 1994; accepted in final form 22 February 1995)

Abstract--Metathesis copolymerixation of norbomene (NBE) with tert-butylacetylene (t-BA) at a feed ratio of 1: 1 initiated by a tungsten carbene complex was investigated under various conditions. Results obtained from time-conversion curves indicated that the reactivity of tert-butylacetylene was several times greater than that of norbomene. The copolymer was obtained in a higher yield and a larger molar mass in the solvent CCL than those of other chlorinated hydrocarbons such as CHCl, and CH,Cl,. A larger molar mass and higher yield of copolymer were also obtained in the presence of a Lewis acid such as AlCl,. The activity of the tungsten carbene complex was obviously affected by Lewis acidity. ‘%2NMR analysis results revealed a content of t-BA unit of the copolymer in the presence of a Lewis acid. When copolymerization of norbomene (M,) with tert-butylacetylene (M2) in toluene was studied at 3o”C, the monomer reactivity ratios were calculated to be r, = 0.42 (kO.02) and r2 = 2.20 (+ 0.05) with the method used by Mayo and Lewis. When in chlorinated solvents the copolymer was irradiated with ultraviolet light, the copolymer degraded and the rate of degradation was found to be the greatest in highly chlorinated solvents such as Ccl,. Thermogravimetric analysis results of the copolymer indicated that there was no significant loss of mass that occurred below 250°C under nitrogen.

INTRODUCTION

In our previous studies, substituted

acetylenes, i.e. 1-heptyne, phenylacetylene, tert -butylacetylene or o trimethylsilyl phenylacetylene, could be polymerized by means of a tungsten carbene catalyst (I) [l-6]. The complex exhibits an intramolecular coordination of a double bond towards the transition metal, resulting in high stability and activity. The active species of this polymerization is tungsten carbene.

our previous investigation, tert-butylacetylene was polymerized in high yield and large molar mass in CC&, or in the presence of AlCl, [5,6]. On the other hand, Fontanille et al. reported that a stable tungsten carbene complex such as I polymerizes norbomene in high yield [9]. However, the catalytic activity of the tungsten carbene complexes in the ring-opening polymerization of cycloolefins remains markedly lower than that of multicomponent conventional systems (such as WCl,/Sn(CH,), or RuCl,/C4H,0H/H,0)

1101.

The monomer tert-butylacetylene, was not polymerized by several transition metal catalysts that cause the polymerization of other acetylenes [S]. In contrast, Masuda et al. succeeded in obtaining poly(tert-butylacetylene) of large molar mass by an MoCl,- and WC&-catalyzed polymerization [7,8]. In *To whom all correspondence should be addressed.

Despite a large difference in monomer structure between cycloolefins and substituted acetylenes, the polymerization has been assumed to proceed by the metathesis mechanism through a metallacyclobutene intermediate [3-6]. It is generally accepted that ringopening polymerization of cycloolefins proceeds via metal carbenes [lO-131. The propagation reaction of the substituted acetylene is also believed to be a metathesis type. A mechanism for the polymerization process has been previously proposed [3-6]. First, the intramolecularly coordinated double bond in the tungsten carbene complex I is replaced by the triple bond of the monomer. The next step is the insertion through a metallacyclobutene intermediate leading to the first carbenic active center [3-6]. If the tungsten carbene is the active species for polymerization for both cycloolefin and substituted acetylene, and the monomers as well as their propagating species have similar reactivities, a possibility arises that random copolymerization will occur between them, to produce a novel type of copolymer. In this study, the copolymerization of a cycloolefin 215

Der-Jang Liaw et al.

216

100

with terr-butylacetylene was investigated. Bicycle [2,2,l]hept-2-2ene (norbornene, NBE), which is very reactive to ring-opening metathesis polymerization (ROMP), was used as the cycloolefin. A tungsten carbene complex was employed as the catalyst because it is relatively easy to prepare and handle, and also has a satisfactory thermal stability. Furthermore, the comprehensive results obtained from the copoly-

merization of norbornene with tert-butylacetylene under various conditions were discussed. The microstructure of the copolymer, monomer reactivity ratios and degradation of the copolymer were also studied.

I 0

I

I

50

100

Polymerization EXPERIMENTAL

The monomer, terf-butylacetylene, was prepared from pinacolone according to a previous approach: b.p. = 36.5-385°C (yield 79%) [14, 151. Gas chromatography revealed that the purity of fern-butylacetylene exceeded 99.6%. Commercial norbornene was distilled twice from calcium hydride under dry nitrogen (purity > 99% by GC). AlCl, was purified by sublimation under vacuum. Next the catalyst, tungsten carbene complex (I), was prepared according to the method of Rudler (m.p. = 77°C) (161. The crude solid obtained was purified by chromatography using silica gel with n-hexane as eluent and recrystallized from n-pentane. Solvents were purified by distillation over a drying agent (CaH, or NaBH,). Copolymerization was carried out under an atmosphere of dry nitrogen. The copolymer formed was isolated by pouring the reaction mixture into a large quantity of methanol, filtering off, and drying it under vacuum. The structure of copolymer was determined from UV. ‘H NMR, ‘)C NMR and IR spectral analyses. Instruments used included a UV spectrometer (Varian model superscan 3). IR spectrometer (Jasco model 8 IO), NMR spectrometer (CDCI, solution 15 w/v %, Bruker AM-300WB FT-NMR), X-ray diffractometer (Philips PW 1710) and a thermogravimetric analyzer (TGA) (under nitrogen, Du Pont 9900). Number and mass average molar mass (tin and Mw) were determined by gel permeation chromatography (GPC). Four Waters Ultrastyragel) columns 300 x 7.7 mm (10~,10~,10~, IO58, in series) were used for GPC analysis with tetrahydrofuran (THF) (1 mlmin-‘) as the mobile phase. The eluents were monitored with a Gilson Model 116 using 254 nm as a detector. Polystyrene was used as standard. Determination of monomer reactivity ratios were performed following the monomer concentration variations by GC, on a Varian 3700 chromatograph equiped with OV- 17 on Chromosorb W-H columns. Bromobenzene was used as an internal standard. RESULTS

Copolymerization butylacetylene

time (min)

Fig. 1. Time-conversion curves for the copolymerization of norbornene (NBE) with rerl-butylacetylene (t-BA) at a 1:1 feed ratio at 30°C in toluene: (a) I-BA; (b) NBE.

addition, random copolymerization these different types of monomers.

occurs

between

Determination of monomer reactivity ratios In order to determine ratios, the copolymerization

the

monomer

reactivity

of NBE (M, ) and t-BA (M,) was carried out in toluene at 30°C. The concentrations of residual monomers in each aliquot were determined by gas chromatography (GC). Copolymerizations were carried out at several monomer feeds, and monomer reactivity ratios were calculated by the Mayo and Lewis method [17]. The copolymerization equation in the integral form is:

r2 =

wf210 1

( >

log

[~,1

-

p

.(l-ZJE)

108 ,(l-P$)

li

where P = (1 - r, )/( 1 - r2), was extensively modified by Yezrielev et al. [18]. [M,], and [M210 represent initial monomer concentration of monomer M, and M2, respectively. [M,] and [M,] refer to the monomer concentration of M, and M, at any given polymerization time. The variation in feed ratios and the resultant unreacted monomer concentration are listed

AND DISCUSSION

of

norbornene

with

tert-

Figure 1 shows time-conversion curves for the copolymerization of NBE with t-BA at a 1: 1 feed ratio at 30°C in toluene. Both monomers were consumed smoothly without any induction period. The reactivity of t-BA was several times higher than that of NBE. This result is consistent with the monomer reactivity ratio (see below), i.e. the monomer reactivity ratios for this copolymerization were: rNBE= 0.42, r,_eA= 2.20, rNBEx r&r&A = 0.92. These VakS indicate that t-BA is approximately 2.3 times as reactive as NBE irrespective of the kind of propagating ends; in

Table I. Results of copolymerization of tea-butylacetylene Monomer

norbornene (M2) at 30°C”

concentration (mmol)

Run no.

w,la

WJo

I 2 3 4 S 6 7

2.00 3.00 4.00 5.00 6.00 7.00 8.00

8.00 7.00 6.00 5.00 4.00 3.00 2.00

(M,)

with

Unreacted monomer (mmol)

MI 1s2 2.28 3.1 I 3.76 4.53 5.25 6.05

M, 4.36 3.73 3.25 2.65 2.04 I .55 I .05

“Copolymerized in toluene for I hr, [Mb/[Il, = 100, [Ml, = 2 mol I-‘, [rJJ[AlCl,] = 2000, [MJ, = [M,b + [A4J0.

Metathesis

copolymerixation

217

Table 2. Result of the copolymerixation of norbornene (NBA) with ferr-butylacetylene complex catalyst m under various conditions’

(I-BA) by the tungsten carbene

[Cocatalyst] Run no.

Solvent

1 2 3 4 5 6 7 8 9 10 II 12 13 14

ccl,” ccl; ccl,’ ccl, CCI, CHCI, CH,CI, n -Hexane n -Hexane n -Hexane n-Hexane Toluene Toluene Toluene

&catalyst AU, AICI, AICI, AICI, AU

Copolymers

m,

Reaction time (hr)

Temp. (“C)

Yield(%)

-

l/6 2 2 2 20 20 20 20 20 20 20 20 20 20

30 30 30 30 30 30 30 60 60 60 60 60 60 60

90 21’ 53 50 53 0 0 0 48 62 37 0 38 64

I 2 3 1 2

E

x IO-’ 151 173 116 110 101 219 30 85 251

‘[M~/~ = 40, [I-BA] = [NBE] = I mol I-‘. bMethanol-insoluble product. ‘Measured by GPC in THF, polystyrene was used as standard. dHomopolymer of rerl-butylacetylene. ‘Homopolymer of norbomene. ‘Tungsten cartwre complex was irradiated in CCI, for 5 min with 300 nm (336 W). sPolymer is scarcely soluble in THF.

in Table 1. The reactivity ratios were determined to be rl = 0.42 (*0.02), r, = 2.20 (+0.05), so that rl x r, = 0.92. These values reveal that t -butylacetylene shows a higher reactivity than norbomene irrespective of the kind of propagating ends; in addition, random copolymerization occurs between these different types of monomers.

dichlorocarbene (CCl,=W(CO),=C:) immediately forms from the reaction of the tungsten carbene complex and CCL [5,6]. This phenomenon was also observed for the homopolymerization of rertbutylacetylene catalyzed by the same tungsten carbene complex [S]. The poly(NBE-co-t-BA) prepared from CC& is soluble in any organic solvent. Notably, the polymer yield slightly increased but molar mass decreased when copolymerization was carried out in CCll for 20 hr (run 5). When copolymerizations were performed in chlorinated aliphatic hydrocarbon (runs 5-7), the copolymer yields decreased with increasing dipole moment of the solvent. Polar solvents (e.g. CHCl, or CH,Cl,) could have decreased the coordination capability between the tungsten carbene complex and monomer, thereby reducing the copolymerization rate [3]. Table 2 (runs g-14) also reveals that the activity of the tungsten carbene complex (I) increased in the presence of a Lewis acid such as AlCl, either in n-hexane or toluene. In order to determine the optimum conditions for the activation of copolymerization by AlC&, the effect of the ratio r = [Lewis acid]/m on the activity of the system was studied. A maximum activity value was obtained for a ratio r = 2 (Table 2, runs 10 and 14).

Copolymerization under various conditions

Table 2 summarizes the results obtained from the homopolymerization and copolymerization of t-BA with NBE by the tungsten carbene complex under various conditions. The results obtained from homopolymerization of t-BA in Ccl, at 30°C indicate a large yield of 90% (Table 2, run 1). However, a lower yield of NBE homopolymer was obtained (Table 2, run 2). As the homopolymer of norbomene is scarcely soluble in THF, the molar mass could not be measured by GPC. When a solution of the tungsten carbene complex was irradiated with UV light (300 mn; 336W) for 5 min, the molar mass was markedly enhanced (run 3). This is despite the fact that the polymer yield was not significantly altered in comparison with the results of run 4. This phenomenon may occur because the tungsten carbene complex (I) decoordinated and tungsten

Table 3. Effect of various cocatalysts on the copolymerixation of norbomene with terr-butylacetylene by the tungsten carbene complex catalyst m Polymef Run no. 1

2 3 4 5 6

Cocatalvst none BCI, AICI, FeCl, (CsH,),AlCl (C,H>),Al

Yield (%)

Kb x 10-r

0 47 34 24 29 3

58 68 35 66 -

Eb

x 10-r 129 122 98 149 -

-M_ IM. 2.22 1.79 2.80 2.26 -

Activitv’ 0.414 0.287 0.212 0.262 0.026

*Methanol-insoluble product. Copolymerixed in toluene at 30°C for 20 hr; [Mb/m0 = 40, [Cocatalyst]JBs = I. bMeasured by GPC in THF; polystyrene was used as standard. cActivity = mass of copolymer/(f&,~t~[~V, + yb). 4BC1, in n-hexane (1 mol I-‘) used.

Der-Jang Liaw et al

218 Table 4. Copolymerization

of norbornene (NBE) with tert-butylacetylene (r-BA) by complex catalyst AICI, or F&I, at various temperatures

[lj with

Polyme+ Run no. I 2 3 4 5 6

Cocatalyst FeCI, F&I, FeCI, AICI, AICI, AICI,

Temperature

(
Yield (%)

30 4s 60 30 4s 60

24 32 36 34 36 38

*x

IO-’

z

x IO-’

35 28 20 68 88 85

MWIM,

98 103 71 122 160 143

“Copolymerized in toluene for 20 hr; [NBE] = [r-BA] = I mol I ‘, [M],/[I], = 40, [Cocatalyst]/[I], bMethanol-insoluble product. ‘Measured by GPC in THF; polystyrene wab used as standard.

2.18 3.69 3.48 1.80 1.82 1.69 = I.

Eflect of various cocatalysts on the copolymerization

As mentioned above in Table 2, the activity of the tungsten carbene complex (I) increased in the presence of a Lewis acid. Table 3 summarizes the effect of various cocatalysts on the copolymerization of t-BA with NBE, initiated by the tungsten carbene complex (I). No yield of copolymer was obtained in the absence of Lewis acid (Table 3, run 1). From the difference of activities (Table 3) the tungsten carbene complex was affected by Lewis acids as: BClj > AlCl, > FeCl, and AlEt, Cl > AlEt,, corresponding to the order of their Lewis acidity [19]. This phenom-

i;;

J-4

, MCI3

M=B,Al,Fe

(II)

C2cc C 3u 7

50

c3tc=3cl

8

%

9

C 211

40

DEPT

30

20

l3 C-NMR

50

100

(PPM) Fig. 2. “C

NMR and DEPT spectra of poly(NBE-co-I-BA) obtained with tungsten carbene complex. Solvent: CDCI,

0

Metathesis copolymerization

219

Table 5. Peak positions in “C NMR spectra of poly(NBErco-IBA) Chemical shift (uoml

Peak no. a(&) a(rrans) b c d

30.97 32.38 37.08 125.55 144.51

Assignment

Peak no.

a(cis) a(trans) b :

I 2 3 4 5 6 8 9 10 11 12

Topolymerized

taken from Fig. 2’

Chemical shift (Dumb

Assigmnentb I,, E, C ICL e1.X

32.21 32.21 32.93 33.13 38.41 38.65 41.36 42.09 42.73 43.13 43.41 133.03 133.84

in n-hexane at 30°C for 20 hr; [M]J,& = 40, &/[AlCl,]

;a CZ C,,=G C, C2u C21c C*, C, = 1, solvent:

CDCI,

bThe first letter denotes the cis or trans structure at the nearest double bond; the second letter, at the next nearest double bond [25].

enon is possibly due to the Lewis acid complexes on the CO ligand of the initiator as shown in complex 0. Such a complexation would induce a decrease in the intramolecular coordination of the double bond, subsequently increasing the activity of the tungsten carbene complex (I). E$et of temperature on copolymerization with FeCl, or AICI,

Results obtained from the copolymerization of NBE and t-BA with FeCl, or AlCl, as a cocatalyst at various temperatures are displayed in Table 4. This table indicates that the activity of initiator (I) in the presence of Lewis acid is enhanced by increasing the temperature. This phenomenon may be accounted for by the efficiency of the Lewis acid interacting with the CO ligand of initiator (I) being increased with an increasing temperature. The molar mass of poly(NBE-co-t -BA) was not significantly altered by

polymerization temperature. This fact may be due to the occurrence of the termination reaction and also to the occurrence of the transfer reaction. Characterization of poly(NBE-co-t-BA)

The copolymer obtained for t-BA with NBE at a 1: 1 feed ratio at 30°C in toluene was characterized by IR, UV, ‘H NMR, 13C NMR and X-ray analyses. The IR analysis of the copolymerization product exhibited key absorptions due to both t -BA and NBE units. The absorptions at 964 and 739 cm-‘, respectively, are characteristic of trans and cis structures of norbomene units in copolymers [20,21]. The absorptions at 2944 and 2860 cm-‘, arise from =CH and C-H stretchings, respectively, of t-BA in the copolymer [5]. The vW stretch absorption of a t-BA unit in the copolymer, is located at 1619cm+’ [5]. The UV-visible spectrum of the copolymerization products exhibiting maximum absorption at 269 nm is quite similar to that of homopolymer of t-BA since poly(NBE) did not show any absorption above 300 run. A similar result was also observed in the copolymerization of norbomene with phenylacetylene by Makio et al. [20,21]. ‘H NMR analysis of copolymer reveals the tram

r

(a) 0 min

15

28 Fig. 3. X-ray diffraction diagram of poly(NBE-co-r-BA).

20

u.v.-time

25

30

Elution volume Fig. 4. Change of molar mass of poly(NBE-co-r-BA) in Ccl, irradiated by UV/visible at various times.

Der-Jang Liaw et al.

220

and cis structures of NBE units in copolymer at 6 = 5.33 and 5.19, respectively. Additionally, the line of ‘H NMR at 6 = 5.94 is attributed to the tram isomer and that at 6 = 6.16 to the cis isomer of t-BA units in the copolymer [5]. Figure 2 shows the “C NMR spectrum of poly(NBE-co-t-BA) obtained with the tungsten carbene complex. Two well-resolved methyl signals are found at 30.97 and 32.38 ppm which correspond to the geometric structures of cis and trans, respectively [4]. The peaks in the DEPT (distortionless enhancement by polarization transfer) spectrum are also shown in Fig. 2. This figure clearly indicates the presence of the high cis content (cu. 90%) of r-BA units in poly(NBE-co-t-BA). Actually, the three larger absorption levels at 1260, 1100, and 800 cm-’ in the IR spectrum also support the high ris content in poly(NBE-co-t-BA) [20,22,23]. This result was similar to that previously reported for the homopolymerization of tert -butylacetylene by the same tungsten carbene complex [5]. Poly(tertbutylacetylene) was reported by Okano et al. to be obtainable with MoCl, and WCl, in which oxygenor nitrogen-containing solvents possessed the high cis structure [22,23]. Additionally, the poly(tertbutylacetylene) prepared with a tungsten carbene, Ph(CH,O)C=W(CO,), by Katz and Lee also appeared to be primarily comprised of the cis structure according to the reported 13C NMR spectrum [24]. Experimental results obtained here indicate that the cis structure is more stable than the tram structure for poly(tert-butylacetylene) [4]. The peak positions in 13CNMR spectra of poly(NBE-co-t-BA) taken from Fig. 2 are listed in Table 5 [25]. Figure 3 shows an X-ray diffraction diagram of

g

75

2 m

50

=

25

200

400

600

Temperature

600

(“C)

Fig. 5. TGA curve for poly(NBE-co-r-BA)

with a heating

rate of 10°C min-‘.

poly(NBE-co-t-BA). The diffraction peaks at 20 = 18.5” and 9.8” indicate that poly(NBE-co-t-BA) is partially crystalline. Degradation of poly(NBE-co-t-BA) Photodegradation. The stability of poly(NBE-cot-BA) was studied in CCL, at 30°C in the absence of light and under ultraviolet/visible light irradiation. In the absence of illumination, the molar mass (Hw) of poly(NBE-co-t-BA) degraded slightly from 35 x lo4 to 27 x lo4 after 8 hr in Ccl, at room temperature. This result shows that poly(NBE-co-t-BA) has a high degree of stability. The changes of molar mass of the poly(NBE-co-t-BA) in CCI, (17.5 mg copolymer/ 5 ml CC14) irradiated by ultraviolet/visible at various times are shown in Fig. 4. This figure indicates that the molar mass has decreased by approximately a factor of 90 after 15 min of irradiation. However, the distribution of molar mass increased with increasing

b

v ‘V

4 c=o

1724

l~II~lIIII1‘1I~l~IIII 4000

3000

I

I

I

2000

Id

I

I

1500

I

I

I 1000

,

1

1

I

,

500

cm-l Fig. 6. IR spectra

of poly(NBE-co-~-BA): (a) pristine polymer; (b) sample sample heated at 30°C for 72 hr.

heated at 120°C for 1 hr; (c)

Metathesis copolymerization illumination time. Moreover, the more diluted solutions (5mg copolymer/5ml Ccl,) show a higher rate of decrease in the molar mass (after 7.5 min of irradiation, and the molar mass has decreased by approximately a factor of 95). This same figure further reveals that the decrease in molar mass with Ccl, as solvent is much more rapid than in CHCI, after 15 min of irradiation. This phenomenon was also observed by Neoh ei al. for the photodegradation of poly(o-trimethylsilyl phenylacetylene) in chlorinated solvents [26]. Thermodegradation. The TGA curve for the poly(NBE-co-t-BA) is shown in Fig. 5 with a heating rate of lO”C/min in nitrogen. This figure reveals that mass loss starts at 250°C. The mass percentage remaining at 800°C is 12% under nitrogen. A comparison is made in Fig. 6 of the IR absorption spectra for the poly(NBE-co-t-BA) and the heated samples. In Fig. 6(a) the three absorptions at 1260, 1100, and 800 cm-’ reflect the high cis content of t-BA unit in poly(NBE-co-t-BA) [22,23]. Figure 6(b) and (c) shows the samples heated at 120°C for 1 hr and at 30°C for 72 hr, respectively. Either with increasing temperature or longer thermal treatment, a peak was observed and increased in magnitude at about 1724cm-‘, which indicates the presence of carbonyl groups. This absorption shows that during thermal degradation, poly(NBE-co -t -BA) reacted with oxygen to form oxygen-containing compounds. However, no line indicative of an OH band was observed in the IR spectra of the copolymer after thermal treatment. The new absorption of carbonyl groups implies that degradation is accompanied by oxidation [271. On the other hand, the appearance of the carbonyl group for poly(rert-butylacetylene) is required above 200°C for 1 hr. This fact revealed that a more rapid degradation of poly(NBE-co-t-BA) occurred when norbomene unit was incorporated in the copolymer chain. Acknowledgemenr-The authors would like to thank the National Science Council of the Republic of China for their financial support.

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