Polymer International 41 (1996) 473-478

Polymerization of a$-Unsaturated Carbonyl Monomers Initiated by

9-Borabicyclo [ 3.3.1 ] nonane

Shuichi Kanno, Masayoshi Hosoi, Tateaki Ogata & Makoto Takeishi"

Department of Materials Science and Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan

(Received 12 April 1996; revised version received 29 May 1996; accepted 17 July 1996)

Abstract: 9-Borabicyclo[3.3.1]nonane (9-BBN) initiated the polymerization of a,/?-unsaturated carbonyl monomers such as ethyl acrylate (EA) without an oxidant at low temperatures (to -9OOC) under argon. Hydroquinone and 2,6-di- tert-butyl-p-cresol had little effect on the polymerization, indicating that the pro- pagating chain end is not a free radical. The rate of polymerization was found to be proportional to [9-BBN] '.' and [EA] 1.5. Electron spin resonance measure- ments using 5,5-dimethyl-l-pyrroline-N-oxide as a spin trap showed the absence of any radical species under polymerization conditions. No copolymerization of EA with styrene occurred. On the basis of the results obtained, this poly- merization was assumed to proceed via a non-radical mechanism.

Key words: ethyl acrylate, 9-borabicyclo[3.3.1]nonane, polymerization, 1,4-addi- tion

I NTRO D UCTlO N

Organoboranes, such as a trialkylborane, initiate redox polymerization of vinyl monomers in the presence of oxygen or other oxidants.'-5 Furukawa et al. have polymerized some vinyl monomers using a tri- alkylborane and initially speculated that the reaction proceeded via an anionic mechanism, because the monomers polymerized were susceptible to nucleophilic attack.6 However, later a radical mechanism for this polymerization was favoured on the basis of copolymer- ization data.' Afterwards, the initiation of the poly- merization by trialkylboranes without an oxidant was attributed to the decomposition of peroxyboranes formed by a reaction of the organoboranes with a trace amount of oxygen in the reaction However, the peroxides are too stable to be responsible for the polymerization attainable at the low temperatures applied.' O Moreover, the reaction order was incompati- ble with that of radical po1ymerization.l' Since hydro- auinone did not inhibit this polymerization, Arimoto

*To whom all correspondence should be addressed.

postulated complex formation of the free radical propa- gating chain end with the organoborane." We found that tributylborane on its own initiated a rapid poly- merization of ethyl acrylate (EA) at low temperatures, below - 60°C. Electron spin resonance (ESR) measure- ments using spin trapping agents showed the absence of radical species. The kinetic analysis of the results sug- gested a mechanism in which the initiation occurred via the 1,Caddition of the alkylborane to the m0n0mer.l~ Graft copolymerization and block copolymerization were investigated by using polymers containing struc- tures of B-alkyl-9-borabicyclo[3.3.l]nonane (B-alkyl-9- BBN), which were synthesized by the addition of 9-BBN to the carbon-carbon double bond in the polymers.14*' 9-BBN was used for air-curable polymer composites ; however, no report on the polymerization initiated by 9-BBN without oxidants has been published.16 We have found that 9-BBN initiates the polymerization of a,/?- unsaturated carbonyl monomers in the absence of an oxidant, even at low temperatures, down to -90°C. The experimental results were suggestive of a non- radical mechanism. In this paper we discuss the 'mecha- nism of the polymerization.

473 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

474 S. Kanno et al.

EXPERIMENTAL

Materials

Ethyl acrylate (EA) was purified by distillation accord- ing to the conventional methods and stored under argon. All solvents used as polymerization media were ultra-pure grade for spectroscopic analysis (Kanto Chemical Co., Inc.), which were used without further purification. 9-Borabicyclo[3.3.1]nonane (9-BBN) (0.5 M solution in tetrahydrofuran (THF), Aldrich) was used as received. Other special grade reagents were used without further purification. Argon was ultra-pure grade, with oxygen content less than 1 ppm.

Polymerization

EA and a solvent were placed in a glass tube, which was stoppered with a rubber septum and cooled in an ice bath. After the tube was purged with argon (250- 350ml min-') using two injection needles through the septum for 15 min, it was kept in a Dewar bottle ther- mostatted at a low temperature. 9-BBN solution was taken out with a micro-syringe and injected into the tube under argon to start the polymerization. At certain time intervals the polymerization reaction was stopped by injecting triethylamine into the reaction solution. This solution was then poured into petroleum ether to precipitate the polymer. The conversion was determined gravimetrically.

RESULTS AND DISCUSSION

Polymerization of EA

The hydroboration of alkenes with 9-BBN is well known and the resulting B-alkyl-9-BBNs also add to carbon-carbon double bonds. Therefore, it is expected that the presence of a small amount of 9-BBN in reac- tion media containing a vinyl monomer would initiate the polymerization of the monomer. As shown in Fig. 1, EA was easily polymerized by 9-BBN at low tem- peratures under argon. The polymerization proceeded even at -9o"C, though at a slow rate. The initial rates increased upon elevation of temperature, but the con- version levelled off when the polymerization was carried out at -5°C. As seen in Table 1, the molecular weights of polymers obtained increased as polymerization tem- perature was lowered. The molecular weights of the polymers increased slightly with increasing conversion at - 5"C, but the polymerization at lower temperatures, below -2O"C, led to a decrease in the molecular weight with increase in conversion. Although this peculiarity is not ~ncomrnon,'~ no reasonable explanation could be given. This polymerization has no living properties because a distinct increase in molecular weight with

5 0 -

40-

-45°C

-60°C

-90°C

-w- 0 1 2 3 4 5

Time (h)

Fig. 1. Effect of temperature on the polymerization of EA with 9-BBN in THF. EA 9.2 mmol, 9-BBN 0.092 mmol, THF

5 ml.

conversion was not observed. These results indicate that termination occurs easily; however, the molecular weight distribution was relatively narrow (1.6-1.7) com- pared with that of polymers obtained in radical poly- merization.

Effect of hydroquinone

The polymerization of vinyl monomers with alkyl- boranes has been discussed by relating the initiation to radical species generated by decomposition of per- oxyalkylboranes formed in the reaction of the alkyl- boranes with trace amounts of oxygen present in the polymerization media.*s9 However, the per- oxyalkylboranes which are assumed to be formed are rather stable at low temperatures, therefore they do not decompose around room temperature." Bulk poly- merization of EA occurred instantaneously after mixing

TABLE 1. Polymerization of EA with 9-BBN"

- -_ Run Temp. ("C) Conv. (%) Mnb Mw/Mnb

1 -5 35 39 000 1.7 2 -5 45 42 000 1.7 3 -20 24 57 000 1 *6 4 -20 53 50 000 1.6 5 -45 21 85 000 1.6 6 -45 43 76 000 1.6 7 -60 29 99 000 1.7 8 -60 57 88 000 1.7

" EA 9.2mmoL THF 5ml, 9-BBN 0.092mmol. ' Determined by gel permeation chromatography with standard polystyrenes (eluant: DMF).

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1 9 9 6

Polymerization of carbonyl monomers 475

the monomer with 9-BBN under argon at -45°C. This suggests that the initiating species is generated by the reaction of the monomer with 9-BBN. This process is different from the generation of initiating species in con- ventional radical polymerization. Therefore, inhibition effects of hydroquinone on the polymerization were examined, because this radical quencher reacts quickly with oxygen-centred radicals which are assumed to be generated by the decomposition of peroxyalkylboranes. As shown in Fig. 2, only a slight decrease in the rate was observed and there was no induction period in the presence of hydroquinone. These results indicate that the initiating species are not oxygen-centred radicals. Furthermore, the molecular weights of the polymers were almost the same both in the presence and absence of hydroquinone. This observation is evidence that hydrogen transfer from hydroquinone is unlikely and the propagating chain-end is not a free radical. 2,6-Di- tert-butyl-p-cresol also showed no inhibition effect, sup- porting the assumption of a non-radical process.

ESR measurements

The polymerization initiated by alkylboranes without oxidants has been discussed on the basis of radical for- mation by the reaction of alkylboranes with oxygen present in the polymerization media. To exclude the effect of oxygen, we completely purged air in the tube with ultra-pure argon, and all operations, including injection of the initiator, were carried out in an argon bag. ESR measurements using 5,5-dimethyl-l-pyrroline- N-oxide (DMPO) as a spin trap revealed that no radical

20

15

h

E

5 ia 2 3

C 0 .-

5

C 20 40 60 80

Time (min)

Fig. 2. Effect of hydroquinone on the polymerization of EA with 9-BBN in THF. EA 9.2mmo1, 9-BBN 0.092mm01, THF 5ml; -45°C; 0, hydroquinone nil; 0, hydroquinone

0.276 m01.

species were present in the polymerization solution at - 60°C. However, a few radicals were detected when the temperature was raised up to -2O"C, and the radical concentration increased very slowly at 0°C. Accord- ingly, some contributions of radical species to the poly- merization should be taken into account for polymerization above 0°C. This polymerization mixture yielded radicals when in contact with air even at low temperatures. Tributylborane, which has been reported to initiate radical polymerization, did not generate rad- icals in toluene solution of EA under argon even at 70"C, whereas butyl radicals were trapped by DMPO when air was introduced at a room temperature around 20°C. Although the polymerization initiated by tri- alkylborane has been reported to proceed via a radical mechanism, it seems that a small amount of oxygen present in the polymerization media generates radical species by a redox reaction with organoborane.

Reaction order

The polymerization rate was first order with respect to the concentration of 9-BBN (Fig. 3). The rate of radical polymerization is proportional to the square root of the initiator concentration. Therefore, this polymerization seems to be different from radical polymerization at this point. Moreover, the polymerization rate was pro- portional to the 1.5 power of the concentration of EA (Fig. 4). The results-that the reaction order with respect to monomer concentration exceeded unity- suggests that the monomer participated in the forma- tion of initiating species. This is in agreement with the experimental observation mentioned above-that the initiating species was formed when 9-BBN contacted EA at a low temperature.

-1.6 I

-2.8 1 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8

log [ 9-BBN 1 (molfl)

Fig. 3. Dependence of the rate of polymerization on 9-BBN concentration. EA 9.2 mmol, THF 5 ml; - 45°C.

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

476 S. Kanno et al.

h

?

2 M 0 -

monomer with 9-BBN and the propagation proceeds via a transfer of the boryl group. Although this poly- merization lacks a living nature as described above, the termination reaction is still unknown. On the assump- tion of unimolecular termination with respect to the propagating chain end, the polymerization can be rep- resented as follows:

K

I + M v I M (1)

IM -M* (2)

M* + (n-l)M -Mn* (3)

Mn* - Polymer (4)

ki

kP

kt

-3.0 I I where I, M and M* are the initiator, the monomer and

The initial rate of polymerization is related to the initial feed concentrations of the monomer and the initi- atory [MI, and [I],. Under the usual condition of [MIo $=- [I], , the rate is given by:

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 the propagating species, respectively. log [EAJ (molll)

Fig. 4. Dependence of the rate of polymerization on EA con- centration. 9-BBN 0.092mm01, THF 5 ml; -45°C.

Polymerization mechanism

The polymerization of. N,N-dimethylacrylamide initi- ated by 9-BBN took place similarly, as did the poly- merization of other acrylic esters such as methyl acrylate and butyl acrylate. However, the poly- merization of styrene and acrylonitrile did not take place with this organoborane at such low temperatures under argon. These results suggest that a$-unsaturated carbonyl monomers are susceptible to 9-BBN-initiated polymerization at low temperatures. Jacob & Brown1* presumed that the reaction of B-vinyl-9-BBN with vinyl ketones proceeded via a cyclic transition state to yield an enol borinate l,4-adduct7 because the 9-BBN deriva- tive reacted with methyl vinyl ketone, which is capable of the cisoid conformation necessary for the transition state, but did not react with transoid ketones such as 2-cyclohexene-1-one and 2-cyclopentene-1-one. Accord- ingly, we assumed a mechanism for this polymerization as shown in Scheme 1, in which the initiating enol borinate is formed via an intermediate complex of the

CH -CH *- I c=o I 0 El

+ H - 0

(5 )

as Fig. 3 shows the rate was first order with respect to the initiator concentration. For 1 < KCMI, the rate can be approximated as:

and for 1 9 KCMI,:

(7)

The experimentally obtained value of R, was pro- portional to the 1.5 power of the monomer concentra- tion (Fig. 4).

Effect of solvent

The effect of solvent on the polymerization was exam- ined and the results are shown in Fig. 5. Although co-

H-CkL-CH II - ki EtO’?o

Scheme 1

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

Polymerization of carbonyl monomers 477

20

15

h

E. 8

3

.- r lo >

5

0 A I A A

0 10 20 30 40 50 60 70

Time (min)

Fig. 5. Polymerization of EA in various solvents: 0, THF 5ml, 9-BBN 0-092mmol; 0, toluene 51-14 9-BBN 0.184 mmol; A, DMF 5 ml, 9-BBN 0.092 mmol. EA 9.2 mmol;

-45°C.

ordination with the ether oxygen of THF stabilizes alkylboranes, the polymerization of EA initiated by 9-BBN proceeded relatively quickly in this solvent. The rate of polymerization decreased markedly in toluene, but the molecular weights of the polymers obtained increased by about one order of magnitude over those in THF. This suggests that the initiation is considerably retarded in this solvent, while the propagation is pro- moted. Since less solvated, the unstable 9-BBN forms a dimer such as diborane in the non-polar solvent and the rate of initiation decreases. On the other hand, the pro- pagating enol borinate end, postulated in Scheme 1, is also unstable in this solvent because of lack of solva- tion; consequently, both the propagation and the termi- nation are accelerated. It seems that the increase of the propagation rate exceeded that of the termination rate to increase the molecular weight of the polymer. The extremely slow polymerization observed in dimethyl- formamide (DMF) is due to solvation, which inhibits the initiation reaction. As described later, amines inhib- ited this polymerization effectively by coordination with 9-BBN. These solvent effects support the postulated mechanism.

Effect of amines and a Lewis acid

Since amines stabilize alkylboranes by complexation, the 9-BBN complex with pyridine is used in organic synthe~es.'~ The effect of amines on the polymerization of EA initiated by 9-BBN is summarized in Table 2. Triethylamine, as a strong Lewis base, completely inhib- ited the polymerization at -45°C. Aniline with a rela-

TABLE 2. Effect of amines and zinc iodide on the EA polymerization with 9-BBN"

Additiveb PK.3 Conv. (%)

- Nil Aniline 4.6 Quinoline 4.8 Pyrid ine 5.4 Triethylarnine 10.7 Znl, -

15.2 3.0 0.9

trace 0

3.4

" EA 9.2rnrno1, T H F 5ml, 9-BBN 0.092rnrnoL -45"C, 60 rnin. '0.276 mrnol.

tively small pK, value effectively retarded the polymerization. Inhibition of the polymerization by amines increased with increasing basicity. In the case of pyridine, only trace amounts of polymer were obtained because of complex formation. The radical poly- merization of MMA initiated by trialkylboranes at higher temperatures has been investigated, in which various amines depressed the rate of polymerization; however, complete inhibition was not observed even with strong amines.20 It is well-known that the addition of Lewis acids, such as zinc bromide, leads to an increase in the rate of radical polymerization because coordination with the Lewis acid accelerates the addi- tion of monomer to the propagating chain end.21 However, as seen in Table 2, the rate decreased in the presence of zinc iodide; therefore, it seems likely that the coordination of the iodo ion with 9-BBN retarded the initiation.

Copolymerization

Copolymerization usually serves to elucidate the reac- tion mechanism; in fact, the radical mechanism was proposed for the polymerization initiated by alkyl- boranes on the basis of copolymerization data.7 The copolymerization of styrene and EA initiated by 9-BBN did not occur: a small amount of styrene (3 mol% to EA) inhibited the polymerization and no polymer was obtained. As described above, 9-BBN-initiated poly- merization of styrene did not take place; only hydro- boration of the vinyl group occurred, because propagation does not proceed. In the copolymerization of EA and styrene, 9-BBN was consumed by adding to the styrene. Copolymerization of EA and methyl meth- acrylate (MMA) also did not occur. The polymerization of MMA initiated by 9-BBN did not take place at the low temperatures applied. Although 9-BBN adds to MMA to initiate the polymerization, the a-methyl group inhibits the propagation at such low tem- peratures. Consequently, after the addition of MMA to the propagating end the chain reaction stops. These

POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996

478 S. Kanno et al.

copolymerization results also support the non-radical mechanism.

Polymerization of vinyl acrylate

As described above, 9-BBN effectively initiated the polymerization of a,p-unsaturated carbonyl monomers at low temperatures; however, the other types of vinyl monomers examined did not polymerize. If the cyclic transition state presumed in Scheme 1, in which the car- bony1 group of an a$-unsaturated carbonyl monomer participates, is essential to initiation of the poly- merization, the vinyl polymerization of the a,p-unsatu- rated carbonyl moiety of bifunctional monomers is anticipated to yield a polymer, which has vinyl groups on the backbone. Immediate gelation was observed in the polymerization of vinyl acrylate initiated by azobis- isobutyronitrile, whereas a soluble linear polymer was obtained by the use of 9-BBN at -45"C, because only the acrylic vinyl groups polymerized. The nuclear mag- netic resonance spectrum of the soluble polymer showed the existence of vinyl groups. As stated above, ESR measurements using a spin trap revealed that no radical species were present in the polymerization solu- tion below -40°C. When the temperature was raised to -2O"C, a very few radicals were detected and the radical concentration increased slowly at 0°C. However, a soluble poly(viny1 acrylate) was obtained by the use of 9-BBN even at 0°C. The small number of radicals gen- erated at this temperature seem to be ineffective in initi- ating radical polymerization. Under air, however, gelation occurred at 0"C, presumably because a redox polymerization of the bifunctional monomer resulted in crosslinking.

CONCLUSION

a,@-Unsaturated carbonyl monomers such as EA were polymerized by 9-BBN at low temperatures (-90°C).

The fact that hydroquinone and 2,6-di-tert-butyl-p- cresol had no effect on the polymerization suggested that the propagating chain end is not a free radical. ESR measurements using a radical trap revealed the absence of radical species at low temperatures, below -20°C. The rate of polymerization was found to be proportional to [9-BBN11" and [EA]"5, thus the reac- tion order is different from that of ordinary radical polymerization. On the basis of these results, a non- radical mechanism in which the initiation occurs via the lY4-addition of 9-BBN to the monomer was presumed. Amines with strong basicity inhibited the poly- merization, and Lewis acids such as zinc iodide retarded the polymerization. The effects of these additives support the presumed mechanism.

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