Mechanism of initiation of polymerization of cyclic ethers by tetrahydrofuranate of boron trifluoride

E,ropean Polymer Jourmd Vol, 17. pp. 1107 to I 110. 1981

(R)14-3057 81 101107-04S02.00 0 Pergamon Press Ltd

P~inled m Grcat Brilain

MECHANISM OF INITIATION OF POLYMERIZATION OF CYCLIC ETHERS BY TETRAHYDROFURANATE OF BORON TRIFLUORIDE* T. V. GRINEVICH,A. N. SHUPIK,G. V. KOROVINA and S. G. ENTEL1S Institute of Chemical Physics, Academy of Sciences of the U.S.S.R., Vorobievskoje Chaussee 2b, Moscow 117334, U.S.S.R. Abstract The mechanism of the initiation of polymerization of epichlorohydrin and its copolymerization with tetrahydrofuran, with boron trifluoride tetrahydrofuranate and triethyloxonium tetrafluoroborate, has been studied by [19F]NMR. It was shown that the initiation by Lewis acid resulted in the formation of an active centre of zwitterionic nature.

INTRODUCTION

Processes involving b o r o n trifluoride (BF3) and its etherates have been dealt with in many publications but the nature of the active centres in such systems is still uncertain. Two possible schemes for initiation of polymerization of cyclic ethers (CE) by BF 3 and other Lewis acids have been suggested: BF 3 + O q --~ [F3B--O] ° R

2BF3 + O ~ ~ F z B - - O R

[]

(1)

A

[]BF~

(2)

B

This paper deals with a [ 1 9 F ] N M R study of the polymerization of cyclic ethers, initiated by tetrahydrofuranate of b o r o n trifluoride (BF3THF) and triethyloxonium tetrafluoroborate (Et3OBF4). An attempt is made to elucidate the mechanism of initiation for B F 3 T H F with formation of a zwitterion (A) or an ion pair (B). Studies were made of the h o m o p o l y m e r i z a t i o n of epichlorohydrin (ECH) and its copolymerization with tetrahydrofuran (THF) for wide ranges of concentrations of the reagents and the initiator.

ance in the spectrum of the reaction mixture of a new signal corresponding to product A would correspond to the zwitterion mechanism. The presence of two signals corresponding to the F z B - O group and BF,, ion respectively would be an indication that the active centre is of the B type. In'selecting the conditions for the reaction, preferably excluding the active centre decay step, we decided to study first t h e copolymerization of ECH with T H F at an equimolar ratio ( [ E C H ] = [ T H F ] = 0.6 mol/l). As shown earlier [2], the decay of the active centre practically does not occur by the abstraction of halogen from the counterion. Figure 1 represents N M R spectra of the reaction mixture at different conversions. It can be seen that the introduction into the initial B F 3 T H F - M C system (spectrum 1) of a c o m o n o m e r mixture is followed by a decrease in the signal corresponding to B F 3 T H F (a), accompanied by the appearance of a new peak (x) in the n e i g h b o u r h o o d of - 1 1 . 3 ppm, ascribed to the [ O B F 3 ] - group in the active centre of type A. N o

X

G

C~F~

EXPERIMENTAL

The reactions were conducted directly in ampoules 5 mm in diameter in an argon atmosphere for recording NMR spectra. Methylene chloride (MC) was used as solvent and hexafluorobenzene as the internal standard. The reagents were purified by methods described earlier I-l]. The [19F]NMR spectra were taken on a "Bruker" spectrometer, model SXP4-100, operating at 84.67 MHz. The monomer concentration ranged from 1 to 10- ~ tool/l, and that of the initiator from 10- ~ to 10 -2 tool/1. The reaction and NMR measurements were conducted at 25 _+ 0.5 °. The accuracy of chemical shift measurement was + 30 Hz.

Jk.

4

3

A

RESULTS AND DISCUSSION

I 12

If the active centre does not decay, reactions (1) and (2) must produce different N M R spectra. The appear*Presented at the 21st Microsymposium on Macromolecules, Karlovy Vary, Czechoslovakia, 22-26 September, 1980.

6

I 8

I 4

2

I 0

plx~

Fig. 1. [19F]NMR spectra of BF3THF (1) and reaction mixtures (2-7) of products of copolymerization of ECH with THF in CHzCI2 at 25:. [THF] = [ECH] = 0.6 mol/1; [BF3THF]=0.14mol/I. Conversion o : 1---O; 2--10; 3--29; 4 ~ 5 5 : 5--64; 6--65: 7--77.

1107

1108

T.V. GRINEVICHet al.

B

Q 4

2

i

30

60

90

120

I

150

t, rain

Fig. 2. Integral intensities (Q) of BF3THF (1) and active centre (2) signals vs standard during the reaction. [THF] = [ECH] = 0.6 mol/l; [BF3THF ] = 0.14 tool/1. other signals in the range of _ l l 0 0 p p m were observed during the reaction. The sum of integral intensities of signals (a) and (x) remains constant, within experimental error, during the reaction and close to the initial intensity of the BFa introduced into the reaction (Fig. 2). This result indicates that only two forms of fluorine-containing components are present in the system; thus we are dealing with reaction (1) rather than (2) for which the three forms BF3THF, F2B-O, and BF,~ would have been involved at the same time. The decrease in the active centre concentration [drop in peak (x)] during the process seems to be due to a chain transfer reaction [2] which generally consists of detachment of the BF3 molecule from the active centre (reaction 3) with subsequent regeneration of BFaTHF or formation of complexes with other donors present in the system. If T H F is such a donor, the chain transfer can be represented thus:

[F3B - 0 ] 7 .%~ q"~ + BFa.THF

~

.t

sion (approx. 77~o), there still remains in the system sufficient THF to participate in the chain-transfer yielding BF3THF [3]. The presence in the system of a single signal corresponding to a B F 3 complex with a donor is indicative of the occurrence of no polymer transfer. The latter was confirmed in another series of experiments with a higher monomer concentration ([ECH] = [THF] = 3 mol/1) where, after the process is over in the presence of polymer at high concentration, there are no changes in the intensity of peak (x) suggesting that the chain-transfer reaction ceases. Under these conditions, copolymerization continues up to complete conversion of both monomers, i.e. at the end of the reaction no THF promoting chain transfer is present. Note that one of the reasons why the transfer reaction discontinues may also be a greater stability of the active centre as a result of its solvation by the polymer molecule. A more complex NMR spectrum is observed under conditions when [ECH] >> [THF]. Then, the NMR spectra of the reaction mixture show, in addition to signal (x), new peaks (b), (c), and (d) (Fig. 4) attributed to the products of decay of the zwitterion active centre as a result of, for example, reaction (4). [FaB__O ~ ] e ~ [ ] __. F 2 B ~ O ~ CF.

(4)

Special experiments have shown that, after the reaction mixture has been treated with water (washing out the initiator), peaks (x) and (b) disappear from the NMR spectra of the polymer (Fig. 4, spectra 2 and 3). This discovery prompted us to ascribe peak (b) to the F2B-O group. The remaining signals (c) and (d) seem to belong to the CF group present in the system in two unhydrolysable forms -CH(CH2CI)CH2F and -CH(CH2CI)F. The hyperfine structure of the (c) and (d) signals (F ~ 50 Hz) is not observed, evidently due to the large width of the hyperfine components (Avl/2 ~- 150 Hz).

(31

HO ~*-'CH = CH2 + BF~.THF

C o m p a r i s o n of Fig. 2 w i t h the kinetic polymerization curve (Fig. 3) shows that, under the experimen-

x

C,F~ c

d

tal conditions, the reaction does not result in complete conversion of the monomers and the active centre concentration continues to decrease even after the polymerization has practically ceased. This effect must be due to the fact that, for the limiting conver-

I

3

t 80 O

=% 60

11

,

4O I

40

I

1

I

0

I

-40

I

I

-80

20

ppm | 30

I 60

I 90

I 120

I 1.50

1', min

Fig. 3. Kinetics of accumulation of ECH-THF copolymers (conditions as in Fig. 1).

Fig. 4. [t9F]NMR spectra of BF3THF (1) and reaction mixtures polymerization of ECH in CH2CI2, 25. I-ECH] = 1 mol/l; [BF3THF] = 3 × 10-2 mol/l. Time hr: 1 ~ ; 2 4 . 7 ; 3~0.7 after water treatment; 4~72 (limit conversion ct~ = 60%).

Initiation of polymerization of cyclic ethers

1109

c~

C6F6

d f ex

1

b

0

6 5

I

4 I

I

I

I

40

20

0

-20

I -4"0

I - 60

3 ppm

2

I

I

I

I

16

8

0

-8

Fig. 7. [19F]NMR spectra of the reaction mixture for polymerization of ECH with Et3OBF4 in CH2CI2 at 25 . [ECH] = 6.4mol/h [Et3OBFa ] = 10 2 mol/l. 1 spectrum of the initiator; 2 spectrum of the reaction mixture (time: 170 mini.

ppm

Fig. 5. [agF]NMR spectra of reaction mixtures for polymerization of ECH with B F J H F in CH2C12 at 25. [ECH] = 0.1 tool/l; [BF~THF] = 0.01 mol/I. Time rain: 1

0;2

4:3

7;4

14;5

24; 6- -34 (94"~).

Comparison of the integral intensities of signals (b) and (c) + (d) shows that the integral intensity of signal (b) is not equal to the doubled intensity of signals (c) + (d); instead, it is less than the intensity expected from (4). Moreover, when the reaction mixture is allowed to stand for a long time, peak (b) disappears completely (Fig. 4, spectrum 4). This effect must be due to the reaction yielding ortho-borate (reaction 5), postulated by Meerwein [4]

/

O~CF

3 F 2 B - - O ~ CF ~ 2BF3 + B - - O ~ C F .

\

(5)

O~CF C6 F6

~

3

I I 20

I I0

I 0

I - I0

I -20

ppm

Fig. 6. [~9F]NMR spectra of reaction mixtures: I---BFaTHF in CH3C12; [BF3THF] = 1.3 x 10 I mol/1; 2--water added to solution 1: [H20] = 1.3mol/1; 3--ECH added to solution 2, [ECH] = 1.3 mol/l.

Figure 4, spectrum 4 shows the absence of polymerization in a system containing both an active centre [presence of peak (x)] and a sufficient amount of unused monomer (limit conversion -60",~). This seems to be associated with the solvation of the growing macrocation by the resulting polymer as a consequence of which the active centre is stabilized. Figure 5 shows a series of N M R spectra of the reaction mixtures, taken at different moments for [ E C H ] = 10 l mol/l and [ B F 3 T H F ] = 10-2mol/l. It can be seen that the emergence of signal (x) from (a) is accompanied by a faster appearance of signals (e) and (f). As was shown earlier in an additional experiment (Fig. 6), according to Diehl [5] these signals belong to hydrated forms of the initiator. Hence, in this case also, signal (x) is not followed by a signal corresponding to the F2B-O group. The decrease in the intensity of signals (e) and (f) during the reaction (Fig. 5) is due to the water being consumed to form terminal hydroxyl groups in the polymer [6]. The lack of signals (b), (c) and (d) in the spectrum (Fig. 5) attributed to the products of active centre decay probably connect with the presence in the system T H F added with the initiator in a quantity equal to ECH [2]. It is interesting that, when ECH is added to a system containing only hydrated forms of the initiator (Fig. 6), the polymerization proceeds on new types of active centres most probably on H O ~ m [ O H ] - . These centres are characterized by appearance of signals (p) and (q) which are not observed under conditions when BF3 is hydrated partially by the water present in the system. The absence of signals (P} and (q) in the case of partial hydration of BF3 may stem from the fact that these centres interact less readily with the monomer than BF3THF. To summerize the above data concerning the polymerization of CE on BF3, it can be stated that initiation by BF 3 yields active centres of a zwitterionic nature. Under the experimental conditions when the monomer (epoxyde) concentration is much greater than that of the initiator, the rate of interaction between product A (reaction 1) and the epoxyde is

1110

T.V. GRINEVICHet al. Table 1. Reaction mixture components and the corresponding signals for polymerization of CE on BF3THF

Group BF3THF [F3B-O] F2B-O ~ - CF

Signal

Chemical shift (ppm)

a x b c d

+ 6.6 + 11.3 +39.1 - 26.0 -69.7

Group HO - [] [BF3OH ] BF3H20 BF~7

much greater than that of its interaction with the second BF3 etherate molecule, yielding an oxonium salt, with the result that the latter is not formed under the polymerization conditions. lnitMtion by oxonium salt

Figure 7 shows N M R spectra of the reaction mixtures, taken during polymerization of E C H initiated by Et3OBF,. In this case, initiation proceeds according to reaction (6) I-7, 8],

Signal

Chemical shift (ppm)

p q e f --

- 3.5 + 3.5 + 13.4 + 15.3 + 10.1

explaining the similarity of the N M R spectra of the reaction mixtures in both cases (Figs 4 and 7). The resulting boron trifluoride will initiate the polymerization in accordance with the zwitterion mechanism. Chemical shift similarities for BF,~ and [ O B F a ] ions can be explained by equivalence of their electronic structures. The signals corresponding to particular groups of the reaction mixture are listed in Table 1.

REFERENCES

EI3OBF,, +

(6)

i.e. the appearance of a single signal corresponding to BF,7 would have been expected. The presence of a greater number of signals must be due to the conversion of the initial active centre according to reaction (7) yielding a Lewis acid: Et-O ~ [ ] BF,~ ---}Et-O ~ C F + BF3.

(7)

This results in a similarity of the fluorine-containing products formed through two initiations paths,

1. T. V. Grinevich, G. V. Korovina and S. G. Entelis, Vysokomolek. Soedin. A21 6, 1244 (1979). 2. S. G. Entelis and G. V. Korovina, Makromolek. Chem. 175, 1253 (1974). 3. A. I. Kuzaev, G. N. Komratov, G. V. Korovina and S. G. Entelis, Vysokomolek. Soedin. A16 5, 995 (1970). 4. H. Meerwein, E. Battenberg, H. Gald, E. Pfeil and G. Willfang, J. Prakt. Chem. 154, 83 (1939). 5. P. Diehl, Heir. phys. Acta 31,685 (1958). 6. G. V. Korovina, D. ~Y. Rossina, D. D. Novikov and S. G. Entelis, Vysokomolek. Soedin. AI6 6, 1274 (1974). 7. T. V. Grinevich, G. V. Korovina, S. G. Entelis and R. N. Potsepkina, Vysokomolek. Soedin. A21 5, 1160 (1979), 8. P. E. Black and D. J. Worsfold, J. Macromolec. Sci. A9, 1523 (1975).