Addition and Condensation Polymerization Processes

22 Block and Random Copolymers by Cationic

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Copolymerization YUYA YAMASHITA Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Nagoya, Japan

Although 3,3-bischloromethyloxetane(BCMO) polymerize

does not co-

with styrene (St), random copolymers

are ob-

tained between 1,3-dioxolane (DOL) with St or BCMO

by

cationic catalyst. The propagating species of cationic polymerization of cyclic formals such as DOL are cations in between a carbonium ion like St and an oxonium ion like BCMO.

The living nature of the cationic

of DOL in DOL

C H Cl 2

4

2

with

Et OBF 3

4

polymerization

was established, and

polymer of molecular weight 160,000 was obtained.

Copolymerization

of DOL-St

attained a molecular weight

of 300,000. By adding BCMO

to the DOL-St

mer, block copolymers of DOL-St

living poly-

and DOL-BCMO

were

obtained.

T n copolymerization b y a radical mechanism, random copolymers are obtained i n almost every case, but true copolymers are not obtained i n copolymerization by a cationic mechanism. Usually copolymers with considerable block character are obtained, or some homopolymer is formed together with the copolymer. It is generally recognized that copolymerization between monomers with different polarities leads to copolymers with marked block character. In an extreme case the copolymerization product practically consists of a homopolymer mixture. O'Driscoll (7) derived a copolymerization equation showing that monomer consumption is determined by the ratio of the active sites formed at the initiation reaction if cross propagation is negligible. W e propose that the difficulty of cross propagation between monomers with different polarities might arise from the different nature of the propagating species. A

350 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

22.

YAMASHiTA

Cationic

Copolymerization

351

The predominant propagating species i n the cationic polymerization of vinyl monomers is a carbonium ion, but the living end of cyclic ether polymerization is an oxonium ion, and the nucleophilic attack of the ether oxygen of the monomer to the α-carbon atom of the oxonium ion should be the rate-determining step of the propagation reaction. H o w ever, bond breaking between the α-carbon atom and the oxygen atom of the oxonium ion might proceed in some cases to give more electrondeficient carbon atom in the transition state. The propagating species i n the cationic polymerization can be exam ined from the copolymerization behavior (21). Cyclic ethers such as tetrahydrofuran ( T H F ) or 3,3-bischloromethyloxetane ( B C M O ) , and cyclic esters such as β-propiolactone ( β - P L ) or c-caprolactone (c-CL) are classified as oxonium ion type monomers. Copolymerizations between these monomers are observed easily as i n the case of B C M O - T H F (12, 13), B C M O - 0 - P L (14, 15), B C M O - c - C L (16), and T H F - c - C L (21). Copolymerization between an oxonium ion type monomer and a carbonium ion type monomer has never been carried out successfully. Styrene (St) does not form a copolymer with T H F ( I ) , B C M O ( I ) , or β-PL (2, 16). The formation of a homopolymer mixture was confirmed for the St-/?-PL system (18,19, 26). The reason for the absence of cross propagation was discussed elsewhere (6), but the reaction of the trityl cation with β-PL and the reaction of the triethyloxonium ion with 1,1diphenylethylene d i d show the absence of the bonding reaction (6). O n the other hand, some intermediate monomers can be copolymerized either with St or with T H F as in the case of cyclic formais. Cyclic formais such as 1,3-dioxolane (JO) ( D O L ) , 1,3-dioxepane (11), and trioxane (3, 4) can be copolymerized randomly with styrene, and the cross sequence can be measured with N M R (10). This suggests that formais and styrene have a simliar propagating species. The oxonium ion from formais is apt to react like a carbonium ion because of the resonance stabilization of the derived carbonium ion. It is difficult to consider the propagating species as a true carbonium ion because the carbon atom i n the presence of neighboring oxygen atom might not be able to have more than fractional positive charge. The formation of random copolymer between formais and oxonium ion type monomers, on the other hand, suggests that the oxonium ion type monomer and formais have a similar propagating species. D O L can copolymerize with T H F , B C M O , and 0 - P L (8). Thus, we concluded that the propagating species of cationic polymerization of cyclic formais is something i n be tween the carbonium ion and the oxonium ion (24, 25). Hence, it should be possible to copolymerize an oxonium ion type monomer with a car bonium ion type monomer by introducing an intermediate monomer with it. This idea was verified by obtaining random copolymer of β - P L - S t -

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

352

ADDITION AND

CONDENSATION

POLYMERIZATION

PROCESSES

D O L (6), although we could obtain only a homopolymer mixture in the copolymerization of β-PL and St. W e intend to obtain a high molecular weight polymer with such a random sequence of cyclic ether and vinyl monomer by using cyclic formais as intermediates. To obtain a high molecular weight block or random copolymer of the oxonium ion type monomer and carbonium ion type monomer, experi mental conditions must be such that termination or transfer reactions are minimized. The living nature of the cationic polymerization of T H F (7) is well established, but it has been difficult to obtain a high polymer of styrene or D O L by cationic mechanism. In this paper we demonstrate the living nature of the polymerization of D O L and the high polymer of St—DOL copolymer. Using this technique, we were able to obtain a block copolymer of vinyl monomer and cyclic monomer. Living

Polymer

of

1,3-Dioxolane

Polymerization of D O L with cationic catalysts such as B F 3 · E t 2 0 , Ac 2 0-HC10 4 , or A l E t 2 C l - H 2 0 in a sealed ampoule under nitrogen yields polymers with molecular weight of less than several thousands (9). To clarify the reason for the low molecular weight, we examined the mecha nism of the polymerization of D O L with B F 3 · E t 2 0 ( 2 2 ) . The poly merization proceeds with complex kinetics, and a complex initiation reaction arises from B F 3 · D O L . In addition, there is evidence for con siderable termination and transfer. O n the other hand, the polymerization with E t 3 O B F 4 proceeds with very simple kinetics, and the following scheme was deduced from the structure of the initiation product and the molecular weight of the polymer ( 2 0 ) . CH2—CH2 EtHOBF4

+ 0

\ /

CH2

Ο

CH2—CH2 .

Et—©Ο

Ο

\ / CH2

CH2—CHU

t

I

Et—©Ο

\ / CH2

Ο

• BFP

CHU—CH2 DOL „ Et—OCH.,CH 2 OCH 2 —Ο β

\

Γ ΟΙ /

· BF 4 ©

CHo


22.

YAMASHiTA

Cationic

353

Copolymerization

The rate of polymerization follows the standard equation obtained for an equilibrium polymerization without termination and with rapid initiation (20):

- ^ P = * p [ C ] „ { [ M ] - [M] e } where [ C ] 0 is the initial catalyst concentration, and [ M ] e is the equilib rium monomer concentration. 100 r

Time (min.) Figure 1. Time-conversion curve for DOL polymerization with Et OBF at 30°C. 3

(a) [M] 0

&

=

4.95 moles I liter, [C] 0 = 1.78 X 10~ mole/liter in ChtCh under N (b) [M] 0 = 8.65 moles/liter, [ C l 0 = 1.56 Χ I0"2 mole/liter in CtHtCL under high vacuum 2

2

The time—conversion curve (Curve a) shown in Figure 1 was ob tained by an experiment in an open system from residual monomer con centration measured by gas chromatography. The thermodynamic data for the equilibrium polymerization were obtained as Δ Η Ρ = —3.6 k c a l . / mole and AS = —14 cal./mole-degree for the benzene solution (23). The induction period depends upon the reaction between the catalyst and the added water. The degree of polymerization determined by vapor pressure osmometer increases linearly as shown in Figure 2 if the added water is kept low. The absolute rate constant for the propagation was estimated as 4.3 Χ 10"2 liter/mole-sec. at 30 ° C . with an activation energy of 11.7 kcal./mole with E t 3 O B F 4 i n methylene chloride—a value com parable with that of T H F . The living nature of D O L is suggested by Figure 2, and the observed degree of polymerization reaches 90% of the P


354

ADDITION AND CONDENSATION

calculated value Ρ == - — °

Ί

POLYMERIZATION

PROCESSES

— - even in this rough open system if

the initial catalyst concentration is sufficiently high to yield polymer molecular weight below 10,000.

0

20 40 60 80 Conversion (%)

Figure 2. conversion

Degree of polymerization of DOL with EtsOBFh CH2Cl2 at 30°C.

100 vs. in

4 . 9 5 moles/liter, [ C ] = 2.93 X 10~* mole I liter Added water: (a) none, (b) 7.2 X 10~ mole/liter, (c) 14.4 X 10~ mole/liter, (d) 25.5 X 10-' mole/liter [M]

0

=

0

s

3

Because the adventitious water reacts with catalyst to decrease the degree of polymerization by transfer or termination, experiments in high vacuum system are necessary. The following experiment was carried out in a vacuum system kept at 10"6 mm. H g , and D O L was distilled from the stock solution colored with anthracene sodium. The solvent, ethylene dichloride, was dried over calcium hydride and then over barium oxide. The catalyst, E t 3 O B F 4 , was also prepared in vacuum from the ether solu tion of epichlorohydrin and B F 3 · E t 2 0 , filtered and dissolved i n ethylene dichloride, and the necessary amount was charged into the polymerizing system. After polymerization for the desired time at room temperature, the polymer solution was poured into methanol. The molecular weight was determined by membrane osmometer. Table I shows some experimental results. The induction period decreases to several minutes as shown by Curve b in Figure 1, but it was not possible to eliminate it. The molecular weight reached a value as high as 160,000, still a little lower than the calculated value. Thus, some


22.

Cationic

YAMASHiTA

355

Copolymerization

termination or transfer reaction might be unavoidable i n this system if the polymerization were carried out for a longer time with lower catalyst concentration, and a true living polymer was hardly visualized i n this cationic system. The melting point of high molecular weight crystalline polymer of D O L reached 66.0 ° C . compared with 55 ° C . of low molecular weight polymer (9), and since the solubility i n benzene decreased, the viscosity measurement was carried out i n dimethylformamide. Table I.

Polymerization of DOL in Q>H4Cl;> at 3 0 ° C . with Et 3 OBF 4

[ M ] 0 = 10.3 moles/liter, [C] 0 = 2.47 Χ 10"3 mole/liter No.

min. Time,

1 2 3

95 135 240

Reduced Viscositya

Conversion,

%

1.22 2.45 2.63

47.5 76.2 85.5 b

—

161,000 e 165,000 e

* W C in D M F at 3 0 ° C , C = 0.3 gram/100 ml. h Equilibrium conversion calculated from [ M ] e = 1.5 moles/liter 85.5%. e Calculated value from Wn = t M 3 ~ t M l « _ 230,000 and 261,000, respectively. Block Styrene,

and

Random

and

Copolymer

3,3 -Bis chlor omet

of

1,3-Dioxolane,

hyloxetane

In a previous paper (10), we reported random copolymer of D O L - S t using B F 3 · E t 2 0 at 0 ° C . The monomer reactivity ratios were determined as fx ( D O L ) = 6.5 ± 0.85, r2 (St) = 0.65 ± 0.07 at 0 ° C . in toluene (11 ), and fi = 1.9 ± 0.2, r 2 = 0.35 d b 0.05 at 25 ° C . i n toluene (10), and rx == 3.8 ± 0.5, r 2 = 1.4 ± 0.3 at 0 ° C . i n nitrobenzene (11). Experiments i n a vacuum system were carried out using rigorously dried D O L , St, and ethylene dichloride with E t 3 O B F 4 . Although pure St dried over barium oxide showed little tendency to polymerize by this catalyst system, copolymerization of DOL—St occurred and gave a fairly high molecular weight copolymer if the St feed were kept low. Table II shows some experimental results. In some cases, where a fairly large amount of St monomer was added i n the initial feed, the molecular weight of the copolymer is considerably lower than the calcu lated value, indicating considerable chain breaking reactions during polymerization. However, from initially low St feed, the molecular weight of the copolymer becomes comparable with the calculated value. Presumably, this is caused by solvation of the growing St end by D O L


356

ADDITION

AND

CONDENSATION

POLYMERIZATION

to prevent chain scission by proton transfer. living nature of D O L - S t copolymer system.

PROCESSES

Thus, we can show the

Table III shows the increase of molecular weight of B C M O poly merization with conversion, although the polymer tends to precipitate. The monomer reactivity ratios of D O L - B C M O copolymerization were previously determined as r x ( D O L ) = 0.65 ± 0.05, r2 ( B C M O ) = 1.5 ± 0.1 at 0 ° C . by B F 3 · E t 2 0 (8). Table I V shows a preparation of block copolymer of D O L , St, and B C M O . In the first step we polymerized D O L and St; in the second step we added B C M O to this living system. The copolymer obtained showed an increase of molecular weight, and considerable B C M O was incorporated in the copolymer still remaining soluble i n ethylene dichloride. The solubility behavior together with the increase of molecular weight with addition of B C M O shows that this polymer consists of block sequences of D O L - S t and ( S t ) - D O L - B C M O . This we call block and random copolymer of D O L - S t - B C M O . W e can deny the presence of B C M O , St, or D O L homopolymers in this system, but some chain-breaking reactions are unavoidable, leading to copolymer mixtures. Thus, the principle of formation of block copolymers by cationic system is partly substantiated. Table II.

Copolymerization of D O L and St in C2H4O2 at 2 0 ° C . with Et 3 OBF 4

Concentration, No.

DOL

St

4 5 6 7

4.30 10.3 5.64 12.1

4.28 1.48 0.565 1.48

moles/liter / I

3.35 3.03 6.50 3.28

Χ 10" Χ 10"3 3

Χ 10"4 X 10^

Time, hrs.

... Yield, %

M

5 4 91 1

26.3 55.9 39.4 85.5

4.19 Χ 10 3 2.11 Χ 10 5 3.06 X 10r> 1.1 Χ 10 4

n

_ -, ^ Λ St Content, mole %

— 6.1 — 8.9

Experimental

Materials. Rectified D O L was dried over calcium hydride and dis tilled from a stock solution of colored sodium anthracene i n a vacuum system. B C M O , St, and ethylene dichloride were purified by the usual procedure and dried with calcium hydride and barium oxide. E t 3 O B F 4 was prepared i n a high vacuum system (10~6 mm. H g ) . Polymerization. A l l experiments were run under a high vacuum system. Monomer was added from a breakseal to the catalyst solution. The polymerization was stopped by pouring the reaction mixture into excess methanol containing phenyl ^-naphthylamine, and precipitated polymer was vacuum dried under room temperature. Characterization of the Copolymer. The copolymer composition was analyzed by elementary analysis and N M R . N M R spectra were run at 7 0 ° C . on a JNM-C-60 high resolution spectrometer at 60 M c . i n C C 1 4 .


22.

Cationic

YAMASHiTA

Table III.

Polymerization of BCMO in at 0 ° O with Et 3 OBF 4

Concentration, No.

moles/liter

BCMO

8-1 8-2 8-3 8- 4

357

Copolymerization

2.21 2.21 2.21 2.21

4.42 4.42 4.42 4.42

, Time, min.

_. Yield, %

Vsp/C*

25 30 40 250

15.7 17.4 23.4 36.1

0.385 0.405 0.449 0.475

m

I

Χ 10"3 Χ 10"3 Χ 10"3 Χ 10"3

C2H4CI2

"Measured in cyclohexanone at 6 0 ° C . (C = 2 grams/liter).

Table IV. Block Copolymerization of DOL, St, and BCMO in C 2 H4Cl 2 at 3 0 ° C . with Et 3 OBF 4 First Step: Copolymerization of D O L and St Concentration, No.

9- 1

DOL

9.82

moles/liter

St

I

1.96

1.05 Χ 10"

2

_. Time,

., Yield,

_ _ , St Content,

hrs.

%

M

2

24

4.35 Χ 10

%

n

4.0

3

Second Step: Addition of B C M O No.

BCMO,

Time,

Yield,

moles/liter

hrs.

%

M

72

35.8

5.8 Χ 10

9-2

1.91

Composition, n

3

mole %

DOL

St

BCMO

85.7

2.6

11.7

As previously described ( 9 ) , the St content is measured from the peak of the phenyl proton at 2.7--3.1 τ, the D O L content from the corrected peak area at 5.3 τ and 6.4 τ , and the B C M O content from the peak area of 6.5 r. The number average molecular weight was measured i n benzene solution b y Mechrolab membrane osmometer 502 and also b y Hitachi vapor pressure osmometer 115. Literature

Cited

(1) Aoki, S., Harita, Y., Otsu, T., Imoto, M., Bull. Chem. Soc. Japan 38, 1922 (1965). (2)

Ibid., p. 1928.

(3) Higashimura, T., Tanaka, Α., Miki, T., Okamura, S., J. Polymer Sci., A-1, 5, 1927 (1967). (4) Höhr, L., Cherdron, H., Kern, W., Makromol. Chem. 52, 59 (1962). (5) Dreyfuss, P., Dreyfuss, M. P., J. Polymer Sci., A-1, 4, 2179 (1966). (6) Ito, K., Umehara, K., Yamashita, Y., Kogyo Kagaku Zasshi 70, 2040 (1967). (7) O'Driscoll, K. F., J. Polymer Sci. 57, 721 (1962). (8) Okada, M., Takigawa, N., Iwatsuki, S., Yamashita, Y., Ishii, Y., Makromol. Chem. 82, 16 (1965).


358

ADDITION AND CONDENSATION POLYMERIZATION PROCESSES

(9) (10) (11) (12) (13)

Okada, M., Yamashita, Y., Ishii, Y., Makromol. Chem. 80, 196 (1964). Ibid., 94, 181 (1966). Okada, M., Yamashita, Y., Kogyo Kagaku Zasshi 69, 506 (1966). Saegusa, T., Imai, H., Furukawa, J., Makromol. Chem. 56, 55 (1962). Saegusa, T., Ueshima, T., Imai, H., Furukawa, J., Makromol. Chem. 79, 22 (1964). (14) Tada, K., Saegusa, T., Furukawa,J.,Makromol. Chem. 71, 71 (1964). (15) Tsuda, T., Shimizu, T., Yamashita, Y., Kogyo Kagaku Zasshi 67, 1661 (1964). (16) (17)

Ibid., p. 2145. Ibid., p. 2150.

(18) Tsuda, T., Yamashita, Y., Makromol.Chem. 86, 304 (1965). (19) Tsuda, T., Yamashita, Y., Kogyo Kagaku Zasshi 70, 553 (1967). (20) Yamashita, Y., Okada, M., Kasahara, H., Makromol. Chem. 117, 256 (1968). (21) Yamashita, Y., Okada, M., Kozawa, S., unpublished work. (22) Yamashita, Y., Okada, M., Suyama, K., Makromol. Chem. 111, 277 (1968). (23) Yamashita, Y., Okada, M., Suyama, K., Kasahara, H., Makromol. Chem. 114, 146 (1968). (24) Yamashita, Y., Tsuda, T., Okada, M., Iwatsuki, S., J. Polymer Sci. A-1, 4, 2121 (1966). (25) Yamashita, Y., Uchikawa, Α., Kogyo Kagaku Zasshi 71, 758 (1968). (26) Yamashita, Y., Umehara, K., Ito, K., Tsuda, T., Polymer Letters 4, 241 (1966). RECEIVED

March 7, 1968.


Addition and Condensation Polymerization Processes

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