Radical Cyclopolymerization of Divinyl Formal: Polymerization

6 Radical Cyclopolymerization of Divinyl Formal: Polymerization Conditions and Polymer Structure

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MITSUO TSUKINO Kitakyushu Technical College, Department of Chemical Engineering, Kokura-minami, Kitakyushu 803, Japan TOYOKI KUNITAKE Kyushu University, Department of Organic Synthesis, Faculty of Engineering, Fukuoka 812, Japan

Radical polymerization of divinyl formal was conducted in benzene and the polymer structure examined byH- and C-NMR spectroscopy. The polymer contains the cis-dioxolane ring as the major structure, along with the trans-ring and the branched structure. The major structures are connected in the meso and racemic fashions. The content of the minor structures becomes negligible by lowering polymerization temperature and by increasing monomer concentration. The unsaturated unit and the six-membered unit are not formed under any condition. 1

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We have been conducting detailed structural studies on the cyclopolymers of several unconjugated dienes. Radical cyclopolymerization of divinyl ether had been known to give polymers with the unsaturated monocyclic unit and the bicyclic unit(j^, 2). Our initial C-NMR study indicated that the polymer was composed of the five-membered monocyclic unit with the pendent unsaturation and the bicyclic unit with the bicyclo[3,3.0]octane skeleton(3.) . According to the stereochemistry of the polymer established by more recent NMR examinât ion (4), the polymerization process was shown to be highly stereoselective. 13

or

—C ;r*CH2

0097-6156/82/0195-0073$06.00/0 © 1982 American Chemical Society Butler and Kresta; Cyclopolymerization and Polymers with Chain-Ring Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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POLYMERS

WITH CHAIN-RING

STRUCTURES

Similar studies were performed for the radical polymers of cis-propenyl vinyl ether and 2-methylpropenyl v i n y l ether, with the conclusion that the polymer structures were basically the same as that of poly(divinyl ether)(5).

cis-propenyl vinyl ether

2-methylpropenyl vinyl ether

Divinyl acetals are also known to give soluble cyclopolymers by radical polymerization. Although the polymer structure had been studied by a variety of spectroscopic and chemical methods (j6-9), the results were inconclusive and no information was obtained as to the stereochemistry. Our C-NMR methodology was thus applied to polymers of divinyl acetal and i t s derivatives (acetaldehyde divinyl acetal and acetone divinyl acetal)· Comparison of C-NMR spectra of the polymers with those of 1,3dioxolanes and 1,3-dioxanes which are model compounds of the cyclic units indicates that these polymers have essentially the same structure(10). The propagation process of these monomers i s shown i n Scheme I with divinyl formal as an example. The i n t r a molecular cyclization produces cis-closed five-membered cyclic radical I predominantly. The radical either reacts with monomer or abstracts hydrogen from the neighboring exocyclic methylene. The newly-formed radical propagates subsequently to produce branched unit III. On the other hand, NMR evidence indicates the absence of the uncyclized structure and the trans-closed rings IV and V. Furthermore, i t i s indicated that the meso and racemic modes exist i n equal amounts for the connection between the major structure I I . Following these previous results, we investigated i n this study the influence of the polymerization conditions(monomer concentration and polymerization temperature) on the structure of poly(divinyl formal). 13

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Experimental Materials. Divinyl formal was prepared as described before (10). Azobis(isobutyronitrile)(AIBN) was recrystallized from methanol. Benzene and methanol were purified by the conventional procedure. Polymerization. The polymerization was conducted i n benzene with AIBN i n i t i a t o r at 50 and 70 C. The i n i t i a t i o n was accelerated at 10 and 30°C by irradiation with a high-pressure Hg lamp. Required amounts of divinyl formal, benzene and AIBN were placed i n ampoules, subjected to the freeze-pump-thaw cycle, and the e

Butler and Kresta; Cyclopolymerization and Polymers with Chain-Ring Structures ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Cyclopoly'merization of Divinyl Formal


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POLYMERS WITH CHAIN-RING STRUCTURES

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ampoules s e a l e d . The polymers were p r e c i p i t a t e d by methanol, r e p r e c i p i t a t e d from benzene and methanol, and d r i e d . They were f r e e z e - d r i e d from benzene, when necessary. M i s c e l l a n e o u s . NMR s p e c t r a o f 1,3-dioxolanes were obtained with a V a r i a n A-60 spectrometer. H- and C-NMR s p e c t r a o f the polymers were obtained with a JEOL FX-100 spectrometer. The molecular weight was determined by g e l permeation chromatography (Toyo Soda HLC-802UR) u s i n g monodisperse p o l y s t y r e n e as r e f e r e n c e . 1

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Results and D i s c u s s i o n I t was p r e v i o u s l y confirmed that the six-membered r i n g and the unsaturated u n i t were not present i n the polymer(10). The five-membered r i n g u n i t may possess the s t r u c t u r e s shown i n F i g u r e 1. S t r u c t u r e A i s the c i s - c l o s e d r i n g i n the main c h a i n and Β i s the branched c i s - c l o s e d r i n g . C and D are the t r a n s c l o s e d r i n g s i n the main c h a i n and i n the s i d e c h a i n , r e s p e c t i v e ly. Ε and F are the c h a i n end r i n g s with the c i s and trans stereochemistries. F i g u r e 2 shows a *H-NMR spectrum of the polymer obtained at low monomer c o n c e n t r a t i o n and high p o l y m e r i z a t i o n temperature. The carbon chemical s h i f t s o f the t r a n s r i n g s are c l o s e t o those of s t r u c t u r e B, and the a c e t a l carbons o f s t r u c t u r e s A t o Ε i n F i g u r e 1 possess almost the same chemical s h i f t . From these data we t e n t a t i v e l y concluded i n our previous study that the trans r i n g u n i t s were absent(10). However, the stereochemical d i f f e r e n c e o f the r i n g i s c l e a r l y r e f l e c t e d i n the chemical s h i f t of the a c e t a l hydrogen i n the H-NMR spectrum o f improved r e s o l u t i o n ( F i g u r e 2 ) . The peak assignment i s performed on the b a s i s of the chemical s h i f t o f 1,3-dioxolanes: peaks a t c a . 1 ppm are a t t r i b u t e d t o the methyl proton of s t r u c t u r e s Β and D, and peaks at 1.5-2.0 ppm a t t r i b u t e d t o the e x o c y c l i c methylene proton o f s t r u c t u r e s A and C. The molecular weight o f t h i s polymer i s r e l a t i v e l y small(MW « 2800), and a methyl proton peak of t h e i n i t i a t o r fragment i s observable a t 1.4 ppm. The peaks o f the r i n g methine protons are l o c a t e d a t 3.3-4.3 ppm, and the peaks of the a c e t a l methylene proton are s e p a r a t e l y found a t 4.5-5.0 ppm, i n correspondence t o the s t r u c t u r e s o f F i g u r e 1. The t o t a l amount of the t r a n s r i n g u n i t ( C + D + F) i s 20 %, as estimated from the r e l a t i v e peak area o f the a c e t a l proton. F i g u r e 3 gives C-NMR s p e c t r a of the polymer prepared a t 70°C w i t h d i f f e r e n t monomer c o n c e n t r a t i o n s . These s p e c t r a are assigned as d e s c r i b e d b e f o r e ( l O ) by r e f e r r i n g t o C-NMR data o f 1,3-dioxolanes. In F i g u r e 3a, a peak a t 14 ppm suggests the presence of the methyl carbon o f s t r u c t u r e s Β and E. Peaks a t 25 ppm are a s c r i b e d t o the e x o c y c l i c methylene carbon of s t r u c tures A and E, and they are s p l i t due t o the two modes of r i n g connection(meso and racemic). The methine carbon peaks are l o c a t e d a t 72-82 ppm. The a c e t a l carbon peak of a l l the r i n g 1

13

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6.

Cyclopolymerization of Divinyl Formal

TSUKINO A N D KUNITAKE

-H C CH 2

v

r

-CH-

77

-H C CH 2

V

2

Β

"CH-

Figure 1.

-H2ÎXPH3

-KACH3

Conceivable five-membered ring units for poly(divinyl formal).

ABE(cis-anti)

//

A RF

(cis-syn)

3 2 ppm from TMS Figure 2.

*H NMR spectrum of polyfdivinyl formal). Table I, run 3:3 w/v % in d -DMSO at 150°C; accumulation, 32 scans. 8


POLYMERS WITH CHAIN-RING STRUCTURES

78

(W

(a) B,E C.E

80

60 ppm

40 from

20

TMS

13

Figure 3. C NMR spectra of poly (divinyl formal). Key: a, Table I, run 3, 28 wt % in C D at 75°C, accumulation, 2000 scans; and b, Table I, run 4, 28 wt % in C D at 75°C, accumulation, 1600 scans. 6

6

6

e

Table I. Radical Polymerization of Divinyl Formal Run

Monomer mol/1

1 2 3 4

2.5 2.5 1.0 5.0

AIBN mol/1 b

0.1 > 0.02 0.02 0.02

0

Temp C

Time hr

Conversion %

M η

10 70 70 70

8.0 1.5 6.0 0.75

8.6 8.1 8.6 8.6

5.500 8,100 2,800 20,000

e

a. i n benzene, b. irradiated with a high-pressure Hg lamp


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structures(A to F) i s at 93.6 ppm. Thus, the branched structure and the trans rings are included i n the polymer apart from the main structure(A), when the monomer concentration i s low. In contrast, the formation of the minor structures are suppressed by conducting polymerization at higher monomer concentrations, as shown by Figure 3b. The fraction of structure A i s estimated from the relative peak area of the acetal carbon (total structures) and the ring methine carbon at 77-78 ppm (structure A), by assuming that NOE's are the same for these peaks. At 70 C, the fraction of A increased from 47 to 78 % upon increase i n the monomer concentration from 1.0 to 5.0 mol/1. Figure 4 i s a H-NMR spectrum of the polymer obtained under the polymerization conditions of relatively low monomer concentration and low polymerization temperature. The formation of structures other than A i s again suppressed. The content of the trans rings(C + D + F) i s 5 % i n this case. Figure 5 compares C-NMR spectra of the polymers prepared at different temperatures. The main structure A becomes predominant at a low polymerization temperature as shown i n Figure 5a, but the content of the other structures, particularly the branched c i s ring B, increases at a higher temperature(Figure 5b). The fraction of structure A increased from 66 to 89 % by lowering the polymerization temperature from 70 to 10°C at a fixed monomer concentration of 2.5 mol/1. The mode of connection of the main structure i s constant (meso: racemic » 1:1), i n spite of the change i n the polymerization conditions. The equal amounts of the meso and racemic connections might be randomly distributed. It i s interesting, however, that the sequence of type c appears least s t e r i c a l l y demanding among the four types of the connections on the basis of the CPK molecular model. (a) . . . r r r r r r r r . . . e

1

13

(b)

· · . mmmmmimnm. · ·

(c) (d)

··.mmrrmmrr.·· • · .mrmrmrmr. · ·

In conclusion, the structure of poly(divinyl formal) was shown to vary with the polymerization conditions (Table I ) . It i s expected that highly stereoregular polymers are obtainable by selecting proper polymerization comditions. These results w i l l be described elsewhere, together with the kinetic analysis of the cyclopolymerization process.


POLYMERS WITH CHAIN-RING

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STRUCTURES

A A 1

J

1

1

I

I

L

5

4

3

2

1

0

ppm

Figure 4.

from

TMS

*H NMR spectra of polyfdivinyl formal) from Table I, run 1: 3 w/v in de-DMSO at 150°C; accumulation, 16 scans.


%

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Figure 5. C NMR spectra of polyfdivinyl formal). Key: a, Table I, run 1, 28 wt % in C D at 75°C, accumulation, 2500 scans; and b, Table I, run 2, 28 wt % in C D at 75°C; accumulation, 3000 scans. 6

e

e

6


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POLYMERS

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Literature Cited 1. Aso, C.; Ushio, S. Makromol Chem. 1967, 100, 100. 2. Guaita, M,; Camino, C.; Trossarelli, L. ibid. 1970, 131, 237. 3. Kunitake, T.; Tsukino, M. ibid. 1976, 177, 303. 4. Tsukino, M.; Kunitake, T. Macromolecules 1979, 12, 387. 5. Tsukino, M.; Kunitake, T. Polym. J. 1981,13,657. 6. Matsoyan, S. G. J. Polym. Sci. 1961, 52, 189. 7. Minoura, Y.; Mitoh, M. ibid. 1965, A-3, 2149. 8. Aso, C.; Kunitake, T.; Ando, S. J. Macromol. Sci. Chem. 1971, A5, 167. 9. Aso, C.; Kunitake, T.; Tsutsumi, F. Kogyo Kagaku Zasshi 1967, 70, 2043. 10. Tsukino, M.; Kunitake, T. Polym. J. 1979, 11, 437. RECEIVED March 8,

1982.


Radical Cyclopolymerization of Divinyl Formal: Polymerization

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