Cationic Ring-Opening Polymerization of Epichlorohydrin in the

25 Cationic Ring-Opening Polymerization of Epichlorohydrin in the Presence of Ethylene Glycol YOSHIHISA OKAMOTO

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch025

BFGoodrich Research and Development Center, 9921 Brecksville Road, Brecksville, OH

44141

Cationic ring-opening polymerization of epichlorohydrin, using triethyloxonium hexafluorophosphate i n i t i a t o r i n the presence of ethylene glycol to produce a low molecu l a r weight polyepichlorohydrin g l y c o l , was examined. A new polymerization mechanism was postulated that: polymerization i n i t i a t e s at the hydroxyl groups i n the ethylene g l y c o l , and the polymer chain propagates simul taneously at both ends through the addition of the monomer. In this polymerization system, the polymer molecular weight increases d i r e c t l y with the polymer conversion, and it can be controlled by the molar r a t i o of epichlorohydrin and ethylene g l y c o l . The obtained polyepichlorohydrin glycols possess predominantly secon dary hydroxyl groups, narrow molecular weight d i s t r i b u t i o n , and a hydroxyl f u n c t i o n a l i t y of 2. Byproducts formed i n polyepichlorohydrin glycol are i d e n t i f i e d as non-functional c y c l i c oligomers, mainly epichlorohydrin cyclopentamer and cyclohexamer.

It i s well known that the low molecular weight polyether glycols such as poly(tetramethylene ether) glycol have been u t i l i z e d i n many polyurethane foams, rubbers, and castable elastomers. Recently halogen containing polyether glycols such as polyepichlorohydrin glycol attracted interest for flame resistance polyurethane applica t i o n . Among the several methods to prepare such polyether g l y c o l , a cationic ring-opening polymerization of epichlorohydrin (ECH) using water and ethylene glycol seems to be a promising metHod (1-2). Recently employing a similar method, poly(3,3-disubstituted t r i methylene ether) glycols have been prepared using b o r o n t r i f l u o r i d e i n i t i a t o r i n the presence of alkanediols for an application i n high energetic polyurethane binders for rocket propellants (3). In order to prepare polyether glycols possessing the desired molecular weight and hydroxyl f u n c t i o n a l i t y , research has been conducted to examine the role of using water and alkanediols i n the cationic ring-opening polymerization of oxetane and tetrahydrofuran (3-5). To extend the investigation of the preparation of low molecular weight polyether 0097-6156/85/ 0286-0361 $06.00/ 0 © 1985 American Chemical Society

In Ring-Opening Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

362

RING-OPENING POLYMERIZATION

glycols, a study on a cationic ring-opening polymerization of ECH using an oxonium type i n i t i a t o r i n the presence of alkanediols was conducted. Results and Discussion


The low molecular weight, 500—3000, polyepichlorohydrin glycol (PECHG) was prepared using triethyloxonium hexafluorophosphate (TEOP) i n i t i a t o r i n the presence of various amounts of molecular weight modifiers, mostly ethylene glycol (EG). Type of Hydroxyl Group i n Polyepichlorohydrin Glycol. First, in order to investigate the role of alkanediol i n the cationic ringopening polymerization, the type of the terminal hydroxyl groups i n PECHG was examined. As shown i n Figure 1, PECHG possesses predomi nantly secondary hydroxyl groups not only at 100% conversion, but also at as low as 40% conversion. The secondary hydroxyl group domination i s also observed i n cases using other glycols as molecular weight modifiers and using b o r o n t r i f l u o r i d e etherate i n i t i a t o r (see Table I ) .

Table I. E f f e c t of Molecular Weight Modifiers and I n i t i a t o r on the Terminal Hydroxyl Groups

Glycol and I n i t i a t o r EG, EG, 2-butene-l,4-diol, 2-butyne-l,4-diol, water,

Secondary OH Group, %

TEOP BF -0(C H ) TEOP TEOP TEOP 3

2

5

93-97 98-100 93.0 92.7 88.4-94.0

2

Next, small amounts of primary hydroxy groups i n PECHG were examined to determine whether they belong to starting glycol or not. As shown i n Table I I , chemical s h i f t s of the methylene protons adjacent to the primary hydroxy group are i d e n t i c a l among the various types of initiators. These results c l e a r l y indicate that a small amount of primary hydroxyl groups i n PECHG are not the primary hydroxyl groups belonging to the glycols used as molecular weight modifiers. Besides H NMR, C NMR also indicates that PECHG prepared using EG does not contain any EG units (H0-CH -CHg-0- @ 61.5 ppm) at the polymer chain terminal p o s i t i o n . X

1 3

3

Disappearance of Molecular Weight Modifier. Disappearance of the molecular weight modifier was monitored by measuring the polymer hydroxyl number at various polymerization stages. As shown i n Figure 2, disappearance of EG used as a molecular weight modifier was so rapid at the beginning of the polymerization that nearly a l l EG was incorporated i n the polymer by -50% polymer conversion. Rapid disappearance of molecular weight modifier was also confirmed by gas


Table I I .

363

Cationic Polymerization of EC Η

25. OKAMOTO

E f f e c t of Molecular Weight Modifiers on the Terminal Primary Hydroxyl Group

1

Molecular Weight Modifier

-CH -0H H NMR Chemical g

H0-CH -CH -0H HO-CH -CH=CH-CH -OH H0-CH -C=C-CH -0H 2

4.43 ppm 4.43 ppm 4.43 ppm

2

2

2

2

Shift*

2


* After derivatized with trichloroacetylisocyanate

chromatography and l i q u i d chromatography, which indicated that a l l of the molecular weight modifier was consumed by ~50% polymer conver sion. Polymer Molecular Weight. Polymer molecular weight was monitored by GPC which shows that the polymer possesses narrow molecular weight d i s t r i b u t i o n (Mw/Mn = 1.1~1.2) and GPC curves s h i f t to higher molecu l a r weight with conversion. When the polymer molecular weight was plotted against polymer conversion (Figure 3), the l i n e a r r e l a tionship which i s a c h a r a c t e r i s t i c of l i v i n g polymerization system was obtained. Thus polymer molecular weight i s readily adjusted by the molar ratio of ECH/molecular weight modifier, and i t s molecular weight i s t h e o r e t i c a l l y calculated by the following equation: Mn = (92.5 [ECH]/[MWM]) · conversion/100 + mol. wt. of MWM MWM:

molecular weight modifier

In a l i v i n g polymerization system, Beste reports that the polymeriza t i o n rate i s d i r e c t l y related to the concentration of i n i t i a t i n g species (6). As shown i n Figure 4, i t seems that polymerization rate of this ECH polymerization i s d i r e c t l y related to the concentration of i n i t i a t o r . No further study on the polymerization kinetics was conducted i n the present study. Non-Functional C y c l i c Oligomers. Formation of c y c l i c oligomers i s an inherent problem with a cationic ring-opening polymerization (7-11), and polymerization of ECH i n the presence of EG i s no exception. PECHG possessing molecular weight less than approximately 1000 contained no c y c l i c oligomers, while PECHG possessing molecular weight more than approximately 1000 showed bi-modal molecular weight d i s t r i b u t i o n and contained 5~20% of oligomers. These oligomers were isolated by preparative GPC and i d e n t i f i e d by field-desorption mass spectrometer as non-functional c y c l i c oligomers, mainly ECH cyclopentamer, Μ ·460, and cyclohexamer, Μ ·552 (see Figure 5). No hydroxylterminated l i n e a r oligomers, such as hydroxyl-terminated ECH pentamer and hexamer, were detected. +

+

Polymerization Temperatures. As shown i n Figure 6, polymer conver sion of 100% i s achieved at the 30°C polymerization temperature i n 8 hr. However, i t seems that the polymerization rate i s slower with the higher polymerization temperature, and polymerization at 70°C


364


lOOh 80h

60H 40

r


20r

20

Figure 1.

40 60 80 CONVERSION (%)

100

Types of hydroxyl groups i n polyepichlorohydrin glycol [ECH] = 11 mol/L, [EG] =1.1 mol/L, [TEOP] = 2xl0" mol/L 3

20

Figure 2.


Disappearance of molecular weight modifier [ECH] = 11 mol/L, [EG] =1.1 mol/L, [TEOP] = 2 x l 0 ~ [0: obtained by M. P. Dreyfuss (1)]

3

mol/L

1000-

800-

m

I

2j

/

600400-

3 UJ

ο

2000

Figure 3.

20


100

Relationship between molecular weight and conversion [ECH] = 11 mol/L, [EG] =1.1 mol/L, [TEOP] = 2 x l 0 ~ mol/L 3



25. OKAMOTO

365

Cationic Polymerization of ECH

0

1

2

3 4 5 TIME (Hr.)

6

7

8

Figure 4. E f f e c t of TEOP i n i t i a t o r concentration on conversion [ECH] = 11 mol/L, [EG] = 1.1 mol/L at 30°. φ = l.lxlO"" , φ = 2.2xl0~ , and Q = 3.7xl(T mol/L 3

3

3

100.01-

50.0h

340 83 ΤΤΤΊ

125 ι I ι ι ι ι ι ι ι ι ι

ioo.oh

ι ι ι ι }

462

I II ι ι ι ι ι ι ι ι ιι ι ι ι ι 200 M/E 250

I

I

293 332 I I'f ι ι ι ι ι ι Γ Υ Γ ι ι ι 300 350

CYCLO PENTAMER CYCLO HEXAMER

50.0

554 CYCLO TETRAMER I

ι

CYCLO HEPTAMER

a 417 424 4Z4 • 5/8 411 - 646 I IιI Γ"Γ-Γ»Π ι ι T*l I ' ιι ι·ι Iί f· Iι l ιI Ί ι ι ι ι I ιr»ι ιι ι ιι ιI Iι IΊ ι Iι Iι •'ι •I ι ^ IΊΗIτIτI II IIι ι I ι ι • ι ι ι • ι · ι 650 600 500 550 400 450 m

m

1

M

Figure 5.

/

E

Field-desorption mass spectrogram of polyepichlorohydrin oligomers.



366


TIME (Hr.)

Figure 6.

Effect of polymerization temperature on conversion. [ECH] = 11 mol/L, [EG] =1.1 mol/L, [TEOP] = 2 x l 0 " mol/L C = 0°, Ο = 30°, = 50°, φ = 70°, and 4 = [TEOA] = 2 x l 0 " mol/L at 70°.

φ

3

3

reaches the plateau of approximately 60% conversion. This slower polymerization rate with the higher polymerization temperature i s probably due to the decomposition of i n i t i a t i n g species. When triethyloxonium hexafluoroantimonate (TEOA), which i s reported to be more thermally stable than TEOP (12), i s used as an i n i t i a t o r , the polymerization reaches approximately 95% conversion i n 4 hr at 70°C. At the polymerization temperature of 0°C, the polymerization rate i s so slow that i t takes about 48 hr to reach 100% polymer conversion. Polymerization Mechanism Based on the results mentioned above, i t i s doubtful that the previ ously postulated polymerization mechanism applies ( 5 ) . In this mechanism alkanediols were postulated to behave l i k e chain transfer agents : HO

+

^-s^-w+o

H0-R-0H

^-CHgCl

HO

0-R-0H

This reaction does not seem consistent with our results which c l e a r l y indicate that the hydroxyl groups i n PECHG are not the hydroxyl groups belonging to the alkanediols. Considering a l l the experimental results, the following polymer i z a t i o n mechanism i s postulated for ECH cationic ring-opening poly merization i n the presence of alkanediols such as EG. I n i t i a t i o n : Since triethyloxonium s a l t i s a strong alkylating agent, i t probably alkylates ethylene glycol which i s a stronger base than ECH monomer (13-14). Then the proton i s abstracted by ECH to form i n i t i a t i n g species 2 for this cationic ring-opening polymerization.



25. OKAMOTO

367

+ + ( C H ) 0 - P F ~ + HOCH CH OH "* C H 0CH CH 0H + ( C H ) 0 2

5

3

6

2

2

2

5

2

2

2

5

2

I Η

1


!

^—rCH Cl

+

- H 0CH CH 0H

2

C2

5

2

2

V " / " ^ +0

+

1

I Η 2 Transfer: The i n i t i a t i n g species 2 i s then attacked by EG, instead of ECH, to produce 3

+ 2 + H0CH CH 0H -> H0-CH-CH -0-CH CH 0H 2

2

2

2

I

2

I

CH C1

Η

2

3 Termination & R e i n i t i a t i o n : The proton i n 3 i s abstracted by ECH to form new ECH-EG adduct g l y c o l , 4, and regenerate the propagating species 2. 3 +

\ /

C

H

2

C

H0-CH-CH -0-CH CH 0H + 2

1

2

2

2

CH C1 2

4 After a l l EG i s consumed, the regenerated propagating species 2 then reacts with the nascent glycol 4 by a manner i d e n t i c a l to that mentioned above to produce the ne\7 ECH-EG-ECH adduct g l y c o l , 5, and regenerated propagating species 2.

4 + 2

~

v

~

+ HO-CH-CH -0-CH CH -0-CH -CH-OH 2

2

2

2

I I

I CH C1

H

2

CH C1 2

-CH C1 2

HO-CH-CH -0-CH CH -0-CH -CH-OH 2

I

2

2

CH C1

2

+2

I

CH C1

2

2

5


368


Overall polymerization proceeds by repeating the "transfer", "termin ation" and " r e i n i t i a t i o n " steps schematically as shown below. HO-CH-CH -0-CH CH -0-CH -CH-OH + \ 2

2

2

y

2

CH C1

2

C 1

I H

CH C1

2

C H

2


0

H-fO-CH-CH *_^ 0-CH CH -0 4 0 Η - 0 Η - Ο ί ^ γ Η 2

2

2

2

CH C1

CH C1

2

2

+

\

/

C

I Η

According to t h i s new postulated mechanism, a l l results obtained are readily explainable. I t i s reasonable that a l l EG was consumed at the beginning of the polymerization stage, and no EG residue i s located at the terminal p o s i t i o n of the polymer chains. Small amounts of primary hydroxyl groups observed i n PECHG are probably due to opening of the oxirane CH-0 linkage i n the propaga ting species: ^/^0-CH -CH-CH Cl ' I predominantly OH 2

• — 7 CH C1 1 ^ pi 2 +0 2

2

v/^0-CH-CH 0H 2

I

CH C1 2

The oligomers are believed to be formed by so-called back-biting or t a i l - b i t i n g which i s supposed to produce hydroxyl-terminated l i n e a r oligomers as well as non-functional c y c l i c oligomers. Absence of any hydroxyl-terminated l i n e a r oliogomers i n PECHG can be explained as follows: since hydroxyl groups are polymer propagating sites, hydroxyl-terminated l i n e a r oligomers can p a r t i c i p a t e back into the polymerization as soon as they are formed, while non-functional c y c l i c oligomers cannot. Thus oligomers found i n PECHG are only non-functional c y c l i c oligomers. These c y c l i c oligomers can form only when polymer molecular weight exceeds approximately 1000. According to the newly postulated mechanism, PECHG possessing the molecular weight of approximately 1000 has two polyepichlorohydrin chains each possessing -500 molecu l a r weight at both ends of EG residue.


25. OKAMOTO

369


Hi0-CH-CH i-5-0-CH2-CH2-0-{CH2-CH-0r-5H

I

I

2

CH C1

CH C1

2

2


PECHG -1000 molecular weight

These two polymer chains are too short to form c y c l i c oligomers such as cyclopentamer and cyclohexamer. No c y c l i c oligomers were, there fore, observed i n PECHG possessing the molecular weight of -1000. C y c l i c oligomers formation starts taking place when molecular weight of PECHG exceeds -1000. The above r a t i o n a l i z a t i o n i s also supported by the other r e sults. For example, three branched polyepichlorohydrin t r i o l having molecular weight of 1500 does not form any c y c l i c oligomers.

H+O-CH-CHgl-r 0-CH -CH-CH -0 -ÎCH -CH-0f~ H

I

I

CH C1 2

2

2

2

I

I CH C1 0-fCH -CH-0f-_H 2

2

~

I

5

CH C1 2

polyepichlorohydrin t r i o l -1500 molecular weight

Polyepichlorohydrin Polyols By u t i l i z i n g the new findings mentioned above, PECHG and polyepi chlorohydrin t r i o l (PECHT), each with a molecular weight of approxi mately 1000, were prepared using ethylene g l y c o l and g l y c e r o l , respectively. The results are summarized i n Table I I I .

Table I I I . Polyepichlorohydrin Polyols

PECHG Molecular weight (Mn) Hydroxy equivalent weight Hydroxy f u n c t i o n a l i t y Hydroxy group, primary secondary Dispersity (Mw/Mn)

954 477 2.0 6.6% 93.4% 1.29

PECHT 1010 335 3.0 6.5% 93.5% 1.12


370


Attempts to prepare polyepichlorohydrin t e t r a o l , pentaol, and octaol using pentaerythritol, glucose, and sucrose, respectively were unsuccessful, probably due to the poor s o l u b i l i t y of s t a r t i n g polyols in ECH.


Conclusion A l l results mentioned above c l e a r l y support the new mechanism of cationic ring-opening polymerization of ECH i n the presence of EG used as a molecular weight modifier. Polymerization i n i t i a t e s at the hydroxyl groups i n EG and the polymer chain propagates simultaneously at both ends through the addition of the monomer. Since this poly merization possessed a c h a r a c t e r i s t i c of l i v i n g polymerization, i . e . polymer molecular weight increased d i r e c t l y with polymer conversion, the polymer molecular weight was readily controlled by adjusting the r a t i o of ECH to EG. The obtained molecular weight PECHG possessed predominantly secondary hydroxyl groups, narrow molecular weight d i s t r i b u t i o n , and a hydroxyl f u n c t i o n a l i t y of two. Experimental Reagents. Epichlorohydrin was dried over molecular sieves. Ethylene g l y c o l , reagent grade, was used as received. 2-Butene-l,4-diol and 2-butyne-l,4-diol were freshly d i s t i l l e d before used. TEOP (rap 140142°C) was reprecipitated from methylene chloride and ether, and borontrifluoride etherate was d i s t i l l e d before use. Polymerization. Polymerization was carried out i n a 250 mL 3-neck flask, equipped with a mechanical s t i r r e r , a thermometer, and a rubber septum for i n i t i a t o r introduction. ECH, 93.8 g, and EG, 6.2 g, were charged into a flask and the flask was purged with dry nitrogen. The i n i t i a t o r solution, 0.045 g of TEOP dissolved i n 5 mL of methylene chloride, was added to the above mixture incrementally (1 mL per every 5 min) by a hypodermic syringe while maintaining the temperature at 30°C. The polymerization was carried out at 30°C and the reaction was monitored by taking small amounts of sample during the polymerization. On completion of the polymerization, the reac t i o n was terminated with -300 μΐ of a mixture of 30% ammonium hydrox ide and isopropanol (1:4 by v o l ) , and then the polymer was dried on a rotary evaporator at 60°C i n vacuo. The obtained PECHG possessed a molecular weight of 950 determined by vapor pressure osmometry (THF solvent), hydroxyl number of 118, and molecular weight d i s t r i b u t i o n , Mw/Mn, of 1.23. Determination of Hydroxyl Groups. Type of hydroxyl groups i n PECHG were determined using a Bruker WH-200 superconducting NMR spectrome ter employing the trichloroacetylisocyanate d e r i v a t i z a t i o n method (15). Typical NMR spectrum was shown i n Figure 7. I d e n t i f i c a t i o n of Oligomers. The low molecular weight f r a c t i o n was separated by a preparative GPC and i d e n t i f i e d using a Finnigan MAT 311A f i e l d desorption mass spectrometer (FD-MS).


25.

OK A MOTO

371


Molecular Weight Determination. Molecular weights were determined using a Waters Model 200 gel permeation chromatograph equipped with a modified Waters R4 d i f f e r e n t i a l refractometer detector. The solvent was THF; the flow was 2.0 mm /min. The column, 25 cm χ 7.8 mm ID, consisted of 10 , 10 , 10 , 10 , 10 A° waters microstyragel. 3

6

5

4

3

2


Disappearance of Molecular Weight Modifier. Hydroxyl number was determined using a standard procedure (1_6) . Disappearance of molecu l a r weight modifier was calculated by dividing the polymer hydroxyl number by the t h e o r e t i c a l hydroxyl number. I t was assumed i n calcu l a t i n g the t h e o r e t i c a l hydroxyl number that a l l molecular weight modifier was incorporated into the polymer obtained.

0 II R-CH -0-CNHC0CI 2

|-— -{-O-CHg-CH-^ CH CI

3

2

^CH-O-CNHCOC^

SECONDARY PRIMARY

I

Figure 7.

BACKBONE

I

*H NMR spectrum of polyepichlorohydrin glycol derivatized with trichloroacetylisocyanate: CDC1 solvent. 3


372


Acknowledgments The author would l i k e to thank Dr. M. P. Dreyfuss who generously shared his expertise, Dr. D. Harmon and Mrs. B. Boose for GPC analy ses, Dr. R. Lattimer for FD-MS analyses, Mr. J . Westfahl for NMR analyses, and Mrs. R. Lord f o r carrying out the experiments. The author wishes to express his appreciation to the BFGoodrich Chemical Group for permission to publish this work.


Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13.

14.

15. 16.

Dreyfuss, P. U.S. Patent 3 850 856, 1974. Young, C. I.; Barker, L. L. U.K. Patent Application 2 021 606, 1979. Manser, G. E.; Guimont, J . ; Ross, D. L. Presented at the 1981 JANNAF Propulsion Meeting, New Orleans, Louisiana, May 27, 1981. Dickinson, L. A. J . Polymer S c i . 1962, 58, 857. Hammond, J . M.; Hooper, J . F.; Robertson, W. G. P. J . Polymer S c i . A-1 1971, 9, 265. Beste, L. F.; H a l l , Η. Κ., J r . J . Phy. Chem. 1964, 68, 269. Ito, K.; Usami, N.; Yamashita, Y. Polymer J . 1979, 11, 171. Dreyfuss, P.; Dreyfuss, M. P. Polymer J . 1975, 8, 81. Robinson, I. M.; Pruckmayr, G. Macromolecules 1979, 12, 1043. Goethals, E. J . "Advances i n Polymer Science", Springer-Verlag B e r l i n Heidelberg, New York, 1977; Vol. 23, pp 104-130. Penczek, S.; Kubisa, P.; Matyjaszewski, K. "Advances i n Polymer Science", Springer-Verlag B e r l i n Heidelberg, New York, 1980; Vol. 37. Jones, F. R.; Plesch, P. H. Chem. Commun. 1969, 1231 and J . Chem. Soc., Dalton 1979, 927. Meerwein, H. "Houben-Weyl Methoden der Organischen Chemie.", E. M i l l e r Ed., Stuttgart, George Thieme Verlog, 1965; Vol VI/3, pp 359. Penczek, S.; Kubisa, P.; Matyjaszewski, K. "Advance i n Polymer Science Cationic Ring-Opening Polymerization", Springer-Verlag B e r l i n Heidelberg, New York, 1980; Vol. 37, p 6. Groom, T.; Babiec, J . S., J r . ; Van Leuwen, B. G. J . of C e l l u l a r P l a s t i c s 1974, January/February, 43. Sorenson, W. R.; Campbell, T. W. "Preparative Method of Polymer Chemistry", 2nd Ed., Interscience Publisher, a d i v i s i o n of John Wiley & Sons, New York, London, Syndey, Toronto, 1968; p 155.

RECEIVED October 4, 1984


Cationic Ring-Opening Polymerization of Epichlorohydrin in the

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