Polyether Glycols from Tetrahydrofuran and Ethylene Oxide

15 Polyether Glycols from Tetrahydrofuran and Ethylene Oxide I. M . ROBINSON, E . PECHHOLD,

and G. PRUCKMAYR

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 2, 2017 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0172.ch015

Chemicals and Pigments Department, Ε. I. du Pont De Nemours & Co., Inc., Wilmington, D E 19898

Polyether glycols have found wide acceptance as soft segment components for polyurethanes. Representative examples of useful polyether glycols are obtained by ring-opening polymeri zation of propylene oxide to polypropylene ether glycol and tetrahydrofuran (THF) to polytetramethylene ether glycol (PTMEG). However, the copolymerization of ethylene oxide (EO) and THF produced copolyether glycols which were reported to give poly urethanes with inferior properties (1). It is now known that these early EO/THF copolymers contained macrocyclic oligomers. The formation of macrocyclics in the cationic polymerization of certain cyclic ethers has been known for some time (2). McKenna et al. (3) have reported on the macrocyclics from THF. Dale (4) has studied the formation of cyclic oligomers from EO, and Hammond (5) has identified some of the macrocyclics produced in the copolymerization of propylene oxide and THF. We have recently reported (6) on the formation of crown ethers from EO and THF using CF SO H as a catalyst. The present paper deals broadly with the copolymerization of ethylene oxide and tetrahydrofuran using cationic ring-opening polymerization catalysts. A comparison is made of EO/THF poly ether glycol with PTMEG and their respective polyurethanes. 3

3

Polymerization In t y p i c a l experiments, THF and EO were mixed i n the presence o f H 0 and polymerized a t temperatures from 0-60°C u s i n g B F 3 · O E t as a c a t a l y s t . Gas chromatograms o f samples showed both l i n e a r and m a c r o c y c l i c oligomers a t the e a r l y stages o f the c o p o l y m e r i z a t i o n . As the c o p o l y m e r i z a t i o n proceeded, the i n i t i a l l y formed l i n e a r low molecular weight g l y c o l s were consumed. M a c r o c y c l i c s were generated s t e a d i l y during the p o l y m e r i z a t i o n . P o l y m e r i z a t i o n ceased when e s s e n t i a l l y a l l the EO was used. Conversion and degrees o f p o l y m e r i z a t i o n were c a l c u l a t e d from 100 MH NMR s p e c t r a u s i n g te trame thy1s i l a n e as an i n t e r n a l reference. The composition o f the copolymer was v a r i e d by a d j u s t i n g the EO/THF r a t i o , and the molecular weight was c o n t r o l l e d by the amount o f H 0 present. G l y c o l s , such as ethylene, tetramethylene 2

2

2

2

0097-6156/81 /0172-0197$05.00/0 © 1981 American Chemical Society

Edwards et al.; Urethane Chemistry and Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

URETHANE

198

CHEMISTRY AND

APPLICATIONS

and hexamethylene were a l s o used f o r molecular weight c o n t r o l and to i n f l u e n c e end group s t r u c t u r e , A range of other c a t i o n i c i n i t i a t i o n s were e f f e c t i v e , such as a c i d forms of Nafion* p e r f l u o r o s u l f o n i c a c i d r e s i n and c e r t a i n c l a y s . f


Results and D i s c u s s i o n Gas chromatograms taken at the end o f EO:THF p o l y m e r i z a t i o n , producing p o l y e t h e r g l y c o l s with number average molecular weights above 100Q, show only the presence of m a c r o c y c l i c s , as discussed e a r l i e r C6). I d e n t i f i c a t i o n o f each of the macrocyclics was made by chemical i o n i z a t i o n mass spectroscopy. Figure 1 shows a t y p i c a l gas chromatogram f o r the EO/THF m a c r o c y c l i c e t h e r s . The c y c l i c cotetramers are the dominant s p e c i e s — a n event i n keeping with other c y c l i z a t i o n phenomena where tetramers are found i n highest c o n c e n t r a t i o n . Some of these c y c l i c s present the p o s s i b i l i t y f o r isomer forms, but none have been found to date. The 28-membered r i n g , EO:THF crown 1:5, i s shown as the l a r g e s t r i n g , although l a r g e r r i n g s are present, as apparent by GC peaks at longer r e t e n t i o n times. I t i s i n t e r e s t i n g that no homologs of EO or THF crowns are detected « with the exception of very small amounts o f dioxane. The r e l a t i v e c o n c e n t r a t i o n of EO: THF crowns formed i s u s u a l l y i n the range of 8-20% of the l i n e a r p o l y e t h e r g l y c o l . V a r i a b l e s i n f l u e n c i n g m a c r o c y c l i c c o n c e n t r a t i o n and composition are p o l y m e r i z a t i o n c o n d i t i o n s , monomer r a t i o , and c a t a l y s t . Figure 2 shows the EQ:THF crown ethers i d e n t i f i e d by chemical i o n i z a t i o n mass spectroscopy from a p o l y m e r i z a t i o n using N a f i o n p e r f l u o r o s u l f o n i c a c i d r e s i n as the c a t a l y s t . A l l of the c y c l i c ethers shown i n F i g u r e 1 were obtained but i n s i g n i f i c a n t l y d i f f e r e n t concentrations. Again, no homologous EO or THF r i n g s were detected. However the dominance o f species high i n the THF component, such as 1;3, 1:4 and 1:5, may be a r e s u l t of the e f f i c a c y of t h i s c a t a l y s t f o r THF propagation. The chemical i o n i z a t i o n mass s p e c t r a f o r two of these EO:THF crowns are shown i n F i g u r e s 3 and 4. The fragmentation p a t t e r n s i n d i c a t e down-sizing by e x t r u s i o n of THF and EO m o i e t i e s . In the example of the E0:THF 3:2 crown, the r e l a t i v e abundance of 1:1 and 2:1 species may p o i n t to d i r e c t formation from the 3:2 r i n g r a t h e r than stepwise fragmentation v i a 3:2—*2:2—>2:1—KL:1 and 3 : 2 — > 3 z l — > 2 : 1 . I n t e r e s t i n g l y , the 1:1 i o n i s seen as a dominant component i n the decay process although t h i s represents a normally s t r a i n e d 8-membered r i n g and one which i s not formed t o any s i g n i f i c a n t extent during the p o l y m e r i z a t i o n process. Formation of EO/THF m a c r o c y c l i c s probably occurs v i a t a i l b i t i n g and b a c k - b i t i n g mechanisms as o u t l i n e d i n Figure 5. The b a c k - b i t i n g r e a c t i o n route appears most l i k e l y f o r t h i s system. Since no THF homocyclic oligomers have been found i n t h i s copolymerization and are extremely l i m i t e d i n THF R

f


15.

ROBINSON E T A L .

Polyether Glycols

199

2:2


I Response

3:1

1:3

2:3 3:2

I

1:2

Retention Time (min)

Figure 1. Gas chromatograph for EO.THF crowns.The nomenclature, for exampl EO.THF crown 2:3, has been found convenient with the smaller monomer de nated as the leading number.

Crown Ether |E0:THF) 1:2 3:1 2:2 1:3 3:2 2:3 4:2 1:4 3:3 2:4 4:3 1:5

Molecular Weight (m/e-l) 188 204 232 260 276 304 320 332 348 376 392 404

GC area % 3 1 16 51 3 5 1 10 2 1 t 4

•small amounts of higher mol. wt. crown ethers

Figure 2. Crown ethers identified by chemical ionization mass spectroscopy EO/THF polymerization using Nafion perfluorosulfonic acid resin as catalyst.



200

U R E T H A N E CHEMISTRY A N D APPLICATIONS

2:2

Relative Abundance]

2:1

1:1 U:l 1:0

Ml

y 1 !» 100j JΓ ; 2:0

π , .111

Ύ ι" i

1-rr

200

Ί

Γ

m/e

Figure 3.

Chemical ionization mass spectrum for EO.THF crown 2:3 with frag mentation pattern to down-sized rings.

Relative Abundance

m/e

Figure 4.

Chemical ionization mass spectrum for EO.THF 3:2 crown with specu lative pathways for ion decay.



15.

ROBINSON E T A L .

Polyether Glycols

201

Macrocycles

Figure 5.

Mechanism for formation of EO/THF macrocyclics involving both back-biting and tail-biting routes.

Composition Viscosity 40 C PA.S

PTMEG

EOrTHF Copolymer

.26- 36

.15

Melting Range C

14-23

Polyether Glycols from Tetrahydrofuran and Ethylene Oxide

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