Carbon-13 NMR Studies on the Cationic Polymerization of Cyclic Ethers

12 Carbon-13 N M R Studies on the Cationic Polymerization of Cyclic Ethers G . P R U C K M A Y R and T . K. W U

Downloaded by UNIV LAVAL on May 4, 2016 | http://pubs.acs.org Publication Date: July 2, 1979 | doi: 10.1021/bk-1979-0103.ch012

Ε . I. du Pont de Nemours & Co., Inc., Experimental Station, Wilmington, DE 19898

The c a t i o n i c ring-opening p o l y m e r i z a t i o n o f cyclic ethers has been the subject of many recent i n v e s t i g a t i o n s (1,2,3,4). Nuclear magnetic resonance (NMR) methods, p a r t i c u l a r l y carbon-13 techniques, have been found most u s e f u l i n studying the mechanism of these polymerizations (5). In the present review we would like to r e p o r t some o f our recent work i n t h i s field. The first p a r t o f this r e p o r t will illustrate how C-NMR has been utilized i n the e l u c i d a t i o n o f the p o l y m e r i z a t i o n mechanisms o f cyclic ethers. In the second p a r t , q u a n t i t a t i v e a p p l i c a t i o n s o f -NMR f o r determinations o f thermodynamic and k i n e t i c constants will be discussed. The last s e c t i o n deals with p o s s i b l e a p p l i c a t i o n s o f q u a n t i t a t i v e C-NMR a n a l y s i s i n copolymerization o f cyclic ethers. 13

13C

13

EXPERIMENTAL Tetrahydrofuran (THF) and oxepane (OXP) were distilled from CaH p r i o r t o use. All other reagents and s o l v e n t s are commer cially a v a i l a b l e i n reagent grade p u r i t y and were used without further p u r i f i c a t i o n . The proton noise-decoupled C-NMR s p e c t r a were obtained on a Bruker WH-90 F o u r i e r transform spectrometer operating a t 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a V a r i a n XL-100. Tetramethylsilane (TMS) was used as i n t e r n a l r e f e r e n c e , andallchemical s h i f t s are reported downfield from TMS. Field-frequency stabilization was maintained by deu terium lock on e x t e r n a l o r i n t e r n a l perdeuterated nitromethane. Q u a n t i t a t i v e s p e c t r a l i n t e n s i t i e s were obtained by gated decoupling and a p u l s e delay o f 10 seconds. Accumulation o f 1000 p u l s e s with phase a l t e r n a t i n g pulse sequence was g e n e r a l l y used. For " r e l a t i v e " s p e c t r a l i n t e n s i t i e s no pulse delay was used, and accumulation o f 200 pulses was found t o give adequate s i g n a l - t o noise r a t i o s f o r q u a n t i t a t i v e data c o l l e c t i o n . A c a l i b r a t i o n curve was obtained from ^C-JJMR s p e c t r a o f a s e r i e s o f polytetramethylene ether (PTME)-THF/CH3NO2 s o l u t i o n s a t 2

13

0-8412-0505-l/79/47-103-237$08.50/0 ©

1979 A m e r i c a n C h e m i c a l Society

Pasika; Carbon-13 NMR in Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

238

CARBON-13 NMR IN POLYMER SCIENCE

d i f f e r e n t concentrations and temperatures. The PTME was obtained by p o l y m e r i z a t i o n o f THF with M e 3 0 + B F 4 ~ i n CH3NO2 (molar r a t i o THF:Me30BF4:CH3N02 = 1.36:0.1:1.07) under c o n d i t i o n s s i m i l a r to the subsequent k i n e t i c study. The r e a c t i o n mixture was quenched with MeONa/MeOH, and the polymer i s o l a t e d by removal of unreacted monomer and s o l v e n t under vacuum, and e x t r a c t i o n of the residue with ether. A f t e r i s o l a t i o n , the r e s u l t i n g dimethoxypolytetramethylene ether MeO—fc^CI^CI^C^Ô}--Me (Μη ~ 600) was used d i r e c t l y i n the c a l i b r a t i o n mixtures. RESULTS AND


I.

DISCUSSION

Polymerization o f C y c l i c Ethers

General Mechanism and Spectra. The c a t i o n i c ring-opening polymerization of c y c l i c ethers has long been known to i n v o l v e oxonium ions (6). For THF i t i s w e l l recognized that under c e r t a i n c o n d i t i o n s a l l the r e a c t i o n s are r e v e r s i b l e and t h a t l i m i t i n g conversions are reached at given temperatures. The p o l y m e r i z a t i o n of THF has t h e r e f o r e been f r e q u e n t l y c h a r a c t e r i z e d as a " l i v i n g " polymerization (7). In the i n i t i a l step of the polymerization, a c y c l i c oxonium ion i s formed by t r a n s f e r of an a l k y l group from the i n i t i a t o r to the c y c l i c ether. Propagation occurs by S N 2 attack of a monomer molecule at a r i n g α-methylene p o s i t i o n of the c y c l i c t e r t i a r y oxonium i o n , followed by opening of the oxonium r i n g and forma t i o n of a new c y c l i c oxonium i o n . The i n i t i a t o r may be a Lewis a c i d , an oxonium s a l t or pre cursor (8), or an e s t e r of a strong a c i d (9). The anion A" i n the formula scheme below may designate, e.g., t e t r a f l u o r o b o r a t e (BF^~) , f l u o r o s u l f o n a t e (FS03~) ' ^ ^ l u o r o m e t h y l s u l f o n a t e tr

(CF3SO3"),

etc.



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K i n e t i c study o f t h i s r e a c t i o n u s u a l l y r e q u i r e s sampling the polymerizing mixture and a n a l y z i n g f o r the concentrations o f the v a r i o u s r e a c t i o n species a t d i f f e r e n t p o l y m e r i z a t i o n times. Vofsi and Tobolsky i n 1965 reported the use o f r a d i o a c t i v e l y tagged i n i t i a t o r (10), while Saegusa and coworkers i n 1968 developed a "phenoxy end-capping" method i n which the oxonium i o n i s trapped with sodium phenoxide and the d e r i v e d phenyl ether a t the polymer chain end q u a n t i t a t i v e l y determined by UV spectrophotometry (11). We have been i n v e s t i g a t i n g s i m i l a r model p o l y m e r i z a t i o n s . In 1973 we reported the use o f 1 H - N M R spectroscopy f o r the i d e n t i f i c a t i o n o f the v a r i o u s species i n such a p o l y m e r i z a t i o n (12). We found t h i s method t o be extremely u s e f u l f o r k i n e t i c i n - s i t u study of polymerizations without d i s t u r b i n g the system. Subse quently we a p p l i e d 1 9 F - N M R t o f o l l o w the p o l y m e r i z a t i o n i n i t i a t e d with c a t a l y s t s c o n t a i n i n g f l u o r i n e atoms (13). A t the same time the s u p e r i o r r e s o l u t i o n o f 1 3 c - N M R was e x p l o i t e d t o i n v e s t i g a t e the v a r i o u s proposed e q u i l i b r i a i n the p o l y m e r i z a t i o n o f c y c l i c ethers (_5) . F i g u r e 1 shows the proton noise-decoupled 1 C-NMR spectrum of a p o l y t e t r a h y d r o f u r a n (polytetramethylene ether g l y c o l , PTMEG) d i s s o l v e d i n THF. In t h i s spectrum the carbons numbered 1, 2 and 3 which are α t o the oxygen appear a t lower f i e l d than the 3-carbons l a b e l e d as 4, 5 and 6. The carbon atoms i n the polymer are c l e a r l y r e s o l v e d from the corresponding carbons o f the THF monomer. The f a c t t h a t carbons 3 and 4 near the hydroxyl endgroups can be e a s i l y i d e n t i f i e d shows the e x c e l l e n t r e s o l u t i o n of t h i s technique. 3

Polymerization E q u i l i b r i a . As mentioned e a r l i e r , e s t e r s o f strong a c i d s , e.g. trifluoromethane s u l f o n i c a c i d ( " t r i f l a t e s " ) , are e x c e l l e n t i n i t i a t o r s f o r the p o l y m e r i z a t i o n o f THF. With such i n i t i a t o r s , however, a complication a r i s e s . In a d d i t i o n t o the normal propagation £ depropagation e q u i l i b r i a o f oxonium i o n s , Smith and Hubin p o s t u l a t e d t h a t the macroion (I) may a l s o convert i n t o a corresponding nonpolar macroester (Ε) by attack o f the anion (14).

+ θ

(I) θ o w

Η

3 3




240

Ο CD


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Subsequently, Penczek f u r t h e r expanded t h i s concept and concluded that the extent of macroion or macroester formation was dependent on the p o l a r i t y o f the p o l y m e r i z a t i o n medium (15). Initial e f f o r t s t o s u b s t a n t i a t e t h i s theory u s i n g proton NMR d i d not l e a d to unambiguous s p e c t r a l assignments (LI) . By r e c o g n i z i n g the l a r g e chemical s h i f t d i f f e r e n c e between f l u o r i n e i n an anion and i n a n e u t r a l species, Saegusa's group and we independently obtained F-NMR data to support t h i s mechanism (13,16) . However, the l ^ F technique i s l i m i t e d t o examination of f l u o r i n e - c o n t a i n i n g i n i t i a t o r s , and we decided to use l^C-NMR to shed f u r t h e r l i g h t i n t o the nature of t h i s problem. F i g u r e 2 shows the complete C s p e c t r a o f two THF/methylt r i f l a t e p o l y m e r i z a t i o n mixtures, one i n a nonpolar s o l v e n t ( C C I 4 , F i g u r e 2,B) and the other one i n a s t r o n g l y p o l a r s o l v e n t ( C H 3 N O 2 , F i g u r e 2 , A). The chemical s h i f t s of the a- and β-methylene carbon peaks o f the polymer and those of the monomer c l o s e l y correspond to those shown i n the spectrum o f p o l y t e t r a hydrofuran (Figure 1 ) . I t i s noteworthy that a t the low f i e l d side o f t h i s polymer peak two strong s i g n a l s appear a t 87.5 ppm i n CH3NO2 s o l u t i o n , while a s i n g l e resonance peak i s observed a t 7 9 . 1 ppm i n C C I 4 s o l u t i o n . We assigned the two l o w - f i e l d s i g n a l s i n CH3NO2 to the endo- and e x o - c y c l i c α-methylene carbons a t the oxonium i o n , r e s p e c t i v e l y , and the s i n g l e l o w - f i e l d s i g n a l i n C C 1 4 to the α-methylene carbon o f the corresponding m a c r o f l u o r o s u l f a t e (5). (Some of the s p e c t r a l assignments were confirmed by o f f resonance decoupling.) The C-NMR s p e c t r a t h e r e f o r e supported the macroion-macroester e q u i l i b r a t i o n proposed by Smith and Hubin (14), and Penczek (15). Further d e t a i l can be seen i n F i g u r e 3, which i s a h o r i z o n t a l expansion of the oxonium r e g i o n from about 85 ppm t o 95 ppm, i l l u s t r a t i n g the spectra of r e a c t i o n mixtures of m e t h y l t r i f 1 ate with 5-, 6-, and 7-membered r i n g compounds. The a-methylenecarbon peaks of the methyl oxonium ions of the 5- and 7-membered c y c l i c ethers are found a t the low f i e l d side of the s p e c t r a . A c o n s i s t e n t r i n g - s i z e e f f e c t i s evident r e s u l t i n g i n a downfield s h i f t of about 1.7 ppm per r i n g expansion by one CH u n i t . In compounds which undergo ring-opening p o l y m e r i z a t i o n the chemical s h i f t of the open chain or " e x o - c y c l i c " methylene carbons of the polymeric oxonium ions i s d i f f e r e n t from the chemical s h i f t of the r i n g , or " e n d o - c y c l i c " methylene carbons. Tetrahydropyran (THP), the s t r a i n l e s s 6-membered r i n g , forms a t e r t i a r y oxonium i o n , but does not subsequently r i n g open. Based on l ^ F - , and l^C-NMR r e s u l t s we have schemati c a l l y represented the e q u i l i b r i u m p o l y m e r i z a t i o n of THF with e s t e r s of trifluoromethane s u l f o n i c a c i d as shown i n Scheme I. I n i t i a t i o n occurs when the a l k y l (R) group of the e s t e r i s t r a n s f e r r e d to THF to form an oxonium i o n . In p o l a r media, the oxygen atom o f another THF molecule w i l l add to the α-methylene p o s i t i o n of the oxonium i o n l e a d i n g to ring-opening propagation. Because the charged species are s t a b i l i z e d i n p o l a r medium, the 19


1 3

13

2




242

JL

Figure 2. C-13 NMR spectra (22.63 MHz) of the polymerization mixture of THF-CH OS0 F (6:1): (A) 64% in CH N0 , after a polymerization time of 20 min. (S indicates the solvent peak); (B) 64% in CCl , after a polymerization time of 20 min. 3

S

2

2

h

90

70

50 PPM

30


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* Downloaded by UNIV LAVAL on May 4, 2016 | http://pubs.acs.org Publication Date: July 2, 1979 | doi: 10.1021/bk-1979-0103.ch012

M e

~°'0 «

Figure 3. C-13 NMR spectra (22.63 MHz) of the oxonium ion region: (A) THF-CH OS0 CF (6:1) in CH N0 (64%), after 15 min; (B) THP-CH OS0 CF (6:2) in CH N0 (67%), after 60 min; (C) OXP-CH OS0 CF (2:6) in CH NQ (67% ), after 30 min. 3

2

3

3

2

3

2

3

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3

3

2

2

3

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244


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C F J S O J Q ^ ^ T ^

1

1

1

1

1

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1

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R O C H

C H

2

C H

2

C H

2

O S 0

2

2

C F

3

R THF

If J

ROCH CH CH CH O 2

2

2

2

|

V

R ( C X : H

2

C H

2

C H

2

C H

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fo-1) T H F

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2

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POLAR M E D I U M

|

Rate o f Formation o fCharged Species Increased

R t O C ^ C ^ C H ^ H ^ ^ O S C ^ C F j

Ifggg P Q I A R

M E D I V M

|

Rate o f Formation o f Charged

|

Sfttcies

Decreased

I Reaction Scheme I.

Polymerization of THF

with esters of fluorosulfonic acids



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e q u i l i b r i u m i s s h i f t e d to f a v o r the macroion. Therefore, the polymerization proceeds l a r g e l y by f o l l o w i n g the steps from top to bottom shown on the l e f t s i d e of the o u t l i n e . In nonpolar media, on the other hand, the newly formed oxonium i o n w i l l e i t h e r q u i c k l y convert to the corresponding s o l u b l e e s t e r , or i t w i l l p r e c i p i t a t e , s i n c e monomeric or s h o r t - c h a i n o l i g o m e r i c oxonium s a l t s have low s o l u b i l i t y i n such media. The s o l u b l e e s t e r i s s t r u c t u r a l l y s i m i l a r to the i n i t i a t o r and may add another THF molecule. The r e s u l t i n g oxonium i o n w i l l again r e v e r t to the e s t e r or p r e c i p i t a t e . In f a c t , p r e c i p i t a t e s are g e n e r a l l y observed durina the e a r l y stages of p o l y m e r i z a t i o n i n media of low p o l a r i t y . They have been i s o l a t e d and charac t e r i z e d as monomeric or short chain o l i g o m e r i c oxonium s a l t s (Γ7). As the polymer chains increase i n length (at longer polymer i z a t i o n times o r very low i n i t i a t o r concentrations), they w i l l tend to s t a b i l i z e the i o n i c ends i n s o l u t i o n . Although the con c e n t r a t i o n of i o n i c species under these c o n d i t i o n s w i l l s t i l l be very low, both types of end-groups may p a r t i c i p a t e i n chain propa g a t i o n (18), s i n c e the propagation r a t e of oxonium ions was found to be much higher than t h a t of the corresponding macroester (19). Chain T r a n s f e r . Dreyfuss and Dreyfuss d i s c u s s e d the p o s s i b i l i t y of chain t r a n s f e r during c a t i o n i c p o l y m e r i z a t i o n o f c y c l i c ethers (20). T h i s can occur when the c y c l i c oxonium i o n i s attacked by an oxygen of a polymer molecule r a t h e r than by mono mer. The oxonium i o n formed i n t h i s case i s a branched s i t e , an open-chain t e r t i a r y oxonium i o n , which has been c a l l e d a "dormant" ion because of i t s lack of r i n g s t r a i n (21).

"DORMANT"



246


Dreyfuss and Dreyfuss reasoned that a s i m i l a r chain t r a n s f e r should a l s o occur with added small a c y c l i c e t h e r s . Indeed i n the presence o f d i e t h y l ether they found that the u l t i m a t e conversion of THF t o polymer was not a f f e c t e d but t h a t t h e i n t r i n s i c v i s c o s i t y o f the polymer decreased with time (20). I n v e s t i g a t i o n o f t h i s chain t r a n s f e r r e a c t i o n i s g r e a t l y f a c i l i t a t e d by using 13c-NMR. In F i g u r e 4 the low f i e l d r e g i o n o f a p o l y m e r i z a t i o n mixture o f THF/MeOSC^F/diethyl ether i s shown. We observe the α-methylene carbons o f the methyl t e t r a h y d r o furanium i o n , the α-carbons o f the two types o f propagating chain heads, the macroion and the macroester (17). The observation o f the α-methylene carbon resonances o f the a c y c l i c t e r t i a r y oxonium ion provides a d i r e c t proof o f chain t r a n s f e r r e a c t i o n i n THF polymerization. Formation o f C y c l i c Oligomers. Chain t r a n s f e r r e a c t i o n s occur by i n t e r m o l e c u l a r attack o f oxygen from another p o l y e t h e r chain on the α-methylene carbons o f the oxonium i o n . In an i n t r a molecular attack a d i s t a n t oxygen o f the growing polymer chain i t s e l f attacks the α-methylene p o s i t i o n o f i t s oxonium center. In t h i s case a m a c r o c y c l i c oxonium i o n i s formed. Subsequent exoc y c l i c attack by a monomer molecule w i l l y i e l d a m a c r o c y c l i c compound c o n t a i n i n g more than one monomer u n i t s (Scheme I I ) . We f i r s t confirmed the formation o f these macrocycles i n the p o l y m e r i z a t i o n o f THF by using coupled gas chromatography/mass spectrometry {22). M a c r o c y c l i c ethers c o n t a i n i n g up t o 8 THF u n i t s could be separated and i d e n t i f i e d by t h i s method (23) . The two predominant macrocyclic species found i n THF p o l y m e r i z a t i o n mixtures are a c y c l i c tetramer and a c y c l i c pentamer. In analogy to the "crown ether" nomenclature, we proposed the name 20-crown-4 for the c y c l i c tetramer and 25-crown-5 f o r the c y c l i c pentamer (22) .

Ο 20-CR0WN-4

25-CR0WN-5


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Ο

American Chemical Society Library 1155 16th St. N. w. Pasika; Carbon-13 NMR in Polymer Science ACS Symposium Series; American Chemical Washington, D. C. Society: 2003fiWashington, DC, 1979.



INTRAMOLECULAR ATTACK:

MACROCYCLIC OXONIUM ION

Reaction Scheme II.


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I d e n t i f i c a t i o n o f these macrocycles was a l s o f a c i l i t a t e d by examining t h e i r C-NMR s p e c t r a . Figure 5 shows a spectrum o f the GC f r a c t i o n o f 20-crown-4. Due t o the symmetry o f t h i s mole cule there are only two d i s t i n g u i s h a b l e carbons: They are those α and β t o the oxygen atoms a t 70.4 ppm and 26.5 ppm, r e s p e c t i v e l y . (The t r i p l e t a t 77 ppm i s due t o the s o l v e n t , CDCI3.) Comparison o f the chemical s h i f t data (Table 1) r e v e a l s t h a t the peak p o s i t i o n s o f α and β carbons o f 20-crown-4 are q u i t e d i f f e r e n t from the corresponding carbons o f THF o r the polymeric PTME. Small but d i s t i n c t chemical s h i f t d i f f e r e n c e s were a l s o found f o r macrocyclic oligomers o f other r i n g s i z e s . 13


II.

13

Q u a n t i t a t i v e A p p l i c a t i o n s o f C-NMR 1

3

c

In view o f the e x c e l l e n t r e s o l u t i o n o f s p e c t r a i t would be o f i n t e r e s t t o use these data f o r q u a n t i t a t i v e c o r r e l a t i o n s . However, q u a n t i t a t i v e a n a l y s i s by proton noise-decoupled F o u r i e r transform ^c-NMR i s complicated by the f a c t t h a t d i f f e r ent carbon n u c l e i may have d i f f e r e n t s p i n r e l a x a t i o n times and nuclear Overhauser enhancement (NOE) f a c t o r s . Therefore, the observed peak areas i n the s p e c t r a are not n e c e s s a r i l y propor t i o n a l t o the number o f carbon atoms i n v o l v e d . Schaefer and Natusch have shown t h a t f o r many s y n t h e t i c high polymers i n s o l u t i o n the NOE f a c t o r s and r e l a x a t i o n times o f carbon atoms i n o r near the main chains are s i m i l a r (24). In such cases the r e l a t i v e peak areas i n the spectra obtained by the noise-decoupled and f a s t p u l s i n g technique can be used as a good approximation f o r q u a n t i t a t i v e m i c r o s t r u c t u r e a n a l y s i s . However f o r our i n v e s t i g a t i o n o f the p o l y m e r i z a t i o n o f c y c l i c ethers we are f r e q u e n t l y i n t e r e s t e d i n the q u a n t i t a t i v e measurements o f monomers and oligomers as w e l l as the concentrations o f the con t i n u o u s l y growing polymeric s p e c i e s . Therefore, the assumption of Schaefer and Natusch i s r o t a p p l i c a b l e . The standard method o f o b t a i n i n g q u a n t i t a t i v e s p e c t r a i n v o l v e s the use o f gated decoupling and long pulse delay, both of which r e q u i r e very long data c o l l e c t i o n times. F i g u r e 6 d e p i c t s the p a r t i a l -*-C spectrum o f the α-carbon r e g i o n o f an e q u i l i b r a t e d p o l y m e r i z a t i o n mixture o f THF/Me30BF4 i n CD3NO2. Gated decoupling and a long pulse delay time o f 10 seconds were employed t o o b t a i n the spectrum. From the monomer and polymer peak areas, the extent o f p o l y m e r i z a t i o n a t e q u i l i b r i u m can be determined. Measurements o f chain end and the polymer peaks pro v i d e information on number-average degree o f p o l y m e r i z a t i o n . The data c o l l e c t i o n time r e q u i r e d t o o b t a i n t h i s spectrum was almost three hours. Since we were a l s o i n t e r e s t e d i n o b t a i n i n g q u a n t i t a t i v e k i n e t i c data f o r which the long data c o l l e c t i o n time technique cannot be used, we devised a second approach using " r e l a t i v e " peak i n t e n s i t i e s i n the s p e c t r a obtained by f a s t p u l s i n g . The two approaches are summarized as f o l l o w s : 3



250


60

Figure 5.

20

4 0

ppm

FROM

TMS

C-13 NMR spectrum of the cyclic THF tetramer 20-crown-4 in

PARTIAL C-NMR SPECTRUM OF THF/M 0BF IN CD N0 (2 5°C)

—1

.

90

4

3

2

1

1

80

70 ppm ( F R O M

Figure

6.

3

(|

13

e?

CDCl

l _ 60

TMS)—*·

Polymerization mixture THF-CD N0 -Me OBF\ 1:0.75:0.08) after equilibration 3

2

3

(mol ratios


=

PRUCKMAYR AND w u


Table I .

Cyclic

Ether s

C-13 N M R Chemi*

Shift of Tetramethylene Ethers

α-CARBONS

THF

jS- CARBONS

68.2

26.2

70.4

26.5

71.1

27.4

20-CR0WN-4

CH 0CH (CH ) CH 0CH . 2

2

2

2

2

2


252



Table II.

Quantitative C-13

NMR

Spectroscopy

A.

"Absolute" Collection Gated Pulse

S i g n a l I n t e n s i t i e s (Long Data Times) Decoupling to Suppress NOE Delay (>10 seconds)

B.

" R e l a t i v e " S i g n a l I n t e n s i t i e s (Short Data C o l l e c t i o n Times) I n t e r n a l Standard Assumptions : No Change i n Relaxation Time No Change i n NOE Viscosity Effects Negligible

In the f i r s t and obvious approach, "absolute" s i g n a l i n t e n s i t i e s are measured. Since v e r y long data c o l l e c t i o n times are r e q u i r e d , t h i s method i s only u s e f u l i n studying e q u i l i b r a t e d , i . e . nonchanging, sys terns. In the second approach, " r e l a t i v e " s i g n a l i n t e n s i t i e s are compared, and data c o l l e c t i o n times of the order of 5 t o 8 minutes per scan were found to be s u f f i c i e n t . In t h i s approach, an i n t e r n a l standard peak, such as a s o l v e n t peak (e.g. C H 3 N O 2 ) , i s used as the r e f e r e n c e and compared w i t h the peak i n t e n s i t y of a carbon o f i n t e r e s t , e.g. of monomer. The u n d e r l y i n g assumptions are t h a t the r e l a x a t i o n time and NOE r a t i o s of the i n t e r n a l s t a n dard and the carbon of i n t e r e s t remain unchanged during the course o f p o l y m e r i z a t i o n , and that v i s c o s i t y e f f e c t s are n e g l i g i b l e . Since we are d e a l i n g with r e l a t i v e l y low conversions and low molecular weight polymers i n s o l u t i o n , t h i s assumption i s not unreasonable. In order to v e r i f y the v a l i d i t y of these assumptions we p r e pared s e v e r a l c a l i b r a t i o n samples c o n t a i n i n g d i f f e r e n t r a t i o s of THF t o C H 3 N O 2 . D i f f e r e n t amounts o f polymer were added t o these samples t o simulate the v i s c o u s p r o p e r t i e s of the polymer^ i z a t i o n mixture. We found t h a t the peak i n t e n s i t y r a t i o o f THF to CH3NO2 obtained by the f a s t p u l s i n g technique can indeed be l i n e a r l y c o r r e l a t e d with the corresponding weight r a t i o s of these two compounds. Moreover, change of temperature from 0 to 35° introduced no a p p r e c i a b l e d e v i a t i o n s . The c a l i b r a t i o n curve i s shown i n F i g u r e 7. The composition range of i n t e r e s t i n our p o l y m e r i z a t i o n study i s i n d i c a t e d by the bracket. Thermodynamic Data. For an e q u i l i b r i u m p o l y m e r i z a t i o n , the e q u i l i b r i u m constant K i s equal to the r a t i o of r a t e of e



PRUCKMAYR AND w u

οι

Cyclic

τ

Ο WEIGHT

Figure 7.

RATIOS

Ether s

ι

1

'

1.0

2.0

3.0

T H F / C H

3

N 0

2

IN

SAMPLE

1

(THF/CH3N02/PTMEG)

C-13 NMR calibration curve: (A), no PTME; (B), 20% PTME; 30% PTME; (D), 40% PTME; (0), 0°C; (O), 35°C.


(C),

254


propagation, k , to t h a t of depropagation, k_p. This constant i s a l s o r e l a t e d to monomer and polymer concentrations by the law of mass a c t i o n . To a good approximation K i s equal to l / [ X ] e ' where £ M ] i s the monomer concentration at e q u i l i b r i u m . From the f r e e energy r e l a t i o n s h i p one may rearrange the terms and make appropriate s u b s t i t u t i o n to o b t a i n the expression as shown (Eq. 1 ) . Measurements of a t d i f f e r e n t p o l y m e r i z a t i o n temperatures should y i e l d the enthalpy and entropy of p o l y m e r i z a t i o n . p

e

e

e


• φ ^ 1

Kp =

AF

p

= -RT lnK

e

= ΔΗρ — TAS

r

Ί

-°»«.-]P

1

ΔΗ

0

p

AS

P

(1)

In Figure 8 the l o g of i s p l o t t e d against the r e c i p r o c a l of p o l y m e r i z a t i o n temperature. Three types of NMR data are shown. The f i l l e d c i r c l e s are from the f a s t p u l s i n g technique, the open c i r c l e from the gated and delayed p u l s i n g technique and the open squares from proton NMR. From the slope and i n t e r c e p t of the least-square f i t t e d l i n e the enthalpy and entropy of p o l y m e r i z a t i o n were obtained, r e s p e c t i v e l y . The thermodynamic constants of THF p o l y m e r i z a t i o n have been i n v e s t i g a t e d by a number of authors. A v a r i e t y of experimental techniques have been u t i l i z e d i n c l u d i n g determinations of conver s i o n to polymer, combustion, heat c a p a c i t i e s and vapor pressure. Comparison of our r e s u l t s with some p r e v i o u s l y p u b l i s h e d data shows t h a t our r e s u l t s are w i t h i n the range of the values reported (Table 3 ) . K i n e t i c Data. Let us now consider the k i n e t i c s of a r e v e r s i b l e c y c l i c ether p o l y m e r i z a t i o n . For such a p o l y m e r i z a t i o n i n progress, the k i n e t i c expression i s

- ψ

- M P * ] M -H.p[p*]

(2)



PRUCKMAYR AND w u

!ο I 3.1

Cyclic

J

J

255

Ethers

1

L

3.3

1

L_

3.5 l/T

x

10"

3.7

3

Figure 8. Determination of thermodynamic constants of THF polymerization (plot of Equation 1): (·), C-13 NMR (decoupled and fast pulsing); (O), C-13 NMR (gated and delayed pulsing); (\J), Ή NMR. AU = -2.2 kcal mol ;AS = —10.4 cal deg' mol' . P

1

1


1

P

256


where and JJ>^] are the molar concentrations o f the monomer and growing polymer r e s p e c t i v e l y , and kp and k_p are the r a t e constants d e f i n e d e a r l i e r . At e q u i l i b r i u m dfjyf]/dt = 0 and k [M] p

e

· k.

O)

p

By proper s u b s t i t u t i o n , Eq. ( 2 ) i s s i m p l i f i e d t o

-ΤΓ·

MP*]{[M]-M } e

(4)


I n t e g r a t i o n o f Eq. (4) leads t o

I f the instantaneous monomer concentrations t l ^ L ZI t 2 can be continuously monitored during p o l y m e r i z a t i o n , and [ p ^ a l s o known, k can then be c a l c u l a t e d with Eq. ( 5 ) . This approach was used by Saegusa and others t o study the p o l y m e r i z a t i o n o f THF M

a n c

p

(2J5,26) .

However, t h i s r e l a t i o n s h i p i s not a p p l i c a b l e when data from 1 3 s p e c t r a are used. s p e c t r a are obtained v i a F o u r i e r transform computation o f data accumulated over a d e f i n i t e time i n t e r v a l , and instantaneous c o n c e n t r a t i o n measurements are not possible. We have t h e r e f o r e modified the k i n e t i c expression t o handle the C-NMR data. For the s p e c i a l case where the c o n c e n t r a t i o n of a c t i v e chain ends Jp*] i s constant, the d e r i v e d k i n e t i c expres s i o n i s reduced t o a very simple form: C

13

ln([H] -[M] ) s -k [P*]t + CONSTANT t

r e

p

e

r e s

0C3t P ^ n t s the monomer c o n c e n t r a t i o n a t time t , as obtained from F o u r i e r transform C-NMR data. The r a t e constant of propagation kp can now be determined by measuring I j i j t f u n c t i o n o f p o l y m e r i z a t i o n time, t . (For the d e r i v a t i o n o f t h i s expression, see the Appendix.) In a k i n e t i c study, we c a r r i e d out a polymerization r e a c t i o n of THF i n CH3NO2 with ( 0 1 3 ) 3 6 ^ 4 " a t 40°C. t o e q u i l i b r i u m and then q u i c k l y c h i l l e d the r e a c t i o n mixture t o 0°C. t o f o l l o w f u r ther p o l y m e r i z a t i o n a t t h i s temperature. The k i n e t i c data obtained are shown i n Table 4. A C scan was obtained once every 8 minutes which was found t o be the optimal s p e c t r a l accumu l a t i o n time. The data o f Table 4 were now p l o t t e d i n terms o f l n f j T f J t LMZ1 e) versus p o l y m e r i z a t i o n time t , counting from the reference 13

a

1 3


s

a

12.

PRUCKMAYR AND w u

Table III.

Cyclic

Summary of Published Data of Enthalpy and Entropy of Polymerization of T H F

-AS

"ΔΗρ

257

Ethers

p

KCal /mol

Cal /deg/mol

METHOD

2.2

10.4

EQUILIBRIUM (

THIS

IVIN


-

4.3

17.0

3.3

10.7

et a l

(1958)

(CONV. to P O L ) 9.1

WORK

C-NMR)

EQUILIBRIUM

17.7

4.6

1 3

REFERENCE

COMBUSTION

CASS

(1958)

EQUILIBRIUM

SIMS

(1964)

( C O N V . to P O L ) EQUILIBRIUM SOLVENT

-

HEAT

14.8

Table I V .

GLEGG

CAPACITIES

VAPOR

etal

BUSFIELD

(1968) et a l

(1972)

PRESSURE

Polymerization of Tetrahydrofuran

etal

(1965)

EQUILIBRIUM

3.9

1.8

IVIN

WITH

CORRECTION

1

in Nitromethane at 0 ° C

t (min)

Bit"

ln([SO -M )

4

2.57

0.25

12

2.09

-0.21

20

1.99

-0.34

28

1.74

-0.77

42

1.50

-1.51

58

1.36

-2.53

EQUILIBRIUM

1.28

—

t

e

1

THF- 7.38 mol Γ , (CH ) 0BF = 0.59 mol Γ

2

INTEGRATED MONOMER CONCENTRATION IN RELATIVE UNITS

1

3 3

4

1


258


time t (Figure 9). These data were then least-square f i t t e d t o a s t r a i g h t l i n e . The slope o f t h i s l i n e i s equal t o the negative value o f product {p*] and kp. The propagation constant k o f THF p o l y m e r i z a t i o n i n CH3NO2 a t 0°C. was found t o be 1.5 χ 10~3l.mol-lsec~l. T h i s i s i n good agreement with the propagation constant o f a s i m i l a r p o l y m e r i z a t i o n mixture a t t h i s temperature c a l c u l a t e d from l^F-NMR data (18_) . 0

p


III.

A p p l i c a t i o n t o Copolymerizations

13c-NMR k i n e t i c a n a l y s i s would appear t o be most u s e f u l f o r studying p o l y m e r i z a t i o n systems which cannot be adequately charac t e r i z e d by proton o r f l u o r i n e NMR methods. Examples o f such systems are e.g. copolymerizations o f c y c l i c ethers, and i n the l a s t p a r t o f t h i s review we would l i k e t o d i s c u s s b r i e f l y some p r e l i m i n a r y r e s u l t s on THF copolymerizations. F i g u r e 10 presents a summary o f the α carbon chemical s h i f t s of oxonium ions and e s t e r s o f some o f the compounds d i s c u s s e d e a r l i e r . The carbon atoms α t o an oxonium center cover a range of about 25 ppm. The peaks due t o a l l the d i f f e r e n t oxonium ions and e s t e r s can be c l e a r l y d i s t i n g u i s h e d , and C-NMR t h e r e f o r e appeared to be an e x c e l l e n t technique f o r studying such copoly merizations . As an example o f a c y c l i c ether copolymerization, we w i l l b r i e f l y d i s c u s s the p o l y m e r i z a t i o n o f THF with OXP i n i t i a t e d with m e t h y l t r i f l a t e . The homopolymerizations o f both c y c l i c monomers f o l l o w a s i m i l a r mechanism, and both were found t o proceed v i a macrooxonium i o n and/or the macroester mechanism depending on the p o l a r i t y o f the p o l y m e r i z a t i o n medium. There should then be 8 p o s s i b l e end-groups, i . e . two types o f methoxy t a i l s having a penultimate THF or OXP u n i t , r e s p e c t i v e l y , two covalent macroe s t e r s , and four d i f f e r e n t oxonium i o n propagating chain heads: two from a THF oxonium center attached t o penultimate THF o r OXP u n i t s , and two from an OXP oxonium center attached t o THF and OXP penultimate u n i t s (Scheme I I I ) . F i g u r e 11 shows the α carbon resonance r e g i o n o f such a THF/ OXP copolymerization i n C H 3 N O 2 . A t about 55 ppm we observe the peak due t o the methoxy methyl carbons o f the chain ends, and f u r t h e r downfield a s o l v e n t peak and then the methylene carbons o f the unreacted monomers, THF and OXP. There are two peaks a t t r i b u t a b l e t o the polymeric methylene carbons. The higher f i e l d one i s due t o THF and the other one to OXP. S i m i l a r l y , two peaks are observed f o r the methylene carbons attached t o the methoxy chain ends. The f a c t t h a t the i n t e n s i t i e s o f these two peaks are s i m i l a r i n d i c a t e s t h a t both THF and OXP p a r t i c i p a t e i n the i n i t i a t i o n step. In the macroester r e g i o n , we have a small s i g n a l due t o OXPe s t e r only, no THF macroester was observed under these c o n d i t i o n s . On the other hand, i n the oxonium r e g i o n there are resonances due to the THF-macroions only but not t o OXP-macroions. Therefore, i n t h i s p a r t i c u l a r p o l y m e r i z a t i o n system we have i d e n t i f i e d 4 out 13


PRUCKMAYR AND w u


12.

Cyclic

0

Figure 9.

20

Ether s

40

t (min)

259

60

80

100

Determination of kinetic constants (plot of Equation 6). Polymeriza tion of THF in CH N0 at 0°C: k = 1.5 χ 10~ L mol sec' . 3

2

P

3

1

1



Figure 10.

C-13 NMR chemical shift assignments of some oxonium ions and fluorosulfonate esters


PRUCKMAYR

AND

wu

Cyclic

Ethers


12.


261



262


12.

PRUCKMAYR AND w u

Cyclic

263

Ethers 1 3


of the 8 p o s s i b l e chain ends. The r e s u l t s of a q u a n t i t a t i v e C a n a l y s i s of a s i m i l a r THF/OXP copolymerization system are sum marized i n Table 5 i n terms of t o t a l % conversion to polymer, k i n e t i c degree of copolymerization, copolymer composition and feed composition. The k i n e t i c degree of copolymerization o f 6.2 i s , w i t h i n experimental e r r o r , the same as the t h e o r e t i c a l l y c a l c u l a t e d value. Therefore, our r e s u l t s i n d i c a t e t h a t each i n i t i a t o r molecule i n i t i a t e s one copolymer chain as i n homopolym e r i z a t i o n s . Furthermore the copolymer composition was found to be very s i m i l a r t o feed composition. This suggests t h a t c a t i o n i c copolymerization of the two c y c l i c ethers may be s t a t i s t i c a l l y random.

Table V .

Copolymerization of T H F - O X P at

Moles of THF/OXP/CD N0 /CH S0 CF 3

T o t a l Conversion,

2

3

3

3

20°C

= 1.04/.75/1.56/.14

%

44

K i n e t i c Degree of Copolymerization

6.2

Copolymer Composition Mol % THF Mol % ΟΧΡ

57 43

Feed Composition Mol % THF Mol % ΟΧΡ

58 42

We are c u r r e n t l y extending t h i s approach to i n v e s t i g a t e the copolymerization of THF with other c y c l i c e t h e r s . CONCLUSION NMR methods are i d e a l l y s u i t e d to study p o l y m e r i z a t i o n r e a c t i o n s of c y c l i c ethers i n s i t u , without d i s t u r b i n g the p o l y merization e q u i l i b r i a . 13c-NMR methods g e n e r a l l y a f f o r d much more d e t a i l e d information than e i t h e r 1H- or f-NMR methods, but q u a n t i t a t i v e e v a l u a t i o n of data i s not as s t r a i g h t f o r w a r d , due to the n e c e s s i t y f o r F o u r i e r transform and noise decoupling t e c h niques. A 13c-NMR method based on r e l a t i v e s i g n a l i n t e n s i t i e s has now been developed f o r o b t a i n i n g q u a n t i t a t i v e information of homopolymerization and copolymerization systems, which may other wise not be e a s i l y a c c e s s i b l e . 19

ACKNOWLEDGMENT We would l i k e to thank Dr. W. W. Yau f o r h i s a s s i s t a n c e i n modifying the k i n e t i c theory, Dr. J . J . Chang f o r t e c h n i c a l a s s i s tance, and Dr. G. E. Heinsohn f o r a c r i t i c a l review of the manuscript.



264

APPENDIX K i n e t i c A n a l y s i s o f C y c l i c Ether P o l y m e r i z a t i o n by F o u r i e r Transform NMR

The k i n e t i c expression g e n e r a l l y a p p l i c a b l e to r e v e r s i b l e equilibrium polymerizations i s : Μ,,-Μ,

•

In [M]

]Z Zl t


M

s

m

o

n

o

m

e

L 2

t

r

2

[P*]dt ΓΡ*1

e

-*- ^ e c o n c e n t r a t i o n at time t , e ^ equi l i b r i u m monomer c o n c e n t r a t i o n , and £P^] i s the c o n c e n t r a t i o n of a c t i v e polymer chain s i t e s . A p p l i c a t i o n of t h i s formula r e q u i r e s instantaneous c o n c e n t r a t i o n measurements a t times t ^ and t2. T h i s r e l a t i o n s h i p i s not a p p l i c a b l e when the data are accumu l a t e d over a d e f i n i t e time i n t e r v a l , such as by m u l t i p l e p u l s i n g i n C-NMR. In order t o handle t h i s type of data, we have modi f i e d t h i s expression, i n t r o d u c i n g an a r b i t r a r y r e f e r e n c e time, t . £MJ and ]jtf]0 are the monomer concentrations a t times t and t . 13

Q

Q

P ( t ) i s used to designate the i n t e g r a l of [ p ^ J . Eq. (2) i s then converted i n t o an exponential form and i n t e g r a t e d through At, the time i n t e r v a l r e q u i r e d f o r s p e c t r a l accumulation.

ίτ-1

+

Ζ

Δ

[M]dr-[M] At

Ι

e

t-γΔΙ

*(N0-Me)f

2

t--

j

Rearrangement of terms of Eq.

wit-Mt

-kpP(T)

e

d

T

() 3

t-

(3) leads to

_jf

t +

*

A t

dr

(4)

The ] J M J represents the i n t e g r a l and i s the monomer c o n c e n t r a t i o n measured from the l ^ C spectrum a t time t . t


12.

PRUCKMAYR AND w u

Cyclic

265

Ether s


Since the i n t e g r a l o f the right-hand term o f Eq. (4) cannot be a n a l y t i c a l l y i n t e g r a t e d , we took the f o l l o w i n g procedure: Let the integrand o f t h a t i n t e g r a l be X and the i n t e g r a t e d X be Y. I f each Y i s expanded i n T a y l o r ' s s e r i e s , we o b t a i n two s e r i e s o f terms i n Eq. ( 5 ) .

XdT * γ ( ΐ + ^ - Δ ΐ ) -

γ(ΐ--|-Δΐ)

+

(jAt)Y'

+

V" +

-

(ΐΔΐ)γ'

+

V

J

2 r - Y "

Y" + . . . .

" ^ j j ^ - Y

-

+ ..-

= (At)Y' + ί ϋ 1 ' " + ^ l i . γ » « (AIM I , Δ

2

Υ

4

= 3/dt. For p o l y m e r i z a t i o n systems i n which the c o n c e n t r a t i o n o f a c t i v e chain s i t e s i s constant, the k i n e t i c expression d e r i v e d i n Eq. (11) can be f u r t h e r s i m p l i f i e d . Since d]jP*]/dt O, t h e second term o f Eq. (11) vanishes and the t h i r d term becomes a constant. By t a k i n g the constant Jp^j o u t s i d e the i n t e g r a l and i n t e g r a t i o n , the k i n e t i c expression i s reduced t o the simple form shown i n Eqs. (12) and (13) and i n the d i s c u s s i o n s e c t i o n . 0

Q

f

=

[ H

t

"Me

J

-

ln([H]T-[M]E)

l

[P*]dr

k [P*] ( t - t ] ) p

s -k [P*]t p

+ CONSTANT

T=t, +

(12)

CONSTANT

+ CONSTANT

Eq. (13) shows t h a t kp can be determined f u n c t i o n o f p o l y m e r i z a t i o n o f time, t .

by measuring J j Q t


(13)

a

s

a

268



L i t e r a t u r e Cited 1.

Saegusa, T., Kimura, Y., Fujii, H., Kobayashi, S., Macromolecules, (1973), 6, 657.

2.

Penczek, St., Matyjaszewski, K., J . Polym. S c i . , Symposium 56, (1976), 255.

3.

Hoene, R., Reichert, K. W., Makromol. Chem., (1976), 177, 3545.

4.

Pruckmayr, G., Wu, Τ. Κ., Macromolecules, (1978), 11, 662.

5.

Pruckmayr, G., Wu, Τ. Κ., Macromolecules, (1975), 8, 954.

6.

Meerwein, H., Delfs, D., Morschel, H., Angew. Chem., (1960), 24, 927.

7.

Dreyfuss, P., Dreyfuss, M. P., Advan. Chem. Ser., (1969), 91, 335.

8.

Dreyfuss, P., Dreyfuss, M. P., "Ring-Opening Polymerizations", K. C. F r i s c h , S. L. Reegen, eds., Marcel Dekker, N.Y., (1969).

9.

Saegusa, T., Kobayashi, S., Polyethers, ACS Symposium 6, (1975), 150.

10.

V o f s i , D., Tobolsky, Α. V., J. Polym. S c i . , (1965), A3, 3261.

11.

Kobayashi, S., Danda, H., Saegusa, T., B u l l . Chem. Soc. Japan, (1973), 46, 3214.

12.

Pruckmayr, G., Wu, T. K., Macromolecules, 6, 33.

(1973),

13.

Wu, T. K., Pruckmayr, G., Macromolecules, 8, 77.

(1975),

14.

Smith, S., Hubin, A. J . , J. Macromol. S c i . , Chem., (1973), 7, 1399.

15.

Matyjaszewski, K., Penczek, St., J. Polym. Sci., Chem., (1974), 12, 1905.

16.

Kobayashi, S., Danda, H., Saegusa, T., Macro molecules, (1974), 7, 415.



12.

PRUCKMAYR AND wu

Cyclic

Ether s

269

17.

Pruckmayr, G., Wu, T. K., ACS Central Regional Meeting, Akron, Ohio, (May 1976).

18.

Kobayashi, S., Morikawa, K., Saegusa, Macromolecules, (1975), 8, 386.

19.

Buyle, A. M., Matyjaszewski, Κ., Penczek, St., Macromolecules, (1977), 10, 269.

20.

Dreyfuss, M. P., Dreyfuss, P., J . Polym. S c i , (1966), (A-l) 4, 2179.

21.

Saegusa, T., Kobayashi, S., Progress i n Polymer S c i . (Japan), (1973), 6, 107.

22.

McKenna, J . M., Wu, T. K., Pruckmayr, G., Macromolecules, (1977), 10, 877.

23.

Pruckmayr, G., Wu, 11, 265.

24.

Schaefer, J . , Natusch, D. F. S., (1972), 5, 416.

25.

Saegusa, T., Kobayashi, S., J . Polym. S c i . , Polymer Symposia, (1976), 56, 241.

26.

Dreyfuss, P., Dreyfuss, M. P., "Comprehensive Chemical K i n e t i c s " , C. H. Bamford, C. H. Tipper, eds., 259, E l s e v i e r , 15, (1976).

T.,

T. K., Macromolecules,

(1978),

Macromolecules,

Discussion W. P a s i k a , L a u r e n t i a n Univ., Ont.: I would l i k e to r e f e r back t o the f a c t t h a t the compositional r a t i o i s the same as the feed r a t i o . You i n d i c a t e d i t was a random process. I f the mecha nism i s an S 2 type mechanism then the compositional r a t i o w i l l depend very much on the character o f the monomer. The c h a r a c t e r of the copolymerization could be other than random. In such a system the compositional r a t i o w i l l not n e c e s s a r i l y be the same as the feed r a t i o . T. K. Wu: True. T h i s i s a p r e l i m i n a r y r e p o r t , and we d i s cussed only one data p o i n t from one p a r t i c u l a r feed r a t i o . The r e s u l t may be f o r t u i t o u s . W. P a s i k a : On the other hand i t may w e l l be t h a t these p a r t i c u l a r two monomers have i n f a c t the same r e a c t i v i t y when i t comes to the p a r t i c u l a r type o f mechanism. D. J . Worsfold, NRC, Ont.: I was g l a d to see t h a t you were able to i d e n t i f y and measure the amount o f dormant polymer present i n the THF p o l y m e r i z a t i o n . The amount o f the dormant c h a i n end N



270


would increase during the course o f the p o l y m e r i z a t i o n as the amount o f polymer produced i n c r e a s e s and one would think t h i s would have some e f f e c t on the f i r s t order k i n e t i c s o f the r e a c t i o n . Yet i t has always been s u c c e s s f u l t o use f i r s t order k i n e t i c s t o d e s c r i b e the disappearance o f the monomer. I s i t because the amount of dormant chain end i s very small or i s i t t h a t there i s some compensatory e f f e c t which s t i l l gives the f i r s t order k i n e t i c s i n monomer disappearance? Τ. Κ. Wu: We t r i e d t o determine the amount of dormant species i n homopolymeriζations. We can measure the r a t i o o f " e x o c y c l i c " v s . " e n d o c y c l i c " methylene carbon resonance, and i f the r a t i o i s not e x a c t l y 1 t o 2 we can get an estimate o f the concentration o f dormant i o n . G. Pruckmayr, Du Pont, Delaware: The c o n c e n t r a t i o n o f dor mant i o n i s low. We can estimate the concentration of macroc y c l i c p l u s l i n e a r dormant ions from the i n t e n s i t y r a t i o o f the d i f f e r e n t α-methylene groups. The t o t a l i s q u i t e small i n homop o l y m e r i z a t i o n s , p a r t i c u l a r l y a t short r e a c t i o n times. P. Sipos, Du Pont, Ont.: In connection with your copolymer i z a t i o n have you seen a d i f f e r e n c e i n the composition o f the polymer and the feed r a t i o when l a r g e r oxy r i n g s are used? T. K. Wu: The l a r g e s t r i n g used so f a r was oxepane, but we are planning t o go i n the other d i r e c t i o n , using four membered r i n g s . However, i n t h i s case complications a r i s e because the s t r a i n e d oxetane r i n g does not undergo e q u i l i b r i u m p o l y m e r i z a t i o n . RECEIVED M a r c h 13, 1979.


Carbon-13 NMR Studies on the Cationic Polymerization of Cyclic Ethers

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