Progress in Polymerization of Cyclic Acetals - ACS Publications

the structure of the end-groups in poly-Diox is descri- bed, and the mechanistic consequences of the alleged macrocyclic or linear structures of the i...
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5 Progress i n Polymerization of C y c l i c Acetals

STANISŁAW PENCZEK and PRZEMYSŁAW KUBISA Polish Academy of Science, 90-362-Łodz, Poland

In our previous review paper, presented at the Rouen Symposium on Cationic Polymerization, we stressed some of the major differences between the c a t i o n i c po­ lymerization of c y c l i c ethers and c y c l i c acetals |1|. These differences are mainly caused by the much larger b a s i c i t y ( n u c l e o p h i l i c i t y ) of c y c l i c ethers, than that of c y c l i c a c e t a l s ; moreover, c y c l i c ethers are more basic (nucleophilic) than t h e i r polymers, whilst poly­ acetals seem to be more basic than t h e i r corresponding monomers. Thus, i n polymerization of c y c l i c ethers (or,at least i n polymerization of T H F ) , t e r t i a r y oxonium ions 1. are the only growing species |2| | 3 | |4|:

whereas i n the polymerization of c y c l i c a c e t a l s , i n c l u ­ ding 1,3-dioxolan (Diox), the equilibrium between the macroalkoxycarbenium ions 2 with t h e i r t e r t i a r y oxo ­ nium 2 counterparts i s i n our opinion the best represen­ of the active snecies : t a t i o n

Unfortunately, our knowledge of the carbenium-oxo­ nium ion e q u i l i b r i a i s very l i m i t e d ; some f i r s t 60

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q u a n t i t a t i v e d a t a from our l a b o r a t o r y |5j were d i s c u s s e d i n the Rouen paper |1|, P r o p e r t i e s o f 2""and 3^ ^ e q u i l i b r i u m (1) may depend v e r y much on tïïe p o l y m e r i z a t i o n c o n d i t i o n s and s t r u c t u r e o f the c y c l i c a c e t a l . At the s u f f i c i e n t l y l a r ge excess o f a p o l y a c e t a l , £ can become the predominant structure. The o t h e r t o p i c , which w i l l be c o v e r e d i n t h i s pap e r , was p r e v i o u s l y r e v i e w e d by P l e s c h a t the IUPAC Symposium i n Budapest |6|, and more r e c e n t l y a t the I - s t IUPAC Symposium on"~Ring-Opening P o l y m e r i z a t i o n h e l d i n J a b l o n n a (1975) | 7 j . In t h i s p a r t o f our paper the s t r u c t u r e o f the end-groups i n p o l y - D i o x i s d e s c r i bed, and the m e c h a n i s t i c consequences o f the a l l e g e d m a c r o c y c l i c or l i n e a r s t r u c t u r e s o f the i s o l a t e d dead macromolecules i s d i s c u s s e d . In 1975 Rosenberg, Irzhakh and E n i k o l o p i a n p u b l i s hed a book e n t i t l e d " I n t e r c h a i n exchange i n p o l y m e r s " I 8 j , summarizing r e s u l t s o f the Moscow group on the p o l y m e r i z a t i o n o f c y c l i c a c e t a l s . A l t h o u g h some o f the conc l u s i o n s o f t h i s book would c e r t a i n l y be p r e s e n t e d today d i f f e r e n t l y i n l i g h t of the new e x p e r i m e n t a l data, the r e a d e r may f i n d t h e r e an unorthodox s o l u t i o n o f the m a j o r i t y o f k i n e t i c problems p e r t i n e n t to the n o n s t a t i o n a r y polymerizations, i n c l u d i n g polymerization of cyclic acetals. We s h a l l s t a r t , however, t h i s r e v i e w o f the p r o g r e s s 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 a c e t a l s from a b r i e f d e s c r i p t i o n o f the new p o l y a c e t a l s p r e p a r e d , and from summarizing o f the new d a t a on the thermodynamics o f polymerization of substituted 1,3-dioxolans. n

Thermodynamics o f P o l y m e r i z a t i o n .

New

Polyacetals.

I v i n and Leonard |9| extended the thermodynamic t r e a t m e n t o f the polymer-monomer e q u i l i b r i u m to the non i d e a l systems, a c c o u n t i n g f o r the polymer-monomer i n t e r a c t i o n d e s c r i b e d by the F l o r y parameter x p « b u l k p r o c e s s , the f o l l o w i n g e x p r e s s i o n was o b t a i n e d f o r the f r e e energy change upon the c o n v e r s i o n o f one mole o f pure monomer i n t o one base-mole o f amorphous polymer ( A G ) : F

o

r

a

m

l c

A

G

il ^c = RT [ i n mΦ • lm+pX ^pU -Φ™)] (2) m/ where φ ( = 1 - Φ ) i s the e q u i l i b r i u m monomer volume f r a c ­ tion, ^computed from the e x p e r i m e n t a l l y d e t e r m i ­ ned e q u i l i b r i u m monomer c o n c e n t r a t i o n a t the g i v e n tem­ p e r a t u r e T. In t h i s method the F l o r y parameter χ is a r b i t r a r l y chosen (e.g. 0.4 f o r D i o x - p o l y - D i o x i n t e r a c t i o n ) and assumed to be independent on temperatu­ r e . L i n e a r i t y o f the p l o t o f AG /RT as a f u n c t i o n o f 1

T

A

V Y

Y

J

Ό

p

1

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1/T i n d i c a t e s t h e r e l i a b i l i t y o f these s i m p l i f i c a t i o n s . More r e c e n t l y i t has been o b s e r v e d | K)|, t h a t i n t r o d u c ­ t i o n o f a term i n c l u d i n g t h e monomer-solvent and p o l y ­ m e r - s o l v e n t i n t e r a c t i o n s a l l o w s t h e polymer-monomer e q u i l i b r i a i n s o l u t i o n t o be u n i f o r m l y t r e a t e d . T h i s term i s r e l a t e d t o t h e heat o f m i x i n g o f t h e s o l v e n t used w i t h monomer and polymer. I t does not depend, however, v e r y much on t h e s o l v e n t s t r u c t u r e f o r Diox, and. t h e r e f o r e , I D i o x I i s almost s o l v e n t independent \l±\ (although, only C H C 1 , C H r C l and C,H, were s t u d i e d ) . I t i s worth * noting, t h a t i n c o n t r a s t t o Diox, d i f f e r e n c e s between t h e e q u i ­ l i b r i u m monomer c o n c e n t r a t i o n s a r e much more pronounced i n the p o l y m e r i z a t i o n o f THF |1j2J . Indeed, |THFl =5.5 mole»l"' i n CH-,N0 s o l v e n t and o n l y 3.5 mole»l-1 i n CC1 s o l v e n t (fTHF| =7.0 m o l e - l " , 2 5 ° ) . These l a r g e d i f f e r e n c e s r e f l e c t t h e h i g h e r b a s i c i ­ t y o f THF and much s t r o n g e r a c i d - b a s e i n t e r a c t i o n s between s o l v e n t s and THF, than s o l v e n t s and D i o x . The thermodynamic n o n - i d e a l i t y o f these systems are s t r e s s e d , because some a u t h o r s a r e s t i l l t e n d i n g to determine what they a r e c a l l i n g t h e thermodynamic q u a n t i t i e s ( l i k e ΔΗ° and AS*?) on t h e b a s i s o f s i m p l e r r e l a t i o n s h i p s , h o l d i n g o n l y * r o r t h e i d e a l systems. On t h e o t h e r hand, i t has t o be remembered, t h a t i n the p o l y m e r i z a t i o n k i n e t i c s , the proper value o f |monomerl has t o be used, and t h a t i t changes w i t h b o t h jmonomer I and s o l v e n t s t r u c t u r e . The combined r e s u l t s of bulk |l_3| and s o l u t i o n p o l y m e r i z a t i o n o f Diox (taken l a r g e l y from R e f e r e n c e |1J_|) a l l o w e d Leonard t o c a l c u l a t e ΔΗ, = -4.0±0.1 k c a l - m o l e " and A S =-11 ,0±0.3 cal*mole"1·deg~1, These r e s u l t s agree w e l l with v a l u e s o b t a i n e d from an e q u i l i b r i u m between gaseous monomer and amorphous polymer |14|. F o l l o w i n g t h e Ivin-Leonara "? method, Okada d e t e r m i ­ ned r e c e n t l y the thermodynamic f u n c t i o n s f o r the p o l y ­ m e r i z a t i o n o f 4-methyl-Diox |15| and, (assuming X . 0.3) found ΔΗ, =-3.2±0,2 k c a l / m o l e and AS° = - 1 2 . 7 ± 0 . 8 cal*mole~'»deg-1 , Another work, performed i n C H C l 2 s o l v e n t f o r t h e some monomer, and not a c c o u n ­ ting f o r t h e d i s c u s s e d above i n t e r a c t i o n s , g a v e t h e apparent v a l u e s ( ΔΗ P P and AS P P ) d e p e n d i n g , as i t c o u l d be e x p e c t e d , on the s t a r t i n g monomer c o n c e n t r a ­ t i o n 116 I , 9

z

9

z

9

z

0

0

9

1

4

e

o

1

X

1

mr

=

m p

9

a

of

Theoretical Dioxolans.

a

I n t e r p r e t a t i o n o f the P o l y m e r i z a b i l i t y

T h e o r e t i c a l i n t e r p r e t a t i o n o f the r i n g - c h a i n e q u i ­ l i b r i a , p u b l i s h e d by Jacobson and Stockmayer i n 1950 117 I c a n o n l y be a p p l i e d t o t h e case when c h a i n s o r

9

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r i n g s a r e so l a r g e t h a t the c o n f i g u r a t i o n a l e n t r o p y i s governed by the G a u s s i a n f u n c t i o n and the energy d i f f e ­ rence between the c h a i n and r i n g forms i s n e g l i g i b l e . F o r a s m a l l r i n g , such as the five-membered r i n g o f Diox, Jacobson-Stockmayer e q u a t i o n cannot be a p p l i e d . In such c a s e , as shown i n many works, summarized r e c e n ­ t l y by H a l l f o r v a r i o u s c y c l i c monomers I l 8 j l l ? . | > s t a b i l i t y o f the r i n g i s c o n n e c t e d o n l y w i t h the s t r a i n i n the r i n g , caused m o s t l y by the d e v i a t i o n i n v a l e n c y a n g l e s . H a l l e x p l i c i t l y showed t h a t the d i f f e r e n c e i n s t r a i n energy between monomer and polymer e q u a l s the e n t h a l p y o f p o l y m e r i z a t i o n , p r o v i d e d t h a t no s u b s t i tuents are p r e s e n t , or e l s e c o n f o r m a t i o n a l s t r a i n s i n the polymer may outweigh the s t r a i n i n the r i n g . I t has a l r e a d y been o b s e r v e d i n the p o l y m e r i z a t i o n o f s u b s t i t u t e d ε-caprolactams\20||2jJ , t h a t the s u b s t i ­ t u t i o n o f hydrogen atoms d e c r e a s e s the p o l y m e r i z a b i l i t y o f monomers. The same phenomena were o b s e r v e d i n the p o l y m e r i z a t i o n o f 4 , 4 - d i m e t h y l - , c i s - 4 , 5 - d i m e t h y l and t r a n s - 4 ,5-dimethyl-Diox |lj>| . These d i f f e r e n c e s were i n t e r p r e t e d i n the p o l y m e r i ­ z a t i o n o f ε-caprolactams |20| from the change o f thermodynamical p r o p e r t i e s caused by the e x i s t e n c e o f r o t a t i o n a l isomers. In a n a l y s i n g p o l y m e r i z a t i o n o f s u b s t i t u t e d d i o x o l a n s Okada took a s l i g h t l y m o d i f i e d approach, comparing e n e r g e t i c a l d i f f e r e n c e s between d i o x o l a n s and t h e i r p o l y m e r s . Low - m o l e c u l a r weight a c e t a l s , e.g. dimethoxymethane and i t s homologues exist p r e d o m i n a n t l y i n the gauche form to a v o i d the r a b b i t -ear e f f e c t s i n the a n t i - f o r m 122|:

gauche

anti

P o l y d i o x o l a n s a r e a l s o assumed to e x i s t i n the gauche form, because the r a b b i t - e a r e f f e c t i n the a n t i form i s l a r g e r (1 k c a l m o l e ~ 1 ) t h a n the gauche i n t e r a c ­ t i o n o f the methyl groups { u s u a l l y c o n s i d e r e d to be from 0.6 to 0.9 k c a l * m o l e " ' ) . S u b s t i t u t i o n o f the H atoms by CH., groups d e s t a b i ­ l i z e s monomers by r e p l a c i n g the c i s - geminal C^-H and C -H bonds o p p o s i t i o n w i t h a g r e a t e r C^-H and C - C H opposition. e

5

5

3

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A l l o f the Okada's c a l c u l a t i o n s were based on the assumption, t h a t the s t a b l e c o n f o r m a t i o n o f Diox i s the " e n v e l o p e " form, i n which one o f the c a r b o n atoms o f the e t h y l e n e group i s l o c a t e d a t the t i p o f the f l a p , g i v i n g the d i h e d r a l angle o f the c i s - n e i g h b o r i n g hy­ drogens o f the e t h y l e n e group e q u a l to 35 degree. S u b s t i t u t i o n i n a polymer c h a i n l e a d s to the i n c r e a s e d energy o f the g a u c h e - i n t e r a c t i o n s and the d i f ­ f e r e n c e between these two e f f e c t s g i v e s e v e n t u a l l y a d e v i a t i o n i n ΔΗ, (ΔΔΗ, ) f o r a s u b s t i t u t e d Diox from unsubstituted monomer. These c a l c u l a t i o n s l e d Okada to the f o l l o w i n g e s t i m a t e d v a l u e s o f - ΔΗ- ( g i ­ ven below i n kcal»mole~1) f o r v a r i o u s methyl s u o S t i t u t e d Diox :

For 4-methyl-Diox t h e r e i s a good agreement w i t h the v a l u e determined e x p e r i m e n t a l l y (3.2 k c a l - m o l e " ) . Thus, these f i n d i n g a r e i n accordance w i t h a gene­ r a l o b s e r v a t i o n t h a t i n the p o l y m e r i z a t i o n o f h e t e r o c y c l i c monomers s u b s t i t u t i o n l e a d s to d e c r e a s e d probabi­ l i t y o f c h a i n f o r m a t i o n . The e x t e n t o f s e n s i t i v i t y o f a g i v e n c l a s s o f monomers t o s u b s t i t u t i o n i s g i v e n by the r i n g s t r a i n o f the p a r e n t , u n s u b s t i t u t e d monomer. Thus, even f o r h i g h l y s u b s t i t u t e d o x i r a n e s ( e . g . t e t r a ­ me t h y l o x i r a n e ) complete p o l y m e r i z a t i o n c a n be a c h i e ved, because the r i n g s t r a i n overshadows any o t h e r effect. J e d l i n s k i a n a l y s e d i n a s e r i e s o f papers the H-NMR s p e c t r a o f v a r i o u s s u b s t i t u t e d 1,3-dioxolans i n o r d e r to u n d e r s t a n d the s t e r e o c h e m i s t r y o f these monomers. Then, f o l l o w i n g e a r l i e r work, d e s c r i b e d p r e v i o u s l y f o r the u n s u b s t i t u t e d d i o x o l e n i u m s a l t s , s t u d i e d the k i n e ­ t i c s o f H" t r a n s f e r from these monomers t o the t r i p h e n y l m e t h y l i u m c a t i o n |23|, as the f i r s t r e a c t i o n , p r e c e ­ d i n g the t r u e i n i t i a t i o n . T h i s approach, i s complemen­ t a r y t o t h a t o f Okada,which g i v e s a thermodynamic i n f o r m a t i o n about the p o l y m e r i z a b i l i t y , w h i l e J e d l i n s k i tends t o c h a r a c t e r i z e the i n f l u e n c e o f s t r u c t u r e ( s t e ­ r e o c h e m i s t r y ) on the r a t e o f reactions pertinent to elementary r e a c t i o n s . There a r e t i l l now, however,no 1

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q u a n t i t a t i v e i n f o r m a t i o n s a v a i l a b l e about t h e r e a c t i o n s related to the a c t u a l polymerization process (initiat i o n , c h a i n g r o w t h ) . Recently,Kops r e p o r t e d on t h e pol y m e r i z a t i o n o f b i c y c l i c d i o x o l a n s , c y c l i c formais o f t r a n s - and c i s - c y c l o h e x a n e d i o l s :

cis-

Only the t r a n s - monomer p o l y m e r i z e d , g i v i n g h i g h molec u l a r weight, s o l i d polymer |24j . T h i s r e s u l t i s i n accordance w i t h a more g e n e r a l phenomenon o f t h e i n c r e a s e d s t r a i n i n t h e t r a n s - j o i n e d r i n g s , due t o t h e enhanced a n g u l a r s t r a i n . In t h e p r e v i o u s p a r a g r a p h we d i s c u s s e d p o l y m e r i z a t i o n o f 1,3-dioxolans s u b s t i t u t e d a t C and C . I n f o r m a t i o n on t h e p o l y m e r i z a t i o n o f d i o x o l a n s s u b s t i t u t e d at C i s v e r y l i m i t e d ; we s h a l l c o n f i n e o u r s e l v e s t o the p o l y m e r i z a t i o n o f 2 - v i n y l - d i o x o l a n s and 2 - v i n y l -dioxans. 4

5

2

Polymerization

o f the Unsaturated

Cyclic Acetals.

Polymerization o f 2-vinyl-1 ,3-dioxolan (4) I 25| |26| 2-vinyl-1,3-dioxane (5) |27j |2J31 |29j and relatecT mono mers, s u b s t i t u t e d a t C : 2

/CH

I

2

J

, 2

9

2

CH

XH

CH -CH 4

CH -CH 5

2

I

2

2

have been i n v e s t i g a t e d d u r i n g t h e l a s t f i f t e e n y e a r s i n a t l e a s t f i v e l a b o r a t o r i e s , A f t e r the o r i g i n a l d i s c o v e r y o f Mukaiyama 125j|, who found t h a t 4 p o l y m e r i z e s , at l e a s t p a r t i a l l y , t o the l i n e a r p o l y e s t e r : {CH CH CH COCH > 2

2

2

2

Tada, Saegusa, and Furukawa |26| i n t e r p r e t e d t h i s result as a consequence o f t h e hydri3ê-shift p o l y m e r i z a t i o n , s i m i l a r t o t h a t e l a b o r a t e d e x t e n s i v e l y by Kennedy |30| f o r branched α-olefins :

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RING-OPENING POLYMERIZATION

CH =CH-CHC' 7

0-CH I 0-CH

ù

^ (RCH CH-C-H

9

R

-

L

9

^ 0 - C H

L 2

o~ çU^r*

O-QL |

+

+

)

S

"

N

I

I

X

~

2

(3) + .0-CH, •RCH-CH-C 2

2

I

Z

^ - O - C H , +

CH-=CH-CH 2

^ O - C H ,

I

\ 0 - C H

Z

+

L

—

RΠCH C-0-ΠCH,CH 0

2 7

9

9

2

II

1

2

2

9

2

-CH j

0

.CH

0

0 1

I

CH CH 2

2

A l t h o u g h the most n u c l e o p h i l i c s i t e o f a t t a c k i n 4 are r a t h e r the oxygen atoms, but, a p p a r e n t l y the c a t i o n a t e d 4, e.g.: CH.

R - C H

2

C H

2

C O C H

2

C H ^ — 0

C H

2

J

I ,0

χ

CH =CH' 2

9

2

N

H

or i t s o p e n - c h a i n isomer, s t a b i l i z e d by the f o r m a t i o n o f the a l l y l i c - type carbenium i o n , are not s u f f i c i e n ­ t l y r e a c t i v e i n the c h a i n growth to compete w i t h the H " ion t r a n s f e r processes. More d e t a i l e d a n a l y s i s o f p o l y m e r i z a t i o n o f £, and p a r t i c u l a r l y an a n a l y s i s o f the ^H-NMR s p e c t r a o f p o l y -5 r e v e a l e d |j27| , t h a t the complete s t r u c t u r e o f p o l y ­ mers i s much more complex. Almost a l l o f the r e p e a t i n g u n i t s t h a t one c o u l d imagine were found, the most im­ p o r t a n t ones b e i n g ( f o r p o l y - 5 ) as shown below:

• · · —CH ~CH—.. · , 2

. . . —OCH CH CH OCH— · · · , 2

2

2

CH

q

ρ

%· · —CH CH COCH CH CH — · . 2

2

2

2

2

·,

0

il C H 2

The two f i r s t s t r u c t u r e s c o n t a i n groups s t i l l r e a c ­ t i v e i n the c h a i n , and t h i s i s why these polymers are o f i n t e r e s t f o r polymer chemists working i n the polymer s y n t h e s e s , The two-stage p o l y m e r i z a t i o n o f these e a s i l y a v a i l a b l e monomers has been expected to p r o v i d e a new group o f r e a c t i v e p o l y m e r s . F r e e - r a d i c a l p o l y m e r i z a t i o n , f o l l o w e d by the c a t i o n i c f o r m a t i o n o f the network (and,

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67

i n p r i n c i p l e , v i c e v e r s a ) was a l s o s u c c e s s f u l l y a p p l i e d by Minato t o ( 1 , 3 - d i o x o l a n - 4 - y l ) methyl a c r y l a t e |31 I. End-Groups i n P o l y a c e t a l s . P o l y - 1 , 3 , 5 - t r i o x a n i s known t o c o n t a i n h e m i a c e t a l -OH groups; some o f t h e s e are formed because o f the c h a i n t r a n s f e r t o water |32|, A c e t y l a t i o n o f the -OH groups g r e a t l y enhances tEermal s t a b i l i t y o f the p o l y oxymethylene polymers,and i s a t the base o f the com m e r c i a l i z a t i o n o f the f i r s t p o l y a c e t a l (a homopolymer o f CH 0) 132a| known, i n fact»many y e a r s ago from the c l a s s i c a l works o f S t a u d i n g e r and Kern. Jaacks a.o. d i s c o v e r e d |32| f o r m a t i o n o f the meth o x y l end groups i n p o l y - 1 , 3 , ! T t r i o x a n , determined by the Z e i s e l method, and assumed, t h a t t h e s e groups result from the H" i o n s h i f t ( i n t r a m o l e c u l a r l y ) o r t r a n s f e r (intermolecularly). S i m i l a r r e a c t i o n was proposed by us more r e c e n t l y i n the p o l y m e r i z a t i o n o f Diox, conducted above 0 ° , t o account f o r the methoxyl end-groups o b s e r v e d i n the 'H-NMR s p e c t r a |33|. 2

::

End-Groups i n P o l y - 1 , 3 - d i o x o l a n . Gresham p o l y m e r i z e d Diox w i t h m i n e r a l and Lewis a c i d s and was unable t o d e t e c t any end-groups |34j . P l e s c h c o n f i r m e d Gresham s o b s e r v a t i o n |35|, assumed t h a t p o l y - D i o x are m o s t l y c y c l i c and on tïïis b a s i s proposed a mechanism o f p r o p a g a t i o n w i t h p r o t o n i c a c i d s ( r i n g - e x p a n s i o n ) . J a a c k s , i n a p p a r e n t l y s i m i l a r cond i t i o n s (HC10 , C H C 1 s o l v e n t ) found e a r l i e r , t h a t polymers are r a t h e r l i n e a r , and q u a n t i t a t i v e l y d e t e r m i ned e t h y l a l c o h o l from the h y d r o l y z e d end-groups, formed when a l i v i n g - l i n e a r (on h i s o p i n i o n ) p o l y c a t i o n was k i l l e d w i t h sodium e t h y l a t e |36|. These r e s u l t s were r e c e n t l y c h a l l e n g e d by P l e s c h T ^ I · In the p o l y m e r i z a t i o n o f Diox i n i t i a t e d w i t h triethyloxoniumhexafluorophosphate ((C H )^0 PF7) W o r s f o l d |38| c l a i m e d t h a t he c o u l d not f i n e any end-groups i n p o l y - D i o x formed, a l t h o u g h a t r i p l e t from a CH^CH 0 group i s seen i n the 'H-NMR spectrum g i v e n i n n i s paper. Ponomarenko a.o. |39|, by u s i n g ( C H ) - 0 S b C l , l a b e l l e d w i t h 14c i n tïïe e t h y l group, c o n c l u d e d , t h a t the number o f moles o f Co^S groups, i n c o r p o r a t e d i n t o the macromolecules, i s c l o s e t o t h e number o f moles o f the used i n i t i a t o r , Okada |40| i n h i s study o f oligomers i s o l a t e d a t low c o n v e r s i o n ( p o l y m e r i z a t i o n o f Diox i n i t i a t e d w i t h w i t h ( C H ) 0 + BF" and k i l l e d w i t h CH^ONa) o b s e r v e d l i n e a r o l i g o m e r s w i t h e t h y l a t e and m e t h y l a t e end-groups, 1

ff

n

4

2

2

7

+

2

q

2

+

2

5

6

2

5

3

68

RING-OPENING

POLYMERIZATION

Semlyen |4jJ i n a r e p o r t d e s c r i b i n g p o l y m e r i z a t i o n i n i t i a t e d w i t h BF^ ° ( 2 5 ) ? t h a t gc-ms method ga­ ve e v i d e n c e f o r tile e x i s t a n c e o f c y c l i c o l i g o m e r s formed i n the p r o c e s s , which l e d to the m i x t u r e o f l i n e a r and c y c l i c p r o d u c t s w i t h a d i s t r i b u t i o n governed f o r l a r g e r m a c r o c y c l i c s by the Jacobson-Stockmayer theory. T h i s i s an i m p o r t a n t r e s u l t , because an agreement between the o b s e r v e d d i s t r i b u t i o n o f the c y c l i c oligomers w i t h a d i s t r i b u t i o n p r e d i c t e d by the Jacobson-Stockmayer t h e o r y (a s l o p e e q u a l to 2.5 f o r the p l o t o f l o g Κ on l o g x, where Κ i s the molar c y c l i z a t i o n - e q u i l i b r i u m c o n s t a n t f o r m a c r o c y c l e s w i t h a p o l y m e r i z a t i o n degree e q u a l to x) s t r o n g l y i n d i c a t e s t h a t p o l y m e r i z a t i o n p r o ­ ceeds w i t h a l i n e a r a c t i v e s p e c i e s , f o r m i n g m a c r o c y c l e s by b a c k - b i t i n g and end-to-end c l o s u r e . Thus, because o f the e x i s t i n g c o n t r o v e r s y , whether and when p o l y - D i o x c o n t a i n the end-groups, and because of the f a r r e a c h i n g c o n c l u s i o n s based on e i t h e r macroc y c l i c o r l i n e a r s t r u c t u r e s o f the i s o l a t e d p o l y - D i o x , we r e i n v e s t i g a t e d r e c e n t l y t h i s problem. F i r s t o f a l l we d e c i d e d to use methods which would not i n v o l v e d e s t r u c t i o n o f the end-groups ( l i k e hydro­ l y s i s used by J a a c k s |36| and then by P l e s c h |37j as a p o s s i b l e s o u r c e o f a m i F T g u i t y . S e c o n d l y , we assumed, t h a t b o t h end-groups s h o u l d be o b s e r v e d ; the i n i t i a l one, formed from an i n i t i a t o r , and the t e r m i n a l one, coming from the k i l l i n g agent. Thus, we i n i t i a t e d p o l y ­ m e r i z a t i o n e i t h e r by b e n z o i l i u m h e x a f l u o r o a n t i m o n a t e ( C ^ H r C 0 S b F 7 ) , assuming t h a t the benzoate end-groups s h o u l d be o D s e r v a b l e i n UV, o r w i t h ( C H c ) ^ 0 S b F 7 , a s ­ suming, t h a t i n the FPT-^H-NMR s p e c t r a ; CH^C^O t r i p l e t from the end-group s h o u l d be seen. C H ONa, N(CH-)- and P ( C , H r ) - k i l l i n g agents were used a f t d ^ s t u d i e d i l l * * F P T - 1 H - N M R . The benzonoate end-groups absorb a t * 0 ( l i k e the low m o l e c u l a r - w e i g h t b e n z o a t e s ) ; thus as suming t h a t ε f o r e t h y l benzoate i s e q u a l to ε of the benzoate end-groups the DP of poly-Diox were c a l c u l a t e d . T a b l e 1 summarizes some o f these results where DP ( c a l c d . ) are compared w i t h DP (UV) and DP (osm.). The former v a l u e s were c a l c u l a t e d assuming, t h a t tne p o l y m e r i z a t i o n i s a l i v i n g one, i . e . t h a t e v e r y molecul e o f i n i t i a t o r g i v e s one macromolecule w i t h no t r a n s f e r , DP (UV) was c a l c u l a t e d as d e s c r i b e d above, and DP (osm.) was measured by h i g h - s p e e d osmometry. S i n c e polymers taken f o r measurements were i s o l a t e d and p u r i f i e d by s e v e r a l d i s s o l u t i o n / p r e c i p i t a t i o n c y c l e s , some amount of the lower m o l e c u l a r - w e i g h t m a t e r i a l c o u l d be l o s t . P o l y m e r i z a t i o n was conducted i n CH-NO- o r i n Cï^Cl? solvents at -15 i n o r d e r to minimize the H" i o n t r a n s f e r ( i t has been shown i n our l a b o r a t o r y t h a t below -20° C

H

s

h

o

w

e

d

+

+

7

9

q

0

= 2 3

n

m a x

m

a

x

m

n

n

a

m

5.

PENCZEK AND KUBisA

Polymerization

of Cyclic

Acetals

69

the H~ion t r a n s f e r f r o c e.g^ dimethoxymethane to methoxycarbenium i o n -CH^OCH^bF^- becomes immeasurably slow) T a b l e 1 |42| _ Comparison o f DP ( c a l c d . ) DP (UV) and DP (osm-l o f p o l y - 1 , 3 - d i o x o l a n s p r e p a r e d w i t h C H -UO SbF i n CH-N0 o r C H C 1 s o l v e n t s a t - 1 5 ° , and « t e r m i n a t e d w i t h C H ONa. | D i o x | =5.4 m o l e - l +

6

2

2

+

6

χ 10,πκ>1β·Γ

5

n

lF (calcd.)

5

n

Ί

6

2

2

|C H CO SbF-|

t

DP (UV)

lF (osm.)

10-3

10-3

n

|Diox| -|Diox| 0

e

i o

_

3

n

+

|C H CO SbF-| 6

5

o

4.05

I

1.15

1.30

1.39

2.75

i

1.69

1 .73

1 .77

2.70

1.72

•

2.07

1 .88

2.65

1.41

I

1.23

1 .33

1.10

4.19

5.24

4.35

0.95

4.81

3.69

4.00

The second end-group, i n t r o d u c e d upon a t e r m i n a t i o n r e ­ a c t i o n , was o b s e r v e d by FPT-1H-NMR f o r samples o f p o l y -Diox, p r e p a r e d from a p e r d e u t e r a t e d Diox(Diox-dg).This approach d e c r e s a s e s an o v e r - a l l number o f p r o t o n s i n the sample and i n c r e a s e s p r o p o r t i o n o f p r o t o n s i n the end-groups. A p p l i c a t i o n o f the F o u r i e r - P u l s e - T r a n s f o r m method f o r a c c u m u l a t i o n o f the s p e c t r a enhanced the s i g n a l to n o i s e r a t i o s u f f i c i e n t l y to observe s t r u c t u r e and c o n c e n t r a t i o n o f the end-groups by FPT-'H-NMR. Some o f the p e r t i n e n t r e s u l t s are shown i n T a b l e 2. An agreement ( w i t h i n 20-251) between DP c a l c u l a t e d and measured by UV and/or ^H-NMR methods i n d i c a t e s t h a t p r a c t i c a l l y a l l o f the i n i t i a t o r used i s p r e s e n t i n the macromolecules. An agreement between DP found from the end-groups and DP measured o s m o m e t r i c a l l y means, t h a t the p r o p o r t i o n o f c y c l i c macromolecules i s low, as i t c o u l d be p r e d i c t e d , f o r i n s t a n c e , from the Jacobson-Stockmayer t h e o r y . T h i s p r o p o r t i o n , i n p r i n c i ­ p l e , _ c o u l d be d e t e c t e d by comparing DP (end groups) and DP '(osmometry) but our a c c u r a c y o f measurements i s ,

70

RING-OPENING POLYMERIZATION

at l e a s t a t p r e s e n t , comparisons.

not s u f f i c i e n t l y

high

f o r these

Table 2 Comparison o f DP ( c a l c d . ) and DP (NMR) o f p o l y -1,3-Diox, p r e p a r e d from p e r d e u t e r a t e d Diox(-d^). P o l y m e r i z a t i o n c o n d i t i o n s : C H N 0 s o l v e n t , -15®, 12 h r s . 7

9

6

Starting concn. of i n i t i a t o r 103 mole*1 1

|Diox-d | 6

n

IDioxI -IDioxI

mole*l ^

0

n

€

I initiator|

+

|C H CO SbF-| 6

5

4.05

o

Initiator

6

5

2

4.75

+

0

750

C

H

2 5°1000

5.2

5

575

650

; Kc^jP3p aF-|

Terminating agent

C H C(0)0 -OC H

700

4.7

I

DP„ found 1 from H-NMR

T3F (calcd.)

Q

-

-

L

-P(C H )5 6

5

920

Thus, we c a n c o n c l u d e , t h a t p o l y - D i o x , prepared with C HrCO SbF7 or ( C ^ H r ) 3 6 are m o s t l y l i n e a r , and macromoleculës c o n t a i n an i n i t i a l end-group coming from an i n i t i a t o r and the t e r m i n a l end-group coming from the t e r m i n a t i n g agent, e.g.: +

0 + s b F

i

n

i

t

i

a

t

o

r

s

6

C H CfOCH CH OCH > OCH CH 6

5

2

2

2

n

2

3

(4) CH CH iOCH CH OCH > P(C H ) 3

2

2

2

2

n

6

5

3

P o l y m e r i z a t i o n degrees measured i n d i c a t e a l s o , t h a t p o l y m e r i z a t i o n ( a t l e a s t i n c o n d i t i o n s g i v e n i n Table 1 and 2) p r o c e e d s w i t h o u t an a p p r e c i a b l e t r a n s f e r a f f e c t i n g the p o l y m e r i z a t i o n degree. S t r u c t u r e o f P o l y - D i o x ( c y c l i c vs l i n e a r ) and Mechanism o f P o l y m e r i z a t i o n |43|. As i t w i l l be shown i n t h i s p a r a g r a p h , n e i t h e r predominantly l i n e a r nor predominantly c y c l i c s t r u c t u r e s o f the i s o l a t e d , k i l l e d macromoleculës a r e the s t r a i g h t f o r w a r d arguments by themselves f o r the l i n e a r or c y c l i c growth o f the l i v i n g macromoleculës. Indeed, l e t us c o n s i d e r an assumed e q u i l i b r i u m between l i v i n g

5.

PENCZEK AND KUBISA

cyclic

Polymerization

and l i v i n g

linear

of Cyclic

71

Acetals

poly-Diox: (5)

X

^linear living "^macromolecule

—• 0 0 ( bl ^ CH —Oww 2

7A 7C E q u i l i b r i u m (5) , d e s c r i b i n g the instantaneous state,should a l s o be supplemented w i t h the t e m p o r a r i l y dead c y c l i c and l i n e a r ( h o l d i n g two ends coming from an i n i t i a t o r X) macromolecules. C y c l i c l i v i n g macromolecules 7A and 7B a r e r e s u l t s o f the b a c k - b i t i n g l e a d i n g to the end-to-end c l o s u r e (7A) o r a b a c k - b i t i n g to any o f the oxygen atoms i n tïïê c h a i n . The p r o b a b i l i t y o f the f o r mer p r o c e s s i s enhanced v e r y much, p a r t i c u l a r l y a t the e a r l y s t a g e s o f p o l y m e r i z a t i o n , when the oxygen atom i n the i n i t i a l end-group ( e . g . oxygen atom i n the ether end-group) i s much more n u c l e o p h i l i c than the oxygen atoms i n the a c e t a l bonds a l o n g the c h a i n . L e t us now examine r e a c t i o n o f a k i l l i n g agent with these l i v i n g macromolecules. The l i n e a r l i v i n g macro m o l e c u l e s w i l l g i v e t h e i r l i n e a r dead r e p l i c a / b u t the c y c l i c - l i v i n g ones may g i v e e i t h e r c y c l i c - d e a d o r l i n e a r - d e a d macromolecules, depending on the i n i t i a t o r used, and t h e r e f o r e on a s t r u c t u r e o f Χ i n 7A. In t h i s s t r u c t u r e t h e r e a r e t h r e e n o n e q u i v a l e n t boncTs: a, b, and c, t h a t can be b r o k e n upon an a t t a c k o f the k i l l i n g agent. I f X=e.g. CH- o r C H r , ^ th i IA r a t h e r s t a b l e bonds a and c, and one much l e s s s t a b l e a c e t a l bond b. Thus, even i f c y c l i c 7A were a predomi­ nant s t r u c t u r e a t some s t a g e o f p o l y m e r i z a t i o n , then t h e i r r e a c t i o n w i t h k i l l i n g agent would g i v e m o s t l y l i n e a r dead macromolecules. Thus, a l t h o u g h i t has been shown i n the p r e v i o u s p a r a g r a p h , t h a t p o l y - D i o x p r e p a r e d w i t h C.HrC0 SbF7 and ( C H ) ~0 SbF""are l i n e a r , t h i s i s not s u f f i c i e n t to argue t h a t - t h e c h a i n growth proceeds w i t h a l i n e a r m a c r o c a t i o n 7C. In o r d e r to d i s t i n g u i s h between the extreme s t r u c 7Λ ^ predominant d u r i n g the c h a i n growth, i t i s , t h e r e f o r e , n e c e s s a r y to o b s e r v e d i r e c t l y the p o s i t i o n o f X; i n 7A i t i s a d j a c e n t to the p o s i t i v e l y c h a r g e d oxygen atom, i n 7_C i t i s a p a r t o f the e t h e r c h a i n end. I f X=C Hr, t h e n the d i f f e r e n c e s between the chemical s h i f t s i n ^H-NMR are as f o l l o w s : Θ

η

e r e

a

r

e

2

+

+

2

t

u

r

e

s

a n

a

5

s

2

+

6 1.75(t)

δ 1 .15(t)

R

t

w

o

72

RING-OPENING POLYMERIZATION

t h u s , the difference between c h e m i c a l s h i f t s i s s u f f i ­ c i e n t l y l a r g e and both s t r u c t u r e s can be i n d e p e n d e n t l y observed by ^H-NMR. T h i s i s shown i n F i g u r e 1, t a k e n from R e f e r e n c e |43|, and i l l u s t r a t i n g the change o f the p o s i t i o n o f a t r i p l e t o f the CH^CH^-protons d i r e c t l y i n the l i v i n g p o l y m e r i z a t i o n system, c o n s i s t i n g o f a d e u t e r a t e d D i o x ( - d ) and ( C H ) - 0 S b F " i n i t i a t o r i n CD^CK s o l v e n t . At the b e g i n n i n g o f p o l y m e r i z a t i o n o n l y tne 61.75 t r i p l e t i s seen, w h i l e a t e q u i l i b r i u m o n l y the 61.15 t r i p l e t ; i n the i n t e r m e d i a t e s t a g e s b o t h t r i p l e t s are o b s e r v e d . A d d i t i o n a l s p l i t t i n g o f the 61.15 t r i p l e t i n ­ to two t r i p l e t s w i t h a d i f f e r e n c e i n c h e m i c a l s h i f t s e q u a l l i n g o n l y 13 Hz (300 MHz spectrum) and the r a t i o o f i n t e g r a t i o n s 1:2, appears as a r e s u l t o f the s i m u l ­ taneous p r e s e n c e o f two k i n d s o f ethoxy groups, namely one from the polymer end-group, and the second one from e t h y l e t h e r , l i b e r a t e d from the i n i t i a t i n g t e r t i a r y oxonium s a l t . E v a c u a t i o n o f the sample i n high-vacuum removes c o m p l e t e l y e t h y l e t h e r , as i t can be judged from the d i s a p p e a r a n c e o f i t s t r i p l e t from the spectrum. A d d i t i o n o f the N ( C H ) k i l l i n g agent to the l i v i n g system does not change the p o s i t i o n o f the 61.15 t r i p l e t . The f i n a l spectrum o f the k i l l e d system i s shown i n F i g u r e 2 ( a l s o t a k e n from R e f e r e n c e 143j . In t h i s spectrum two s i n g l e t s due to the (CH-r) N and (CH-)-N p r o t o n s are o b s e r v e d , the r a t i o |CH CH 0-|/|-N (CH )-| (from the c o r r e s p o n d i n g i n t e g r a ­ t i o n s ) i s e q u a l to 1:1.1. R e s u l t s r e p o r t e d i n t h i s pa­ r a g r a p h , and based on the r e c e n t work from our l a b o r a ­ t o r y , s t r o n g l y i n d i c a t e t h a t p o l y m e r i z a t i o n o f Diox, i n i t i a t e d by ( C H r ) 0 S b F 7 p r o c e e d s , a t l e a s t i n C H N 0 s o l v e n t , on the l i n e a r a c t i v e s p e c i e s . Systems w i t h p r o t o n i c a c i d s i n i t i a t o r s may behave d i f f e r e n t l y , because i f X=H i n 7A, then the bond a (Η-δ | . Perhaps, 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 a c e t a l s the m a c r o e s t e r - m a c r o i o n - p a i r e q u i l i b r i u m , d e s c r i b e d f o r the p o l y m e r i z a t i o n o f THF, a l s o takes p l a c e , as p r o p o s e d r e c e n t l y f o r the C I O 4 a n i o n |46 |. +

6

e

P

2

2

p

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