Ring Opening Polymerization: Make the Initiator Work for You - ACS

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Ring Opening Polymerization: Make the Initiator Work for You P. DREYFUSS The University of Akron, Institute of Polymer Science, Akron, OH 44325

When you choose an initiator for ring opening polymerization, you may also unwittingly be choosing the properties of the polymer produced. Currently known initiation methods for ring opening polymerization are reviewed in a systematic way with special emphasis on their influence on the properties of the resulting polymer. The importance of the chemical elements that comprise each group of initiators is demonstrated and it is shown that the behavior of the initiators is related to the position of these chemical elements in the Periodic Chart of the Elements. The ring opening polymerization of tetrahydrofuran is used as a model for the review. When you choose an initiator for ring opening polymerization, you may also unwittingly be choosing the properties of the polymer produced. Initiators have a large influence on the endgroups, processability, molecular weight, molecular weight distribution, long term stability, reactivity in different chemical/ physical environments and even total cost of the resulting polymer. The goals of this short review are first, to illustrate the kinds of information that can and should be obtained from the literature when you want to make an initiator work for you in a ring opening polymerization and second, to provide some guidelines for organizing that information in a useful way. There is such a vast literature in this field (See e.g. 1-5 and references therein) that it is not practical to try to examine simultaneously how different initiators work with a l l the kinds of rings that are known to undergo ring opening polymerization. Therefore, detailed discussion will be limited to only one group of rings; namely, cyclic ethers. Furthermore, since there is an effect of ring size, most of the discussion will be related to the single monomer, tetrahydrofuran (THF, a five-membered ring heterocycle with one oxygen atom in the ring) and its polymer, poly(tetrahydrofuran). 0097-615 6/ 8 3/0212-0115$06.00/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INITIATION OF POLYMERIZATION

116 Initiation

lowing

In the present instance i n i t i a t i o n i s d e f i n e d by the f o l equation: CH"—CH .ι— , ,, CH—CH I 2 ι 2 "Initiator" j2 , 2 CH CH Inert CH CH 2


\Q/

2

2

Atmosphere

2

R φ Η

\y

Pure, dry monomer i n t e r a c t s with a species to produce a t e r t i a r y oxonium i o n . Often the s o - c a l l e d " i n i t i a t o r " i s a precursor t o the true i n i t i a t i n g s p e c i e s . Formation of a secondary oxonium i o n by proton a d d i t i o n has d e l i b e r a t e l y been excluded from t h i s d e f i n i t i o n f o r the f o l l o w i n g reason. The propagating species i n THF polymerizations i s a t e r t i a r y oxonium i o n and u n t i l such an ion forms a steady s t a t e i s not present. (Formation of secondary oxonium ions by proton a d d i t i o n to THF i s a f a s t r e a c t i o n but ad d i t i o n of the next molecule of THF i s slow.) Since the polymer i z a t i o n of THF proceeds v i a a c a t i o n i c r i n g opening mechanism, p r e r e q u i s i t e s f o r polymerizations to occur are pure, dry monomer and an i n e r t atmosphere such as n i t r o g e n , i n e r t gas or high vac uum. A l s o because the p o l y m e r i z a t i o n proceeds v i a c a t i o n s , the t o t a l system a l s o contains an equivalent number of anions, X . Thus, making the i n i t i a t o r work f o r you r e q u i r e s knowledge not only about how to form the c a t i o n but a l s o about the i n t e r a c t i o n of the c a t i o n formed with i t s corresponding anion and any other m a t e r i a l s that may be present i n the r e a c t i o n mixture. e

As shown by Figure 1, a b e w i l d e r i n g number of d i f f e r e n t ma t e r i a l s have been added to THF i n an e f f o r t to induce i t s polym e r i z a t i o n . F i g u r e 1 shows j u s t the elements i n these m a t e r i a l s and the p o s i t i o n of those elements i n the P e r i o d i c Chart. The nature of the compounds and e s p e c i a l l y the o x i d a t i o n s t a t e of the elements i n the compounds a r e very important i f a given m a t e r i a l i s t o be an i n i t i a t o r , but as we s h a l l see, the kinds of a c t i v e compounds that are u s e f u l and the manner i n which a c t i v e com pounds can be used i s r e l a t e d to the p o s i t i o n i n the P e r i o d i c Chart of the p r i n c i p a l elements of which the compound i s com prised. Counterions

and Endgroup C o n t r o l by Termination

I t i s p o s s i b l e to c a r r y out a THF p o l y m e r i z a t i o n so that a " l i v i n g " polymerization r e s u l t s (1). Under these c o n d i t i o n s the endgroups, but not the headgroups, can be c o n t r o l l e d by termi n a t i o n . The headgroups i n polymers with only one growing end a r e determined during i n i t i a t i o n and w i l l be discussed l a t e r . T y p i c a l examples of endgroup c o n t r o l by termination are shown below:

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


10.

DREYFUSS

Ring Opening Polymerization

117

Figure 1. Periodic chart of elements used in initiation of THF polymerization. The elements in open squares were used as perchlorate salts (6). Those in shaded squares were used in the compounds given in the key below. A diagonal across the square indicates that, although the compounds containing the element have been examined, polymerization was not initiated (I). a

+

h

c

Key: As H with a variety of counter ions such as ClOr, PF ~, S0 F~ . . . ; as BeCh (7); as BCh, BFs, AlCh, AlBr , AIR , Bids, Al(ClO0* (6, 7, 8); as R C\ RC(OR'),* (7, 8); as NRf, NO\ NOi ; as R O , R S , or Ar S (7, 8, 10, 11), also in numerous anions; "as F~, very common as nonmetallic element in complex anions and as RF in conjugation with metal salts; ^as SiF SnCU, TiCh (7); 'as PF , AsF , SbF , SbCU, NbCU, TaCU, NH , RNH PR , . . . (8, 12-14); 'as (R)R'X (15-18); as Cl~, Br, I~ in complex anions; ClOfl as RX in conjunction with metal salts; "as SeF or WCh or Ar Se+ (12, 19-21); Ce is element 58 and the first member of the lanthanide series. In all cases R,R' = alky I or H; Ar = aryl; X = Cl, Br, I. 6

3

d

s

+

f

+

s

e

S

3

+

+

S

3

S

h

s

s

S

if

S

+

l

6

s



118 HOH

^(0Η )ι^0Η 2

+

ROH

^(CH )ifOR

NH

^(CH )zfNH2

2

3

RNH

2


+

HX

2

'tfKCH^NHR

Endgroup c o n t r o l by termination i s only p o s s i b l e with those coun t e r ions that produce s t a b l e oxonium ion-counterion complexes. Simple anions l i k e C l " , Br", I " are not s u i t a b l e counterions be cause the cation-anion complex i s so unstable with respect t o THF and a l k y l h a l i d e s that no p o l y m e r i z a t i o n a t a l l occurs. Complex ions l i k e P F e " , AsF6~, SbF6" a r e i d e a l . The cation-anion complex i s s t a b l e under normal polymerization c o n d i t i o n s and yet loose enough so that p o l y m e r i z a t i o n and r e a c t i o n with a terminating species can occur. Notice that these anions a l l a r e derived from Lewis A c i d s of elements from Group Va i n t h e i r higher valence s t a t e of +5 and the very e l e c t r o n e g a t i v e h a l i d e , f l u o r i n e from Group V i l a . Lewis A c i d s and other compounds of elements from Group Va i n t h e i r lower valence s t a t e of +3 terminate THF polym e r i z a t i o n s . Ammonia, amines, and triphenylphosphine, f o r example, have a l l been used t o introduce f u n c t i o n a l endgroups i n t o PTHF (1). Complexes from the Lewis A c i d s derived from elements i n groups l e f t of Group Va and h a l i d e s below f l u o r i n e i n Group V i l a become i n c r e a s i n g l y l e s s s t a b l e as the d i s t a n c e t o the l e f t or below i n c r e a s e s . Thus when SbCl6~ counterions are present, both termination and t r a n s f e r r e a c t i o n s with counterion occur; polym e r i z a t i o n s c o n t a i n i n g BFi>" s u f f e r from termination r e a c t i o n s ; and y i e l d s i n polymerizations c o n t a i n i n g A l C l i * " counterions are low due t o the r a p i d d e s t r u c t i o n of the growing c e n t e r s . Elements from Group V i a can form some very s u i t a b l e coun t e r i o n s f o r use when endgroup c o n t r o l by termination i s d e s i r e d . Polymerizations c o n t a i n i n g the anions S O 3 C F 3 " , S O 3 F " or Nafion (S03~form) are " l i v i n g " . However, these anions are able to r e a c t r e v e r s i b l y with the THF t e r t i a r y oxonium i o n to form e s t e r s . These e s t e r s have been c a l l e d " s l e e p i n g " species (22) or i n a s t a t e of "temporary termination" (23) because the r a t e of polym e r i z a t i o n of the e s t e r s i s n e g l i g i b l e compared to that of f r e e ions and i o n p a i r s . When comparable numbers of growing centers are present, polymerization r a t e s are lower i f e s t e r s form (JL). P e r c h l o r a t e anion i s s i m i l a r . Sulfonate anion i s not s u i t a b l e when c o n t r o l of endgroups by termination i s d e s i r e d . The THF t e r t i a r y oxonium ion r e a c t s i r r e v e r s i b l y with the s u l f a t e anion to form s u l f a t e s and d i s u l f a t e s , which can be hydrolyzed but a r e otherwise s t a b l e . The HX formed as a byproduct of the termination r e a c t i o n can be a nuisance. I f l e f t i n the polymer, these strong a c i d s r e s u l t i n degradation and d i s c o l o r a t i o n of the PTHF. Furthermore, the strong a c i d s a r e a l s o very c o r r o s i v e t o polymerization r e a c t o r s . One anion that overcomes these problems i s Nafion ( S 0 ~ form) ( 2 4 ) , which i s a polymeric r e s i n produced by duPont. A f t e r 3


10.

DREYFUSS

Ring Opening

119

Polymerization

termination Nafion can be f i l t e r e d o f f . Choosing an " i n i t i a t o r " that w i l l lead to a p o l y m e r i z a t i o n with s u i t a b l e counterions i s an important p a r t of making the i n i t i a t o r work f o r you.


Endgroup C o n t r o l by T r a n s f e r Another way to c o n t r o l endgroups i n THF polymerizations i s by means of t r a n s f e r r e a c t i o n s . These r e a c t i o n s u s u a l l y l e a d to i d e n t i c a l headgroups and endgroups. Some examples are shown i n the f o l l o w i n g equations:

HSbF

Ο Ο

+

+

6

Et 0PF 3

NOPFe

0 II (CH C) 0

0 Il CH C-r0(CH M

0 I! 0CCH

(CH 0) CH

CH 0(CH ),}

0CH

3

0 II CH CiO(CH M

0 II 0CCH

3

3

6

+

+

3

2

3

3

2

3

CH CC1 3

3T

3

2

2

T h i s i s a very good way to obtain d i f u n c t i o n a l PTHF of any de s i r e d molecular weight and gives polymer with__comparatively low molecular weight d i s t r i b u t i o n s i n a d d i t i o n . M^/M^s of 1.6 to 1.7 at moderate conversions to PTHF are q u i t e commonly observed. Some examples of the degree of molecular weight c o n t r o l that i s pos s i b l e are shown i n Tables I and I I . Since molecular weight i n t h i s type of p o l y m e r i z a t i o n i s c o n t r o l l e d by the t r a n s f e r agent r a t h e r than by the number of a c t i v e centers generated by the i n i t i a t o r , smaller amounts of a c i d are generated on termination, more s t a b l e polymers form, and c o r r o s i o n of equipment i s reduced. TABLE I Molecular Weight C o n t r o l by A c e t i c Anhydride Polymerization Time (Hr) 1.5 5.5 21

(24)

Conversion (%) 47 74 82

Mn (g/mol) 2808 1270 1002

The standard r e c i p e f o r a l l experiments c o n s i s t e d of 5.58 g Nafion Resin, 5.6 g a c e t i c anhydride and 0.6 g a c e t i c a c i d .


THF,


120 TABLE I I

C o n t r o l of Molecular Weight by Trimethylorthoformate mM E t 0 P F mole THF 0.152 0.156 0.151 3

mM(CH 0) CH mole THF 5.39 2.75 1.35

6

3

(25)

Calcd. f o r PTHF 10,000 19,000 38,000

3

[n] dl/g 0.38 0.53 0.79

a


M e a s u r e d a t 25°C i n benzene OH Endgroups During Polymerization As stated above, i n i t i a t i o n by Br^nsted A c i d i s a two-step process:

Ο

+m

^ 0 ^ Η

χ

θ

X

A polymer with an OH headgroup i s produced, and chain coupling occurs repeatedly by r e a c t i o n of the OH endgroup with the THF t e r t i a r y oxonium i o n (11) u n t i l high molecular weight polymer i s formed (26): H0(CH ) 2

H τ

0(CH )^^(>(P^]

X°

2

•Ο

H0(CH ) ^oe 2

lt

I

χ

θ

High Molecular Weight Polymer The combination of slow i n i t i a t i o n and of chain c o u p l i n g makes molecular weight c o n t r o l very d i f f i c u l t . At f i r s t glance t h i s problem would appear to be one that i s e a s i l y avoided by choosing a d i f f e r e n t " i n i t i a t o r " . However, the problem i s f r e q u e n t l y en countered and the s c i e n t i s t needs to be aware of these r e a c t i o n s because many " i n i t i a t o r s " l e a d to i n s i t u generation of Br^nsted A c i d . Among these are 0 C P F , £-Cl-0N PF , N0PF , and N 0 P F alone, R^NClOj^ p l u s e l e c t r i c c u r r e n t ; AgPF plus l i g h t or a f r e e r a d i c a l source and heat or l i g h t ; A r S ^ , A r S e ^ , R Br^, R C1^, and R I $ plus l i g h t (J_) . As a general r u l e carbocations do not add to THF while Ν 0 and N0 $ may add; but i n a l l these cases there i s strong evidence f o r the formation of Η and i t i s the H$ that s t a r t s the chain of r e a c t i o n s that leads to the p o l y m e r i z a t i o n of THF. Tetraalkylammonium c a t i o n does not add to THF but i n the presence of an e l e c t r i c current i t s accompanying p e r c h l o r a t e anion undergoes a s e r i e s of i n t e r a c t i o n s with THF that again gives H$. AgPF alone doesn't i n i t i a t e THF p o l y m e r i z a t i o n but again there 3

6

2

6

6

2

g

3

3

2

2

2

Φ

2

φ

6


6

10.

DREYFUSS

Ring Opening

Polymerization

121

are r e p o r t s that l i g h t or l i g h t / h e a t and a f r e e r a d i c a l source can l e a d to the formation of H$ and polymerization of THF. The c a t i o n s derived from S and Se i n Group V i a and the c a t i o n s from CI, Br, and I i n Group V i l a do not add to THF e i t h e r but i n the presence of l i g h t , H$ i s formed p h o t o l y t i c a l l y and polymerization occurs. The l a t t e r " i n i t i a t o r s " were developed f o r the polymer i z a t i o n of epoxides i n the form of t h i n f i l m s (11, 15, 16).


C o n t r o l l i n g the Headgroup Species that add to THF without any a c t i v a t i o n i n the form of energy or other chemicals i n c l u d e R O0, R C ( = = 0 R ) ^ , RC^=0, super a c i d e s t e r s , P F , SbF , S b C l , N b C l , WC16 and superacid anhydrides (1). The c a t i o n s and the superacid e s t e r s give a l k y l or a c y l headgroups depending on the nature of the R groups. In the few cases where the mechanism of i n i t i a t i o n has been s t u d i e d , the Lewis A c i d s give PTHF that i s growing on both ends. The same i s true f o r the superacid anhydrides. The endgroups i n the l a t t e r polymerizations are determined by the species added f o r t e r mination except i n cases l i k e SbCls, where some s i d e r e a c t i o n s a l s o occur. N o t i c e that the c a t i o n s that add are t y p i c a l l y ox onium or carboxonium i o n s . An oxonium ion can add to h e t e r o c y c l e with s u l f u r as a heteroatom but the reverse does not occur. The Lewis Acids that add and l e a d to PTHF without f u r t h e r a c t i v a t i o n are p r i m a r i l y from Group Va. N b C l i s from Group Vb and WC1 and SeF are from Group VI. Lewis A c i d s from Group I l i a w i l l add to THF but polymerization does not occur unless a "promoter" such as ethylene oxide or e p i c h l o r o h y d r i n i s added a l s o . Lewis A c i d s from elements i n other groups do not i n i t i a t i o n THF polymerization even i n the presence of a promoter. Instead a l c o h o l a t e s form. T i C l ^ and e p i c h l o r o h y d r i n , f o r example, give ( C 1 C H ) C H 0 T i C l (1) . f

3

5

5

2

5

5

5

6

6

2

The T r a n s i t i o n Metals and

2

3

Halides

Soluble s a l t s from the t r a n s i t i o n metals g e n e r a l l y do not polymerize THF without some other a c t i v a t i o n . I t has already been noted that AgPF6 gives polymer i n the presence of l i g h t or a f r e e r a d i c a l source p l u s heat or l i g h t . In t h i s case the species that i s r e s p o n s i b l e f o r polymerization i s p h o t o l y t i c a l l y generated HPF6. In the presence of an a c t i v e a l k y l h a l i d e the mechanism i n v o l v e s the AgPF6 more d i r e c t l y : + AgCl

R

PF

θ 6

Other examples of combinations with h a l i d e s that i n i t i a t e the polymerization of THF i n c l u d e CH 0CH C1 with F e C l , CH =CHCH C1 3

2

3

2


2


122

with AgPF or H g C C l O i ^ , CH C0C1 w i t h Pb(C10i ) and CH COF w i t h N0PF6. Studies have shown that among the metal s a l t s s i l v e r s a l t s give the best r e s u l t s . NO^ i s not a t r a n s i t i o n metal, of course, but i t i s included i n t h i s group i n order to i l l u s t r a t e yet another way of making an " i n i t i a t o r " work f o r you. When the polymerization of THF i s i n i t i a t e d with any combination of mater i a l s i n v o l v i n g a t r a n s i t i o n metal s a l t , removal of c a t a l y s t r e s idues i s o f t e n very d i f f i c u l t . And i f any metal r e s i d u e remains the PTHF soon becomes d i s c o l o r e d . The combination NOPF and C H 3 C O F has the advantage that the NOF byproduct i s a very low b o i l i n g gas that i s r e a d i l y removed or more a c c u r a t e l y , spontan eously i s evolved from the PTHF. A l s o C H 3 C O F i s not a t r a n s f e r agent and molecular weight depends on the amount of N0PF6 added. Reactions i n v o l v i n g a c t i v e c h l o r i d e / s a l t combinations are espec i a l l y u s e f u l f o r the p r e p a r a t i o n of g r a f t copolymers from hydro carbons and PTHF (1). In f a c t a v a r i e t y of g r a f t copolymers that are not otherwise a c c e s s i b l e can be prepared using t h i s chemistry. 6

3

f

2

3


6

Concluding Comments As we have moved across the p e r i o d i c chart of the elements from r i g h t to l e f t we have found f i r s t t o t a l l y i n e r t compounds i n the form of the r a r e gases, next elements of Groups V i l a and V i a (except oxygen) that are u s e f u l mainly i n counterions, then the very a c t i v e Lewis A c i d s of Group Va ρ en ta va l e n t elements, and f i n a l l y i n c r e a s i n g need f o r a c t i v a t i o n of any s o l u b l e compounds com bined with i n c r e a s i n g i n s t a b i l i t y of the counterions that might form. Nothing has been s a i d so f a r about compounds of elements from Groups l i a and I l l b . These compounds appear to be i n a c t i v e even i n the presence of a very r e a c t i v e h a l i d e l i k e a c e t y l c h l o r i d e . Compounds from Group l a e x h i b i t complex behavior i n the presence of THF. The a c t i v i t y of H i s l a r g e l y r e l a t e d to i t s accompanying anion and the same i s probably true to a l e s s e r ex tent of s o l u b l e l i t h i u m and sodium s a l t s . LiClOi*, f o r example, can be a supporting e l e c t r o l y t e f o r THF p o l y m e r i z a t i o n i n the presence of an e l e c t r i c current but i t i s again the CIO^ that gives the compound a c t i v i t y . P o l y m e r i z a t i o n w i l l occur i n the presence of L i P F but i t has been suggested that the true i n i t i ator i s P F 5 . S a l t s of potassium are l a r g e l y i n s o l u b l e i n THF. +

6

By c a r e f u l c o n s i d e r a t i o n of schemes l i k e the f o r e g o i n g , i t i s p o s s i b l e to p r e d i c t how to make an i n i t i a t o r work f o r you i n a r i n g opening p o l y m e r i z a t i o n .

Literature Cited 1. 2.

Dreyfuss, P. "Poly(tetranydrofuran)", Gordon and Breach, New York, N.Y., 1982. "Ring-Opening Polymerization", Saegusa, T.; Goethals, E. Eds., ACS Symposium Series 59, American Chemical Society, Washington, D.C., 1977.


10.

DREYFUSS

3.

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