Thermally or Photochemically Induced Cationic Polymerization

15 Thermally or Photochemically Induced Cationic Polymerization J. V. CRIVELLO

Downloaded by EAST CAROLINA UNIV on November 13, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch015

Corporate Research and Development Center, General Electric Company, Schenectady, NY 12301

Diaryliodonium salts are a new class of versatile initiators of cationic polymerization which are characterized by their exceptional latency. On irradiation with UV light, these compounds are very efficient photoinitiators whose reactivity and absorp tion characteristics can be tailored at will by struc tural modification as well as through the use of photosensitizers. When diaryliodonium salts are com bined with catalytic amounts of Cu(II) compounds, they can be used to thermally initiate cationic polymeriza tion at temperatures above 80°C. The further addition of reducing agents produces initiator systems in which cationic polymerization occurs spontaneously on mixing at 25°C. Examples of polymerizations carried out using these photochemical and thermal initiator systems are given along with the proposed mechanism of the reac tions involved. In recent years, research at t h i s laboratory has centered about the development of new i n i t i a t o r s for cationic polymerization. Among the most interesting and useful of these i n i t i a t o r s are diaryliodonium s a l t s whose structure i s shown below. Ar I 2

+

X"

where X~ = BF^~, PF^", AsF ~, SbFg", etc. 6

These compounds are stable, c o l o r l e s s , c r y s t a l l i n e , ionic s a l t s which are readily soluble i n organic solvents but nearly insoluble i n water. Especially useful i s t h e i r excellent s o l u b i l i t y i n a wide variety of c a t i o n i c a l l y polymerizable monomers. Unlike carbenium s a l t s such as t r i t y l and tropylium s a l t s and trialkyloxonium s a l t s which spontaneously i n i t i a t e cationic polymerization on contact with susceptible monomers, solutions of diaryliodonium s a l t s i n these same monomers are stable and show no tendency to polymerize even when heated to temperatures up to 150°C. 0097-6156/ 85/ 0286-0195506.00/ 0 © 1985 American Chemical Society

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

196

RING-OPENING POLYMERIZATION

Photoinitiated Cationic Polymerization Although diaryliodonium s a l t s are stable toward thermolysis, they exhibit a s u r p r i s i n g degree of p h o t o s e n s i t i v i t y . On i r r a d i a t i o n with 254 nm l i g h t , these compounds undergo a f a c i l e i r r e v e r s i b l e photolysis as shown i n Equation 1 to give an a r y l r a d i c a l , a r y l iodoinium c a t i o n - r a d i c a l pair (1). Ar I

+

X~


2

î^—»

[ A ^ I * X"]*

> A r l t x"

+

Ar-

(1)

The quantum y i e l d for the above process has been estimated to be approximately 0.2 on the basis of the a r y l i o d i d e formed. Work by Pappas and Gatechair(2) and by Timpe and h i s coworkers(3) indicates that the quantum y i e l d for t h i s reaction may be as high as 0.7 based on the amount of protonic acid which i s formed. I f the above photolysis i s carried out i n the presence of a monomer, spontaneous cationic polymerization i s observed. The species responsible for i n i t i a t i n g cationic polymerization i s the aryliodinium cationr a d i c a l which may undergo d i r e c t e l e c t r o p h i l i c attack on the monomer (Equation 2). A l t e r n a t i v e l y , t h i s c a t i o n - r a d i c a l can react with other species present i n the reaction mixture to generate Brrfnsted acids which may subsequently i n i t i a t e polymerization (Equation 3). Attack on M

>

„ , Polymer

(2)

Art"!" χ" HX Formation

Attack of

The observation that only a very small portion of the polymer chains which were produced using diaryliodonium s a l t s contain end groups which are derived from i n i t i a t o r fragments' suggests that the process shown i n Equation 3 i n which Brrfnsted acids are formed i s dominant. The rate of photolysis of diaryliodonium s a l t s and hence the number of i n i t i a t i n g species generated per given i r r a d i a t i o n time and l i g h t intensity i s related to the structure of the cation which i s the l i g h t absorbing species. A bathochromic s h i f t i n the absorp tion bands i s observed when electron releasing substituents are introduced into the ortho and para positions of the aromatic rings. Conversely, the absorption bands are s h i f t e d to shorter wavelengths when electron withdrawing substituents are placed at these positions Using these general guidelines, i t i s possible to design p h o t o i n i t i ators whose absorption c h a r a c t e r i s t i c s l i e i n v i r t u a l l y any desired portion of the u l t r a v i o l e t spectrum. Although the anion of a d i aryliodonium s a l t plays no r o l e i n i t s photochemistry, i t i s the dominant factor i n the subsequent polymer chemistry since i t deter mines the r e a c t i v i t y of both the i n i t i a t i n g and propagating species as well as c o n t r o l l i n g which termination processes occur. Among the most useful diaryliodonium s a l t p h o t o i n i t i a t o r s are those which bear the very weakly n u c l e o p h i l i c anions such as BF ", A~» ^ A~» * p f

S F

A


A

N

C


15.

Thermally or Photochemically Induced Cationic Polymerization

CRIVELLO

SbFg . These p h o t o i n i t i a t o r s are capable of polymerizing almost every known type of c a t i o n i c a l l y polymerizable monomer. Due to the very weakly n u c l e o p h i l i c character of these anions, termination i s very slow i f not absent and i n c e r t a i n cases such as i n the polymeri zation of tetrahydrofuran, l i v i n g cationic polymerizations are ob served with such i n i t i a t o r s . The photolysis of diaryliodonium s a l t s can be carried out i n the long wavelength UV and i n the v i s i b l e region of the spectrum although they do not absorb at these wavelengths provided that photosensitizers are employed(4,5). Diarylketones, condensed r i n g aromatic hydrocarbons and phenothiazines are excellent photosensi t i z e r s f o r use i n the UV, while the acridinium and benzothiazolium dyes, acridine orange and setoflavin-T are a c t i v e photosensitizers for the short wavelength v i s i b l e region. A mechanism involving electron transfer has been implicated i n photosensitization and i s depicted i n Equations 4-7.

-ISU

Ρ P*

+

Ar I

[P-.-Ar I

+

X"

2

+

p* >

X~]*

[P---Ar I 2

> P"f"x"

+ η M

(4)

P

X

+

+

X"]*

Ar I-

(5) (6)

-

* >

-{M>-

(7)

η

The key feature of t h i s mechanism i s that the excited photosensitizer, Ρ , i s oxidized by the diaryliodonium s a l t which i s correspond ingly reduced. This mechanism i s substantiated by f i r s t , the d i r e c t experimental observation of photosensitizer c a t i o n - r a d i c a l species by UV and ESR spectroscopy(6) and second, by a d i r e c t c o r r e l a t i o n between the a c t i v i t y of a photosensitizer and the reduction p o t e n t i a l of i t s excited state r e l a t i v e to the diaryliodonium salt(3>,5). It i s interesting to note that the c a t i o n - r a d i c a l , PÎ, derived from the photosensitizer rather than from the p h o t o i n i t i a t o r i s responsible for i n i t i a t i n g polymerization i n t h i s instance. Thermally I n i t i a t e d Cationic Polymerization I n i t i a t o r s Activated by Elevated Temperatures. Although, as men tioned e a r l i e r , diaryliodonium s a l t s possess considerable thermal s t a b i l i t y and, therefore, cannot be used d i r e c t l y i n thermally activated polymerizations, we sought to f i n d some way i n which the thermal latency of these i n i t i a t o r s could be broken. This appeared to be possible on the basis of a recent general reaction discovered i n our laboratory(7,8). Diaryliodonium s a l t s undergo f a c i l e reac tion with nucleophiles whereby the nucleophile i s arylated as de picted i n Equation 8. Ar I ^

+

X"

+

Nu

Cu(II) >

+

ArNu x"

+

Arl

(8)

A

This reaction proceeds smoothly at temperatures from 100-125°C i n the presence of a c a t a l y t i c amount of a copper compound to give high y i e l d s of the arylated product. Furthermore, as shown i n Scheme 1,


197

198


the reaction i s applicable to a wide variety of substrates. Even compounds as poorly nucleophilic as diphenylsulfide are quantita t i v e l y arylated i n one hour at 125°C.


Scheme 1

Realizing that c a t i o n i c a l l y polymerizable monomers are, by d e f i n i t i o n , nucleophiles, i t appeared that i t might be possible to i n i t i ate cationic polymerization using the same a r y l a t i o n reaction. In deed, when c a t i o n i c a l l y polymerizable monomers were heated at tem peratures i n excess of 80°C i n the presence of diaryliodonium s a l t s containing a trace of cupric benzoate as a catalyst, spontaneous polymerization was observed(9). The polymerization i s completely general with respect to the types of monomers which can be used. Among those representative monomers which have been thermally polymerized using t h i s new catalyst system include: cyclohexene1,2-oxide, s-trioxane, 2-chloroethyl v i n y l ether, ε-caprolactone, α-methylstyrene, and tetrahydrofuran. Figure 1 gives the r e l a t i o n ship between the reaction time and the conversion of monomer to polymer i n the polymerization of ε-caprolactone. In the polymeriza t i o n of t h i s p a r t i c u l a r monomer, an i n h i b i t i o n period can be c l e a r l y seen. In Figure 2 i s shown the effect of the concentration of the diaryliodonium s a l t on the rate of conversion of phenyl g l y c i d y l ether to polymer. As the diaryliodonium s a l t i s increased, the rate of polymerization i s also correspondingly increased. A l l d i aryliodonium s a l t s examined behaved s i m i l a r l y , provided they pos sessed the weakly nucleophilic anions mentioned above. In contrast to the marked influence of the diaryliodonium s a l t concentration on the polymerization rates, the effect of the con centration of the copper compound was found to be c a t a l y t i c . In general, 10 mole % with respect to the diaryliodonium s a l t was found to be s u f f i c i e n t . Although many d i f f e r e n t t r a n s i t i o n and nont r a n s i t i o n metals i n various oxidation states were examined, only


CRIVELLO

Thermally or Photochemically Induced Cationic Polymerization 199


15.



Figure 2.

E f f e c t of the concentration of ( C ^ I ^ ^ I AsF^ on the polymerization of phenyl g l y c i d y l ether at 85°C for 30 min catalyzed by 1.6 χ 10"^ mol Cu(II) benzoate.


15.

Thermally or Photochemically Induced Cationic Polymerization

CRIVELLO


copper compounds were catalysts for the polymerization reaction. V i r t u a l l y any copper compound can be used as a c a t a l y s t ; however, those compounds such as cupric stéarate and cupric benzoate which have appreciable s o l u b i l i t y i n organic media were most u s e f u l . A number of experiments designed to elucidate the nature of the c a t a l y s i s by copper were carried out. I t was observed that when Cu(II) compounds were combined with diaryliodonium s a l t s i n com p l e t e l y unreactive solvents such as chlorobenzene, there was no reaction even at elevated temperatures. In contrast, diaryliodonium s a l t s reacted rapidly and q u a n t i t a t i v e l y even at 25°C i n various solvents i n the presence of c a t a l y t i c amounts of a Cu(l) compound. Analysis of the products of t h i s l a t t e r reaction shown i n Equation 9

6%

tr.

are consistent with the p r i o r suggestion that a r y l a t i o n of a nucleop h i l e , i n t h i s case the solvent methanol, takes place during the reaction. Given the observation that only Cu(I) species are active as c a t a l y s t s i n the above a r y l a t i o n reaction, i t appeared that when Cu(II) compounds are employed i n these i n i t i a t o r systems, a reduction must occur to generate the c a t a l y t i c a l l y active Cu(I) oxidation state. In l i g h t of the above observations, the mechanism shown i n Equations 10-13 has been proposed for the thermal i n i t i a t i o n by d i aryliodonium s a l t s i n the presence of Cu(II) c a t a l y s t s ( 9 ) . Red-H Ar I 2

+

X~

+

Cu(II)L

+

Cu(I)L

[ArCu(III)LX] +

Ar-M X"

»

2

Red

+

Cu(I)L

> [ArCu(III)LX] +

+

M

>

Ar-M X~

+

+

ηM

>

Ar-(M)-M χ" η

+ +

HL Arl

Cu(I)L

+

(10) (11) (12) (13)

The reaction of the Cu(I) species with the diaryliodonium s a l t re sults i n the formation of a proposed organometallic intermediate, [ArCu(III)LX], whose structure has not been f u l l y elucidated due to i t s l a b i l i t y . This intermediate undergoes as i t s primary reaction, an e l e c t r o p h i l i c attack on the monomer, M, to i n i t i a t e cationic polymerization. The above mechanism predicts and i t has been con firmed that only trace amounts of reducing agents are required to convert a c a t a l y t i c quantity of Cu(II) compound to i t s lower valence state. Once the Cu(I) i s formed, i t i s continually recycled between Equations 11 and 12 u n t i l a l l the diaryliodonium s a l t has been con sumed. Another consequence of t h i s mechanism i s that polymers pre pared using these catalysts should possess aromatic end groups which


201

202


originate from the diaryliodonium s a l t . Indeed, bands due to the presence of aromatic end groups can be observed i n the UV spectra of polycyclohexene oxide and poly-e-caprolactone prepared using these i n i t i a t o r s . Additional work with model compounds shown i n Equations 14 and 15 has v e r i f i e d that the chief mode of i n i t i a t i o n involves a r y l a t i o n of the monomer.


(14)

(15) The nature of the reducing agent, Red-Η, has been the subject of a considerable amount of research. In most cases, and especially when oxygen containing heterocyclic monomers are used, the major r e ducing agents are alcohols which are present i n these monomers as impurities or as a result of hydrolysis. Further, Cu(II) compounds are known to oxidize a l i p h a t i c alcohols at elevated temperatures(10). L a s t l y , the addition of small amounts of such alcohols results i n a reduction i n the i n h i b i t i o n period at the start of the polymeriza t i o n and increases the o v e r a l l rate. I n i t i a t o r s Active at Room Temperature. The a b i l i t y of Cu(I) com pounds to catalyze the quantitative reduction of diaryliodonium s a l t s has l e d to the design of a number of novel i n i t i a t o r systems which can be used at low temperatures. The most simple of these systems consists of adding a Cu(I) compound d i r e c t l y to an appro p r i a t e monomer containing a diaryliodonium s a l t . Spontaneous poly merization i s observed on mixing. A l t e r n a t i v e l y , the Cu(I) species can be generated by an i n - s i t u reduction of the corresponding Cu(II) compound. This can be accomplished by the addition of e a s i l y o x i dized alcohols such as benzoin or ascorbic acid, which reduce Cu(II) compounds at room temperature. Again, when these reducing agents are added to reactive monomers containing diaryliodonium s a l t s and Cu(II) c a t a l y s t s , spontaneous cationic polymerization occurs at 25°C on mixing(11,12). Another very useful class of reducing agents which can be used are Sn(II) carboxylates(13)· In the presence of a Cu(II) c a t a l y s t , Sn(II)-2-ethylhexanoate quantitatively catalyzes the decomposition of diaryliodonium s a l t s . Model reactions have shown that the i n i t i a l step i n t h i s reaction i s the f a c i l e reduction of Cu(II) to Cu(I) by the Sn(II) compound as depicted i n Equation 16. 2Cu(II)L

2

+

Sn(II) L 2

f 2

> 2 Cu(I)L +

Sn(IV)L' L


2

2

(16)

15.

CRIVELLO

Thermally or Photochemically Induced Cationic Polymerization 203

Free Radical I n i t i a t o r s as Reducing Agents for Diaryliodonium S a l t s . A f i n a l method by which diaryliodonium s a l t s can be used as thermal i n i t i a t o r s of c a t i o n i c polymerization has recently been reported by Ledwith and h i s coworkers(14,15) and i s shown i n Equations 17-19. R-R R- + A r I 2

+

— > 2RX"

Ar- + THF

> R

(17) +

X" +

> ArH + THF-

Arl

+

Ar-

etc.

(18) (19)


\

Free r a d i c a l s produced by the thermolysis of t y p i c a l r a d i c a l i n i t i a tors as AIBN, benzopinacole and phenylazotriphenylmethane reduce the diaryliodonium s a l t generating a r y l r a d i c a l s and solvent derived r a d i c a l s which i n a chain reaction induce the decomposition of more diaryliodonium s a l t . Through the s e l e c t i o n of p a r t i c u l a r r a d i c a l i n i t i a t o r s with s p e c i f i c decomposition rates, i t i s possible to ad just the i n i t i a t i o n temperature of the cationic polymerization with considerable l a t i t u d e . Conclusions Diaryliodonium s a l t s are a novel and highly v e r s a t i l e class of i n i t i a t o r s for c a t i o n i c polymerization. These compounds are e f f i cient p h o t o i n i t i a t o r s of c a t i o n i c polymerization whose structure may be readily modified to achieve a wide degree of photosensitivity and r e a c t i v i t y . While these compounds are unique i n that they do not thermally i n i t i a t e polymerization even at elevated temperatures, they can be converted to excellent thermal i n i t i a t o r s simply through the addition of c a t a l y t i c quantities of a copper(II) compound. The further discovery that the addition of reducing agents markedly accelerates the i n i t i a t i o n and lowers the i n i t i a t i o n temperature allowed the design of systems which i n i t i a t e polymerization spon taneously at 25°C on mixing or at any desired temperature. T y p i c a l reducing agents which have been explored are: ascorbic a c i d , benzoin, Sn(II) carboxylates i n combination with Cu(II) compounds and common free r a d i c a l progenitors. Literature Cited 1. Crivello, J. V.; Lam, J. H. W. Macromolecules 1977, 10, 1307. 2. Pappas, S. P.; Gatechair, L. R. Proc. Soc. Photogr. Sci & Eng. 1982, 46. 3. Timpe, H.-J.; et al Z. Chem. 1983, 3, 102. 4. Crivello, J. V.; Lam, J. H. W. J. Polym. Sci., Polym. Chem. Ed. 1978, 16, 2441. 5. Pappas, S. P.; Jilek, J. H. Photogr. Sci. Eng. 1979, 23, 140. 6. Crivello, J. V.; Lee, J. L. unpublished results. 7. Crivello, J. V.; Lam, J. H. W. J. Org. Chem. 1978, 43, 3055. 8. Crivello, J. V.; Lam, J. H. W. Synth. Comm. 1979, 9, 151. 9. Crivello, J. V.; Lockhart, T. P.; Lee, J. L. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 97. 10. Clarke, H. T.; Dreger, Ε. E. Org. Syn., Coll. Vol. 1 1941, 87. 11. Crivello, J. V.; Lee, J. L. J. Polym. Sci.,Polym. Chem. Ed. 1981, 19, 539.



204

12. Crivello, J. V.; Lee, J. L. J. Polym. Sci., Polym. Chem. Ed. 1983, 21 1097. 13. Crivello, J. V.; Lee, J. L. Makromol. Chem. 1983, 184, 463. 14. Abdul-Rasoul, F. A. M.; Ledwith, Α.; Yagci, Y. Polymer 1978, 19, 1219. 15. Abdul-Rasoul, F. A. M.; Ledwith, Α.; Yagci, Y. Polymer Bull. 1978, 1, 1.


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Thermally or Photochemically Induced Cationic Polymerization

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