Recent Developments in Cationic Polymerization - ACS Symposium

5 Recent Developments in Cationic Polymerization VIRGIL P E R C E C

Downloaded by UNIV OF ARIZONA on May 18, 2013 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch005

Department of Macromolecular Science, Case Western Reserve University, Cleveland, O H 44106

Ring Opening Polymerization General Considerations Initiation Propagation Termination and Transfer Processes L i v i n g Cationic Ring-Opening Polymerization New Polymers by Cationic Ring-Opening Polymerization Graft Copolymers Block Copolymers Cationic Polymerization of Olefins General Considerations Graft Copolymers I n i f e r Technique Q u a s i - l i v i n g Carbocationic Polymerization Proton Traps Block Copolymers Heterogeneous Graft Copolymerization Polymers with Functional End Groups Polymers with Two Functional End Groups: Telechelics Polymers with One Functional End Group: Macromonomers

C a t i o n i c p o l y m e r i z a t i o n of heterocyclic and vinylic monomers i s currently one of the most active areas of polymer c h e m i s t r y . In the past two years four monographs dedicated to different topics i n t h i s field were published (1-4). Cationic polymerization is a l s o one of the few areas of polymer science that has its own scientific meeting. The fifth meeting was organized i n Kyoto (1979) (5); the previous meetings were i n Akron (1976) (6); Rouen (1973) (7); Keele (1952) (8); and Dublin (1949) (9). At the 26th International Union of Pure and A p p l i e d Chemistry (IUPAC) International Symposium on

0097 6156/85/0285-0095S09.75/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


96

A P P L I E D P O L Y M E R SCIENCE

Macromolecules in Mainz (1979), four of the 33 main lectures were dedicated e n t i r e l y to the field of c a t i o n i c polymerization (10-13). A short history of cationic p o l y m e r i z a t i o n was p u b l i s h e d i n 1975 (14, 15). An excellent collection of classic papers was published i n 1963 (16). The first recorded c a t i o n i c polymerization was d e s c r i b e d i n 1789 (17), and the first industrial polymer prepared by c a t i o n i c polymerization, b u t y l rubber, appeared on the market in 1943 (14). In s p i t e of these achievements and many o t h e r s , our b a s i c knowledge about c a t i o n i c p o l y m e r i z a t i o n s t a r t e d to d e v e l o p o n l y very recently. The main reason i s that the electrophilic species through which cationic p o l y m e r i z a t i o n takes p l a c e (carbenium, oxonium, s u l f o n i u m , phosphonium, and ammonium i o n s ) are very reactive. C o n s e q u e n t l y , i n a d d i t i o n to propagation r e a c t i o n s , chain transfer, termination, and reactions with traces of nucleo-philic impurities take place. Only h i g h l y sophisticated techniques such as high-vaccuum r e a c t i o n c o n d i t i o n s , adiabatic calorimetry, and Fourier transform NMR measurements made progress p o s s i b l e i n t h i s area of chemistry. Even so, a large difference e x i s t s between our knowledge of r i n g - o p e n i n g p o l y m e r i z a t i o n and v i n y l i c polymerization. Cationic polymerization of v i n y l i c monomers takes p l a c e w i t h carbenium i o n s , which are more r e a c t i v e than oxonium, s u l f o n i u m , phosphonium, or ammonium i o n s . Chain t r a n s f e r to monomer can be decreased o n l y at very low polymerization temperature. Consequently, polymers with high molecular weights can be obtained by reaction conditions that are not of i n t e r e s t to industry. Ring-opening polymerization can be followed by NMR techniques; therefore, d i r e c t evidence for the polymerization mechanism could be obtained (1, _4, 17). Our knowledge about v i n y l i c polymerization mechanisms i s obtained mainly from i n d i r e c t evidence. The present s t a t e of both r i n g - o p e n i n g (X, 4_, J J , 17-20) and v i n y l i c p o l y m e r i z a t i o n (_2, _3, 14) was r e c e n t l y r e v i e w e d . The present review w i l l present the most recent developments i n both areas m a i n l y i n regard to the p r e p a r a t i v e power of c a t i o n i c polymerization. Only a few basic achievements of the mechanistic aspects w i l l be considered. Ring-Opening Polymerization General Considerations. The r e a c t i v i t y of h e t e r o c y c l i c monomers i s governed by the s i z e of the r i n g , nature of the heteroatom and i t s e l e c t r o n e g a t i v i t y and bond s t r e n g t h w i t h the carbon atom, and steric factors. A d e t a i l e d d i s c u s s i o n of a l l these f a c t o r s i s presented by Penczek et a l . (_1). Two b a s i c p r i n c i p l e s w i l l be o u t l i n e d here. They w i l l r e f e r to the most s i m p l e h e t e r o c y c l i c monomers only. The s i z e of the r i n g , that i s , the number of atoms i n the r i n g , c o n t r o l s the r i n g s t r a i n by two factors. The f i r s t factor refers to the difference between the bond angles that r e s u l t from normal o r b i t a l o v e r l a p and the bond a n g l e s t h a t are a f u n c t i o n of the number of atoms i n the r i n g , that i s , the angle s t r a i n . The second f a c t o r i s the consequence of the i n t e r a c t i o n s of the nonbonded atoms. For a c e r t a i n geometry of the molecule, nonbonded atoms are s i t u a t e d i n a c l o s e p r o x i m i t y t h a t g i v e s r i s e to t h i s k i n d of i n t e r a c t i o n . Both bond a n g l e s and nonbonding i n t e r a c t i o n s are In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


5.

Recent Developments in Cationic Polymerization

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97

r e s p o n s i b l e f o r the h e t e r o c y c l e r i n g s t r a i n . Consequently, with the e x c e p t i o n of the three-membered r i n g s , a l l r i n g s e x h i b i t a noncoplanar conformation of minimum energy. As a function of the number of atoms i n the r i n g , the r i n g s t r a i n i s dominated by angle s t r a i n or by nonbonded atom i n t e r a c t i o n s . Table I summarizes the r i n g s t r a i n values for the most conventional rings. According to the r i n g s t r a i n energies presented i n Table I , the h e t e r o c y c l i c d e r i v a t i v e s c o n t a i n i n g s i x atoms i n the r i n g are g e n e r a l l y u n p o l y m e r i z a b l e . An e x c e p t i o n i s s y m - t r i o x a n e which polymerizes under conditions i n which the polymer can c r y s t a l l i z e . The r e a c t i v i t y of the r i n g opening toward a c a t i o n i c mechanism i s m a i n l y d i c t a t e d by the n u c l e o p h i l i c i t y or the b a s i c i t y of the monomer. The n u c l e o p h i l i c i t y of a monomer, that i s , i t s a b i l i t y to combine with e l e c t r o p h i l i c species, i s determined by k i n e t i c a l l y c o n t r o l l e d c o n d i t i o n s , and u n f o r t u n a t e l y , no g e n e r a l order of n u c l e o p h i l i c i t y i s known. The monomer b a s i c i t y , t h a t i s , i t s a b i l i t y t o i n t e r a c t w i t h a p r o t o n , can be measured from thermodynamically c o n t r o l l e d c o n d i t i o n s . The most common method used to determine the b a s i c i t y i s based on the p r o p o r t i o n of hydrogen-bonded compound measured at e q u i l i b r i u m . The b a s i c i t y decreases i n the f o l l o w i n g order: R3N > R3P > R 0 > R S , although R S i s more basic than R 0 when the a b i l i t y of bonding with softer a c i d s i s measured. In the case of c y c l i c ethejrs, the b a s i c i t y order i s as f o l l o w s : C @ H ) > tf(CH ) > # ( C H ) > ^ £ p H ) . U s u a l l y the b a s i c i t y o f ^ h e t e r o c y c l i c compound i s a f f e c t e d i n t h e e x p e c t e d o r d e r by the i n d u c t i v e e f f e c t s , c o n j u g a t i o n , s t e r i c e f f e c t s , and r i n g s i z e (I). The order of b a s i c i t i e s i s the only estimation of the monomer n u c l e o p h i l i c i t i e s , and i t r e f l e c t s f a i r l y w e l l the o v e r a l l r e a c t i v i t y observed i n ring-opening c a t i o n i c polymerization. 2

2

2

2

2

2

3

2 4

2

5

2

4

2

Initiation. The most r e c e n t c l a s s i f i c a t i o n of i n i t i a t o r s f o r c a t i o n i c ring-opening polymerization was presented and discussed by Penczek et a l . (T). Only a few c l a s s e s of i n i t i a t o r s that are very u s e f u l both f o r m e c h a n i s t i c s t u d i e s as w e l l as f o r s y n t h e s i s of w e l l - d e f i n e d polymers w i l l be presented here. Protonic Acids. The simplest way of i n i t i a t i o n and polymerization by a protonic acid i s the f o l l o w i n g : k HA

H

+ zQ

—

^QA~

H - ( Z - ) —

H

+

- zQ

+ n Z +

Z 3 A "

A-

(Z—)—*\^)k - ^ H — ( Z — >

N

+

1

~

A

When A~ i s a noncomplex anion, that i s , Cl"~, FSO3", CF3COO"", C10 "~, or CF0SO3"", competition always e x i s t s between the propagation (k ) and the recombination o f the g r o w i n g m a c r o c a t i o n w i t h t h e counteranion (k*.). The r a t i o k /k^ w i l l c o n t r o l the polymerization degree of the obtained polymer. The k /k^ i s determined mainly by t h e r a t i o o f t h e monomer n u c l e o p h i l i c i t y t o t h a t o f the counteranion. F1uorosu1fonic acid (FSO3H and trifluoromethanesulfonic acid (CF3SO3H) are the most conventional i n i t i a t o r s used both for k i n e t i c studies as w e l l as for new monomer 4


98


r e a c t i v i t y testing studies. They a l r e a d y have r e p l a c e d the c o n v e n t i o n a l Lewis a c i d s such as A I C I 3 or B F f o r two r e a s o n s : Their anions are weak n u c l e o p h i l e s , and i n the case of h e t e r o c y c l i c monomers the i n i t i a t i o n t a k e s p l a c e by d i r e c t and q u a n t i t a t i v e protonation. At the other extreme of t h i s s i m p l e i n i t i a t o r c l a s s i s H C l . I t s anion i s a strong n u c l e o p h i l e , and only h i g h l y n u c l e o p h i l i c N7 substituted amines can be polymerized by H C l . Simple a d d i t i o n of the i n i t i a t o r to the f i r s t monomer m o l e c u l e takes p l a c e i n other cases: 3


+ HCl

> C1-CH -CH -0H 2

2

Stable Carbenion and Onium Ions and Their Covalent Precursors. The most representative i n i t i a t o r s from t h i s c l a s s are the f o l l o w i n g : 1. 2. 3. 4. 5.

+

+

Carbenium i o n s : R C , [(C^Hc) C A"] Alkoxycarbenium i o n s : ROCH , (CHoOCH? ^") Oxocarbenium i o n s : R-C=0 , (C^HrCio+A ) Onium i o n s : R X+, [(C H )o0 A ] Covalent i n i t i a t o r s : RA, [CH 0S0 CF , ( C F S 0 ) 0 , C H I , 3

3

4

2

+

+

n

2

5

2

3

3

3

2

2

3

etc.]

Carbenium Ions. S t a b l e carbenium i o n s ( t r i p h e n y l m e t h y 1 and tropylium s a l t s ) were developed by Ledwith (21-23). Their merit i s t h a t they can i n i t i a t e the p o l y m e r i z a t i o n of c e r t a i n o l e f i n s by d i r e c t a d d i t i o n . These i n i t i a t o r s are very u s e f u l i n k i n e t i c studies, e s p e c i a l l y when weak n u c l e o p h i l e s such as SbF^~ or AcF^~ are used as counteranions. Stable t r i t y l s a l t s , which might lead to systems devoid of side reactions, do not, however, i n i t i a t e the polymerization of the majority of h e t e r o c y c l i c monomers by d i r e c t a d d i t i o n (24, 25). On the other hand these i n i t i a t o r s can be produced i n s i t u by the reaction of a s u i t a b l e organic h a l i d e with a silver salt: R R-C-X + AgSbF R

6

R-C ! R

+

SbFl + AgX +

Richards et a l . (26-29) i n v e s t i g a t e d the i n i t i a t i o n of t e t r a hydrofuran (THF) p o l y m e r i z a t i o n induced by a l a r g e v a r i e t y of organic h a l i d e s i n conjunction with AgPF^. The r e a c t i v i t y of the saturated a l k y l h a l i d e s are i n the anticipated sequence: i o d i d e > bromide > c h l o r i d e > f l u o r i d e . Cynnamyl bromide and jp_raethylbenzyl bromide are the most useful i n i t i a t o r s (28, 29), and 1 , 4 - d i b r o m o - 2 - b u t e n e and a , a ' - d i b r o m o x y l e n e are e x c e l l e n t difunctional i n i t i a t o r s . Franta et a l . (30) studied the e f f i c i e n c y and the mechanism of i n i t i a t i o n of the polymerization induced by s e v e r a l a l k y l h a l i d e s and AgSbF^. The i n i t i a t i o n occurs by a d d i t i o n , proton e l i m i n a t i o n , and/or hydride abstraction.


5.


PERCEC

Addition:

^

-f

+ fj>

CH„0 - &I SbF~ + HCl 0

CH OCH 3

2

A more d e t a i l e d discussion w i l l be presented i n another chapter. l , 3 - D i o x o l a n - 2 - y l i u m (Dioxolenium) S a l t s . Triphenylmethy1ium s a l t s react with dioxolane by hydride transfer to form the corresponding dioxolenium s a l t s (33), which react with nucleophiles e x c l u s i v e l y by a d d i t i o n . Ph Ph4

Ph

+

SbF^

+

< £ ] - >

Ph

Ph S b F

O

+n-l

«* >£-i

26.9 — 5.8 -0.15 —

19.8 19.7 1.97 -0.3 3.5

2

Reaction Path i n the System RX + THF + AgSbF (a=addition, H =proton e l i m i n a t i o n , and H = hydride abstraction)

6

+

A l k y l Halide (corresponding cation) +

(C H ) C 6

5

3

Mechanism of I n i t i a t i o n (X=Br) (X=C1) (X=I) H"

H"

H"

+

(C H ) C H 6

5

C H CH 6

a

2

5

+

a(29)

p—CH C^H CH2^" 3

4

(CH ) C 3

+

H

3

(CH ) CH 3

+

2

=

CH2 CH—CH2^" a

AgPF

6

H+

a

2

a

+

a

H+ H+ a/H

+

salt


H

+

5.


PERCEC

101

+

Onium Ions. Trialkyloxonium ions (R 0 A~) became the conventional i n i t i a t o r s f o r the c a t i o n i c r i n g - o p e n i n g p o l y m e r i z a t i o n of a l l c l a s s e s of h e t e r o c y c l e s ( c y c l i c a c e t a l s , e t h e r s , s u l f i d e s , l a c t o n e s , phosphates, and amines). They are prepared by two methods d e v e l o p e d by Meerwin (38) and Olah (39). Another more general and convenient synthesis method was recently developed by Penczek et a L (40): 3

R-C

ft


N

R f

+ 6

X

MtX

V

rI

R-CH5:

n

V

R—C—of

2 +/

+ Mtx

R

\

N

+

1

F

, + Q

n-1

MtX . . 1

R - C - O R ' + R ~0 MtX . ,

3

R f

n+l

Trialkyloxonium ions are strong a l k y l a t i n g agents and i n i t i a t e the polymerization of h e t e r o c y c l i c monomers by simple a l k y l a t i o n of the most n u c l e o p h i l i c s i t e of the monomer. The i n i t i a t i o n occurs q u a n t i t a t i v e l y and without side reactions when s t a b l e anions are used. Consequently, these i n i t i a t o r s are very useful for k i n e t i c measurements. The i n i t i a t i o n was f o l l o w e d d i r e c t l y by NMR spectroscopy i n the case of THF (41), c y c l i c s u l f i d e s (42), and c y c l i c e s t e r s of phosphonic a c i d such as 2 - m e t h o x y - 2 - o x o - l , 3 , 2 dioxaphosphorinane (43). +

+

2

5

C H - QQBF

3

2

5

_

+

A

(C H ) O 2

5

2

C o v a l e n t I n i t i a t o r s . The i n i t i a t i o n w i t h a l k y l a t i n g compounds depends both on the a b i l i t y of the i n i t i a t o r to form a c a t i o n and on the monomer n u c l e o p h i l i c i t y . Strong a l k y l a t i n g agents such as e s t e r s of s u p e r a c i d s ( C F S 0 R , F S 0 R , and C1S0 R) are a b l e to i n i t i a t e d i r e c t l y w i t h o u t s i d e s r e a c t i o n s both s t r o n g and weak n u c l e o p h i l i c monomers (44, 45). 3

C H OS0 CF 2

5

2

3

3

+ ( Q = =

3

3

+

C H - oQcF S0 2

5

3

3

Weak cationating agents such as a l k y l iodine, benzyl h a l i d e s , and methyl-j>-toluenesulfonate are able to i n i t i a t e the polymerization of strong n u c l e o p h i l i c monomers such as c y c l i c amines (46, 47) and c y c l i c imino ethers (48-52). C o m p e t i t i o n always e x i s t s between the n u c l e o p h i l i c i t y of the counteranion and that of the monomer when these types of i n i t i a t o r s are used. In the case of THF polymerization with superacid ester


102


i n i t i a t o r s , macroions and macroesters are i n e q u i l i b r i u m (44, 45). The polymerization of c y c l i c imino ethers takes place with either macroions or covalent species. The c l a s s i c example i s 2-methyl-2o x a z o l i n e , which p o l y m e r i z e s e x c l u s i v e l y v i a c o v a l e n t l y bonded a l k y l c h l o r i d e s p e c i e s ( b e n z y l c h l o r i d e i n i t i a t o r ) or v i a oxazolinium species (benzyl bromide i n i t i a t o r ) (51). C H CH X + 6

5

2

N

V

C

0

H

H

C

O

0

CH

X -

D I

|

Z

Z

c=o

I

I CH,


C^H CH -N-CH -CH ~X

X

6 5^ 2-V/ c

I CH

3

3

Cl

Cl,Br

The mechanistic difference i s e x p l a i n e d by the d i f f e r e n t n u c l e o p h i l i c i t i e s of the c o u n t e r a n i o n s C l ~ and B r " . D i c a t i o n i c a l l y terminated macromolecules can be o b t a i n e d by the i n i t i a t i o n w i t h anhydrides of s t r o n g p r o t o n i c a c i d s such as trifluoromethanesulfonic anhydride (53, 54). CF.SO^ ¥ ° 2 \ 0 + 0 CF S0 3

21 THF*

CF S0 3

|CF S0 3

2

3

THF

2

vl

CF -S0 -K)-(CH ) -O, 3

2

2

4

CF S0 3

3

P h o t o i n i t i a t o r s for Cationic Polymerization. Recently a c l a s s of p h o t o i n i t i a t o r s f o r c a t i o n i c p o l y m e r i z a t i o n was d i s c o v e r e d by C r i v e l l o et a l . (55). This c l a s s includes diaryliodonium ( S t r u c t u r e I) (56, 57), t r i a r y l s u l f o n i u m ( S t r u c t u r e I I ) (58-62), d i a l k y l p h e n a c y l s u l f o n i u m (Structure I I I ) (63), d i a l k y l - 4 - h y d r o x y phenylsulfonium s a l t s (Structure I V ) (64), and t r i a r y l s e l e n o n i u m s a l t s (Structure V) (65). Ar

Ar

V

Ar-S

+

ArCCH^-S 0

II Ar I + Ar-Se I Ar

where:

R'

III

BF,

AsF,

PF,

SbF,

In the absence of l i g h t these s a l t s are s t a b l e even at h i g h temperatures and do not e x h i b i t c a t a l y t i c a c t i v i t y . On i r r a d i a t i o n , for example i n the case of d i a r y l i o d o n i u m s a l t s , the major photochemical process that occurs i n v o l v e s the homolytic cleavage of a c a r b o n - i o d i n e bond to produce a s t r o n g a c i d HX ( i . e . , HBF^, HAsF^, HPF^, or HSbF^). These acids are among the strongest known


5.

PERCEC


and are e x c e l l e n t i n i t i a t o r s f o r c a t i o n i c p o l y m e r i z a t i o n . mechanism i s o u t l i n e d as f o l l o w s :

103

This

hv Major

+

+

Arl X~

> [Ar I X~] 2

+

[Ar I X~]*

> A r - I * + Ar + X""

A r - I * + Y-H

-> Ar-I -H+Y'

2

+

where Y i s a solvent or a monomer. +


Ar-I -H Minor

> A r - I + H+

+

+

[ A r l X ~ ] * + Y-H

> [Ar-Y-H] + A r l + X"

f

+

[Ar-Y-H] > ArY + H Both v i n y l i c (styrene, a-methylstyrene, and v i n y l ethers) and h e t e r o c y c l i c monomers ( c y c l i c ethers or epoxide, oxetane, THF, and trioxane; c y c l i c s u l f i d e s or propylene s u l f i d e and thietane; and lactones and spiro b i c y c l i c orthoesters) were polymerized at room temperature by p h o t o i n i t i a t i o n at wavelengths shorter than 360 nm. The photodecomposition of diaryliodonium s a l t s can be s e n s i t i z e d at wavelengths longer than 360 nm by the use of dyes such as Acridine orange, Acridine y e l l o w , Phosphine R, B e n z o f l a v i n , and S e t o f l a v i n T (66). T r i a r y l s u l f o n i u m , dialkylphenacylsulfonium, and d i a l k y l (4hydroxypheny1) s u l f o n i u m s a l t s were s e n s i t i z e d by p e r y l e n e and other p o l y n u c l e a r hydrocarbons (65, 67). Under these conditions, p h o t o i n i t i a t e d c a t i o n i c p o l y m e r i z a t i o n can be p e r f o r m e d by incandescent l i g h t sources or even ambient s u n l i g h t . The polymerization rate depends on both the monomer r e a c t i v i t y and the n u c l e o p h i l i c i t y of the counteranion of the i n i t i a t o r s a l t . The order of r e a c t i v i t y i n photoinduced polymerization c o r r e l a t e s w e l l with the known r e l a t i v e n u c l e o p h i l i c i t i e s of the anions, that i s , SbF^" > A s F " > P F - > B F ~ . During the photodecomposition, f r e e - r a d i c a l species (Ar* and Y ) are a l s o produced as transient intermediates. Therefore, the p h o t o l y s i s of the sulfonium s a l t s should a l s o i n i t i a t e the freer a d i c a l p o l y m e r i z a t i o n (59). The a m p h i f u n c t i o n a l c h a r a c t e r of s u l f o n i u m s a l t s was demonstrated by the f o l l o w i n g s e r i e s of experiments. I r r a d i a t i o n of an equimolar mixture of l,4cyclohexene oxide and methyl methacrylate with P h g S ^ b F ^ as the p h o t o i n i t i a t o r g a v e a m i x t u r e of two homopo1ymers. Thus, both c a t i o n i c (cyclohexene oxide) and f r e e - r a d i c a l ( m e t h y l methacrylate) p o l y m e r i z a t i o n s took p l a c e i n d e p e n d e n t l y . The same system containing 2 , 6 - d i - t e r t - b u t y l - 4 - m e t h y 1 phenol ( r a d i c a l i n h i b i t o r ) gave only poly(cyclohexene oxide). A l t e r n a t i v e l y the system with t r i e t h y l a m i n e (poison for c a t i o n i c species) y i e l d e d only poly(methyl methacrylate). Monomers such as g l y c i d y l a c r y l a t e and g l y c i d y l methacrylate that c o n t a i n f u n c t i o n a l groups c a p a b l e of c a t i o n i c and 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 s are c o n v e r t e d i n t o a c r o s s - l i n k e d i n s o l u b l e polymer. The use of these hybride photoi n i t i a t o r s i s very i n t e r e s t i n g i n the synthesis of interpenetrating network s t r u c t u r e s . 6

6

4

#

-



104

I r r a d i a t i o n of d i a l k y l p h e n a c y l s u l f o n i u m s a l t s a l s o produces strong protonic acids (63). 0

0 X

Ar-O-CH^-S^

—

Ar-C-CH»S' N


"R

+ HX R

However, u n l i k e the triary1su1fonium s a l t s , these compounds undergo r e v e r s i b l e photoinduced y l i d f o r m a t i o n r a t h e r than h o m o l y t i c carbon-sulfur bond cleavage. Because the rate of the thermal back reaction i s appreciable at room temperature, o n l y those monomers t h a t are more n u c l e o p h i l i c than the y l i d w i l l p o l y m e r i z e . Epoxides, v i n y l ethers, and c y c l i c a c e t a l s undergo f a c i l e c a t i o n i c p o l y m e r i z a t i o n when i r r a d i a t e d i n the presence of d i a l k y 1 phenacy 1 su 1 fonium s a l t s as p h o t o i n i t i a t o r s . The photodecomposition of d i a l k y 1-4-hydroxy phenyl sulfonium s a l t s (64) g i v e s r i s e to a r e s o n a n c e - s t a b i l i z e d y l i d and an a c i d HX. Styrene oxide, 1,4-cyclohexene oxide, trioxane, and v i n y l

/ S-Q-OH R

A

K

+ HX

^

3

S

+ / + \ J ether were polymerized with s a t i s f a c t o r y rates. However, THF, ecaprolactone, and a-methylstyrene could not be polymerized (64). Recently, Ledwith (68) continuing h i s i n t e r e s t i n the chemistry of cation r a d i c a l s (69, 70) demonstrated that the p h o t o i n i t i a t i o n by t r i a r y l a m i n i u r a , s u l f o n i u m , and iodonium s a l t s o c c u r s by a mechanism that i s different from that proposed by C r i v e l l o . Ar N 3

+ Ar N + H + 3

A r

A r

2

N

JF\ 3 I H ( J - R ^

0* Both c a t i o n r a d i c a l s and protons are r e s p o n s i b l e f o r the p h o t o i n i t i a t i o n w i t h these i n i t i a t o r s . S t a b l e c a t i o n r a d i c a l s based on phenothiazine or i t s d e r i v a t i v e s , and t r i a r y l p y r y l i u m and t h i o p y r y l i u m s a l t s are e x c e l l e n t p h o t o i n i t i a t o r s f o r d i f f e r e n t h e t e r o c y c l i c monomers (68). P h o t o i n i t i a t e d c a t i o n i c p o l y m e r i z a t i o n s are w i d e l y used f o r p h o t o c u r a b l e c o a t i n g s f o r c o a t i n g s of metal c o n t a i n e r s , wood, paper, and f l o o r t i l e s , and a l s o have c o n s i d e r a b l e promise i n a p p l i c a t i o n s i n v o l v i n g photoimaging. Epoxy-based photoresists with high r e s o l u t i o n have been developed, and the use of these m a t e r i a l s i n photography and p l a s t i c flexographic p r i n t i n g p l a t e s has been demonstrated. I n i t i a t i o n of C a t i o n i c P o l y m e r i z a t i o n by Free-Radical I n i t i a t o r s . A new procedure for the i n i t i a t i o n of c a t i o n i c polymerization was developed by Ledwith (13, 23, 2 L 21). This procedure consists of


5.

PERCEC


105

the o x i d a t i o n of the e l e c t r o n - d o n o r r a d i c a l s by a r y l d i a z o n i u r a , d i a r y l i o d o n i u m , and t r i a r y 1 s u l f o n i u m s a l t s c o n t a i n i n g a s t a b l e c o u n t e r a n i o n . The parent r a d i c a l can be o b t a i n e d by t h e r m a l or photochemical decomposition. AIBN

or hv

f

R' + THF

Ar I PF7 — ^ & n 2

RH +

•


\ PF~ + A r l + Ar

e

A

*

I

+

R + CH *CH —RCH„-CH — 2 i 2 | -e OR OR' 2

1

P

F

6

+

*> RCH -CH PF, 2 I 6 OR' 0

-

+ A r l + Ar

f

2, 2 - A z o b i s ( 2 - m e t h y l p r o p i o n i t r i l e ) (AIBN), b e n z o y l p e r o x i d e , phenylazotriphenylmethane, and b e n z p i n a c o l were used as thermal r a d i c a l i n i t i a t o r s . Phenylazotriphenylmethane i s e s p e c i a l l y an i n t e r e s t i n g r a d i c a l i n i t i a t o r because by i t s r a d i c a l oxidation, a well-known s t a b l e carbenium s a l t i s obtained. hv PhN = NCPho J

+

Ph C* + P h S P F " 3

3

-> Ph* + N + *CPho 9

z

or A 6

3

+

> Ph C PF ~ + Ph S + Ph* 3

6

2

2,2-Dimethoxy-2-phenylacetophenone, benzophenone, b e n z i l , and many o t h e r r a d i c a l p h o t o i n i t i a t o r s were used to induce the c a t i o n i c polymerization i n the presence of different oxidants. Transformation of Anionic P o l y m e r i z a t i o n i n t o C a t i o n i c P o l y m e r i z a t i o n . R i c h a r d s et a l . (26, 27, 73-75) proposed s e v e r a l methods for the transformation of a l i v i n g anionic polymeric chain end i n t o a c a t i o n i c one. Such a process r e q u i r e s t h r e e d i s t i n c t s t a g e s : polymerization of a monomer I by an anionic mechanism, and capping of the propagating end w i t h a s u i t a b l e but p o t e n t i a l l y r e a c t i v e functional group; i s o l a t i o n of polymer I , d i s s o l u t i o n i n a s o l v e n t s u i t a b l e f o r mechanism (2.), and a d d i t i o n of monomer I I ; and reaction, or change of conditions, to transform the f u n c t i o n a l i z e d end i n t o propagating species I I that w i l l polymerize monomer I I by a c a t i o n i c mechanism (73). Two s i m p l e ways are the r e a c t i o n of p o l y s t y r y l l i t h i u m w i t h excess bromine or ot,a -dibromoxylene. The c a t i o n i c i n i t i a t i o n can be c a r r i e d out by r e a c t i n g the l a b i l e h a l i d e end group w i t h a s i l v e r s a l t containing a weak n u c l e o p h i l i c anion. f



106

«M L i + B r

A*. MBr + L i B r +

n

— MBr + AgX —>*-M X + AgBr +

^~*f L i

+

- M C H - ^ J ; - C H B r + LiBr

+ BrCH -«)>-CH Br 2

2

2


—CH^Br + AgX

2

-CH

MCH 2

X + AgBr +

2

In both cases Wurtz condensation reactions do not a l l o w high chain end f u n c t i o n a l i t y to be obtained. •MBr

>«

•M-M

-+ LiBr

The t r a n s f o r m a t i o n of a n i o n i c l i v i n g c h a i n ends i n t o a G r i g n a r d l e s s r e a c t i v e chain end a l l o w s the Wurtz condensation reaction to be completely eliminated (75). Y i e l d s as high as 95% were obtained by using t h i s procedure. M"Li

+

-MMgBr + L i B r

+ MgBr

Even so, f o r p o l y s t y r e n e , b e n z y l bromide c h a i n ends were p r e f e r r e d over 1-bromoethylbenzene c h a i n ends f o r c a t i o n i c i n i t i a t i o n . 1-Bromoethylbenzene f u n c t i o n a l groups possess h y d r o gens on the 3-carbon atom and do not i n i t i a t e the p o l y m e r i z a t i o n e n t i r e l y by an a d d i t i v e p r o c e s s . The p r i n c i p a l s i d e r e a c t i o n i s one of B-hydrogen e l i m i n a t i o n (28). CH.-CHBr + AgPF J I , o r

»AgBr* + CH.-CH PF, J I o

CK =€H + HPF, 0

Propagation. The s t r u c t u r e of the growing s p e c i e s ( t e r t i a r y oxonium i o n s ) i n r i n g - o p e n i n g p o l y m e r i z a t i o n of s e v e r a l monomers was a l r e a d y c h a r a c t e r i z e d by NMR spectroscopy ( T a b l e I I I ) . Carbenium-oxonium e q u i l i b r i a were a l s o evidenced and measured. c/-jiast^ -OCH CH CH CH -O^J| — Slow The S^2 mechanism of propagation i n polymerization of h e t e r o c y c l i c monomers was g e n e r a l l y accepted. ^OCH CH CH CH 2

2

2

2

N

1

2

2

2

2

One of the most important achievements was the demonstration that the rate constant of propagation by free ions does not d i f f e r from t h a t by i o n p a i r s . A l s o the r a t e constant of p r o p a g a t i o n i s not affected by the counteranion nature (45).


5.


PERCEC

107

T a b l e I I I . Growing s p e c i e s i n C a t i o n i c P o l y m e r i z a t i o n of Heterocyclic Monomers Observed D i r e c t l y by H-NMR Spectroscopy Monomer

Structure of growing species (anions omitted)

r\

+

V

H

2-CH,

~~-CH -V

2

»

2

2


C

/

76

2

v

CH CH CH 0

0

0

-v^CH-V

77 C

,

,

0

0

H

,CH„CH 2-2-2 (

CH -0-CH 9

32

2

CH -0-CH 2

C

H

CH. H.C CH \ / 3

3

\

C

« 3 /

C

H

3

+

/

C

f

3

C t H

>

CH.

\

/ CH — S +

••

78

2

6CH3

3

2

y

— - C H -v- r

P 0

9

/ ^ ~ C H - 0

N

0

Reference

H

t H

2

2

X

/

C

79

3

H

3 80

— CH -N 2

CH CH

C H 3

m

2

3

3

ai -ai 2

N.

.6

V C H

3

-—CH -N' 2

+

N

f

CH


51

108


The l o n g d i s p u t e of the growing s p e c i e s s t r u c t u r e i n the polymerization of c y c l i c a c e t a l s seems to be at i t s end. Penczek et a l . (32) showed c l e a r l y t h a t propagation proceeds on l i n e a r growing species. Growth by r i n g expansion seems to be u n l i k e l y on the basis of the e x i s t i n g experimental evidence. The presence of the end groups was c l e a r l y demonstrated by H - , P [ H ] - N M R , and UV. D P c a l c u l a t e d from the end groups agrees w e l l w i t h the UP determined osmometrically. A l l e q u i l i b r i u m constants were measurea recently for t h i s propagation scheme: A

3 1

1

n

CH OCH


3

+ 2

+ S~J

^

C H

3

O C H

2

-

+

0 ^ ?

CH-^'^J

2

kj/ke^ = 3 • 10 , and explains the tendency of isomerization of l e s s s t a b l e (more s t r a i n e d ) five-membered r i n g s i n t o l e s s s t r a i n e d seven-membered ones. A more d e t a i l e d discussion on t h i s topic can be found i n a recent review (T). T e r m i n a t i o n and T r a n s f e r P r o c e s s e s . C l e a r e v i d e n c e about the mechanism of t e r m i n a t i o n and c h a i n t r a n s f e r processes can be obtained from the polymer chain end structure. One polymer chain end i s c o n t r o l l e d by the i n i t i a t i o n mechanism, and the second one i s c o n t r o l l e d by termination and/or chain transfer. The chemical s t r u c t u r e of the end groups has been s t u d i e d i n a few cases o n l y (I). S e v e r a l p e c u l i a r i t i e s of these r e a c t i o n s w i l l be o u t l i n e d here. Temporary T e r m i n a t i o n : R e v e r s i b l e Recombination with Noncomplex Anions. Temporary termination was evidenced for the f i r s t time i n the polymerization of c y c l i c imino ethers (51).

z»

~^ ~N-CH CH Br 2

2

2

0=0 CH

0

The same r e a c t i o n was r e c e n t l y evidenced i n the case of THF polymerization with CF3SO3" or F S O 3 " counteranions (I):

•V~O-(CH ) -O£) 2

4

/S^CH(CH ) - R - PTHF+PF " + N0C1 6

Recently Dreyfuss et a l . (113, 114) developed a new method for the d e t e r m i n a t i o n of the number of poly(THF) branches i n a g r a f t copolymer. The method i s based on the t e r m i n a t i o n of the l i v i n g c a t i o n i c c h a i n ends w i t h NH^OH-NH^Cl buffer and r e a c t i o n w i t h fluorescamine. The c h a i n ends c o n c e n t r a t i o n i s determined by fluorescent spectroscopy. Franta et a l . (115) synthesized graft copolymers by i n i t i a t i o n of THF polymerization from chloromethylated polystyrene, p a r t i a l l y brominated 1,4-polybutadiene, and a random copolymer of styrene and methacryloyl c h l o r i d e i n the presence of AgSbF^.


5.


PERCEC

113

A l a r g e number of g r a f t copolymers were obtained by Dreyfuss and Kennedy (126-128) by g r a f t i n g of pendant epoxy groups of a v a r i e t y of polymer backbones. This grafting mechanism i s based on Saegusa's f i n d i n g s t h a t s e v e r a l r i n g compounds are a b l e , i n conjunction with Lewis acids, to generate t e r t i a r y oxonium ions, the true i n i t i a t i n g species (129). Q>:BF

tjH

+

3

— ^

2

[>MH


0

2

60BF

3

(CH ) S] CH CH CH CH 0-CH -CHOBF " 2

4

n

2

2

2

2

2

3

Chloromethylated c r o s s - l i n k e d polystyrene was used to i n i t i a t e the g r a f t c o p o l y m e r i z a t i o n of s e v e r a l 2-substituted-2-oxazoline d e r i v a t i v e s (130, 131). A l l y l i c c h l o r i d e from 1 - c h l o r o - l , 3 butadiene-butadiene copolymer and from p o l y ( v i n y l c h l o r i d e ) was used to i n i t i a t e the graft copolymerization of 2-methyl-2-oxazoline (132, 133). The grafting onto method was used to prepare graft copolymers by deactivation reaction onto a backbone f i t t e d with n u c l e o p h i l i c sites. Franta et a l . (134) used t h i s technique to synthesize graft copolymers of poly(THF) w i t h n u c l e o p h i l i c backbones poly(j>dimethylaminostyrene) and p o l y ( 2 - v i n y l p y r i d i n e ) . f

?

h

^ 3

6

\

CH

3

CH

3

SbF

6

Richards et a l . (135) succeeded i n q u a n t i t a t i v e graft copolymeriz a t i o n of poly(THF) onto p o l y ( 4 - v i n y l p y r i d i n e ) . Goethals et a l . (89) a l s o used the grafting onto method to prepare graft copolymers i n a q u a n t i t a t i v e y i e l d by r e a c t i n g a l i v i n g p o l y ( N - t e r t b u t y l a z i r i d i n e ) w i t h p o l y ( 2 - v i n y l p y r i d i n e ) as a " d e a c t i v a t i n g " polymer. Block Copolymers. Several methods have already been used for the synthesis of block copolymers. The most conventional method, that i s , the addition of a second monomer to a l i v i n g polymer, does not produce the same spectacular r e s u l t s as i n anionic polymerization. Chain t r a n s f e r to polymer l i m i t s the u t i l i t y of t h i s method. A recent example was afforded by Penczek et a l . (136). The addition of the 1,3-dioxolane to the l i v i n g b i f u n c t i o n a l poly(l,3-dioxepane) leads to the formation of a block copolymer, but before the second monomer p o l y m e r i z e s c o m p l e t e l y , the t r a n s a c e t a l i z a t i o n process ( t r a n s f e r to polymer) l e a d s to the c o n v e r s i o n of the i n t e r n a l homoblock to a more or l e s s (depending on time) s t a t i s t i c a l copolymer. Thus, c o m p e t i t i o n of h o m o p r o p a g a t i o n and t r a n s a c e t a l i z a t i o n i s not i n f a v o r of f o r m a t i o n of the b l o c k copolymers with pure homoblocks, at l e a s t when the second block, being b u i l t on the a l r e a d y e x i s t i n g homoblock, i s formed more s l o w l y t h a n t h e p a r e n t h o m o b l o c k t h a t i s r e s h u f f l e d by transacetalization. In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.



114

In systems d e v o i d of c h a i n t r a n s f e r to polymer, f o r example, c y c l i c imino e t h e r s , t h i s method g i v e s r i s e to pure b l o c k copolymers (137, 138). A r e l a t e d method was d e v e l o p e d by Pepper and G o e t h a l s (139). Taking advantage of the dormant character of polystyrene prepared at low temperature w i t h p e r c h l o r i c a c i d ( i n the form of a macroester) they prepared w e l l - c h a r a c t e r i z e d b l o c k copolymers of polystyrene with c y c l i c amines. ABA and AB block copolymers were synthesized by the i n i t i a t i o n of 2 - o x a z o l i n e p o l y m e r i z a t i o n by a polymer c o n t a i n i n g t o s y l a t e (140-143) or a l k y l h a l i d e end groups (144-147). I n i t i a t i o n of THF polymerization from p o l y s t y r e n e or p o l y b u tadiene containing l a b i l e h a l i d e s as chain ends i n conjunction with s i l v e r s a l t s was used by R i c h a r d s et a l . (26, 27, 74) to i n i t i a t e the block copolymerization of THF. Richards et a l . (148) developed a new route f o r p r e p a r i n g b l o c k copolymers by a macrocation to macroanion t r a n s f o r m a t i o n . T h i s process c o n s i s t s i n r e a c t i n g l i v i n g poly(THF) w i t h the l i t h i u m s a l t of c i n n a m y l a l c o h o l to prepare a polymer p o s s e s s i n g a s t y r y l t e r m i n a l group. This reaction i s q u a n t i t a t i v e . The second stage i n v o l v e s the reaction of t h i s product with n-butyl l i t h i u m i n benzene to form an adduct to which a monomer such as styrene or isoprene i s added to prepare a b l o c k copolymer a n i o n i c a l l y . T h i s l a s t stage u n f o r t u n a t e l y operates with only 20% e f f i c i e n c y . The c o u p l i n g of an a n i o n i c l i v i n g polymer w i t h a c a t i o n i c l i v i n g polymer gives r i s e to AB or (AB) block copolymers. In the case of polystyrene with poly(THF) the coupling e f f i c i e n c y seems to depend on the n u c l e o p h i l i c i t y of the c o u n t e r a n i o n and of the a n i o n i c c h a i n ends. For example, the g r a f t i n g y i e l d i s very low when poly(THF) w i t h BF^~ c o u n t e r a n i o n i s used (188), and i t i n c r e a s e s i n the case of FSO^" c o u n t e r a n i o n (179). The y i e l d i s q u a n t i t a t i v e when c a r b o x y l a t i n g p o l y s t y r e n e anions are used (37, 150). M u l t i b l o c k copolymers ( A B ) were obtained by c o u p l i n g of d i a n i o n i c p o l y s t y r e n e w i t h d i c a t i o n i c poly(THF) (151, 152). The c o u p l i n g of l i v i n g a n i o n i c p o l y ( a - m e t h y l s t y r e n e ) w i t h c a t i o n i c l i v i n g poly(THF) occurs with only 20% e f f i c i e n c y . Proton transfer and h y d r i d e t r a n s f e r g i v e s r i s e t o p o l y ( T H F ) and p o l y ( a methylstyrene) with v i n y l i c end groups as byproducts (153). Attempts to produce b l o c k copolymers by c o u p l i n g of l i v i n g c a t i o n i c p o l y ( | l - t e r t - b u t y l a z i r i d i n e ) with l i v i n g anionic p o l y styrene f a i l e d because the r e s u l t was a mixture of the two homopo 1 ymers. H o w e v e r , when the c a r b a n i o n of the a n i o n i c polystyrene was f i r s t converted into a t h i o l a t e anion by reaction w i t h propylene s u l f i d e , c o u p l i n g with l i v i n g p o l y ( N - t e r t - b u t y l a z i r i d i n e ) was s u c c e s s f u l i n producing a b l o c k copolymer (89). ABA-type block copolymers were synthesized by reaction of a l i v i n g p o l y ( N - t e r t - b u t y l a z i r i d i n e ) w i t h t e l e c h e l i c amino- or c a r b o x y terminated polymers h a v i n g p o l y b u t a d i e n e or p o l y b u t a d i e n e - c o a c r y l o n i t r i l e as backbones (89). Chain t r a n s f e r to a second polymer can be e x p l o i t e d as a possible avenue for block copolymer synthesis. The homopolymerization of a given monomer A i n the presence of a preformed polymer B or the i n t e r a c t i o n between two homopolymers i n the presence of a c a t i o n i c i n i t i a t o r (_1, 153, 154) produce i n the f i r s t step of the reaction block copolymers. Synthesis of other block copolymers was n

n


5.

115


PERCEC

reviewed by Yamashita et a l . (152). Trimethy l c e l l u l o s e - [ ] > p o l y ( T H F ) ] - s t a r b l o c k copolymers were s y n t h e s i z e d by Feger and Cantow (156, 157). T r i m e t h y l c e l l u l o s e containing a l a b i l e c h l o r i n e end group was used to i n i t i a t e the l i v i n g polymerization of THF i n the presence of AgSbF^. The l i v i n g c h a i n end of t h i s AB b l o c k copolymer was reacted with p o l y ( 4 - v i n y l p y r i d i n e ) oligomers to form star-shaped block copolymers.


Cationic Polymerization of Olefins General Considerations. This f i e l d was presented i n a few recent monographs (_2, 3», L4, 158). Carbenium species are more a c t i v e than oxonium, sulfonium, or ammonium. Side reactions, or transfer to monomer and n u c l e o p h i l i c attack of the aromatic r i n g i n the case of styrene polymerization (to produce a 3-phenylindane type end groups) lead to low molecular weight polymers. The only way to avoid these side reactions i s to decrease the polymerization temperature. The f i r s t carbenium ion was observed by C - N M R spectroscopy i n 1979 (159). In these conditions a l l the mechanistic approaches are supported by i n d i r e c t evidence o n l y . In the case of r i n g - o p e n i n g p o l y m e r i z a t i o n , s e q u e n t i a l and f u n c t i o n a l polymers are s y n t h e s i z e d by u s i n g our knowledge of p o l y m e r i z a t i o n mechanisms. In the case of o l e f i n p o l y m e r i z a t i o n , s e q u e n t i a l copolymers are e v i d e n c e f o r the suggested mechanism of polymerization. Information about the propagation rate constants with free ions are o b t a i n e d , as i n the case of r i n g - o p e n i n g p o l y m e r i z a t i o n , by u s i n g s t a b l e carbenium i o n s as i n i t i a t o r s (23). Unfortunately these i n i t i a t o r s can be used only for the polymerization studies of very a c t i v e c a t i o n i c monomers ( i . e . , s t r o n g bases such as v i n y l e t h e r s or v i n y l d e r i v a t i v e s c o n t a i n i n g s t r o n g e l e c t r o n donor pendant groups) (23). Another way to determine propagation r a t e c o n s t a n t s w i t h free i o n s i s to use r a d i a t i o n - i n d u c e d i o n i c polymerization techniques (160). A new method for the study of nonstationary polymerization i s the flow and stopped-flow spectroscopy developed by Sawamoto and Higashimura (161, 162). A l t h o u g h these methods o f f e r the o n l y a v a i l a b l e data about polymerization k i n e t i c s through known species, t h e i r p r e p a r a t i v e a p p l i c a t i o n s are v e r y l i m i t e d . A number of useful discoveries are coming from Kennedy's laboratory (163-175). P a r t of these d i s c o v e r i e s w i l l be presented l a t e r . Kennedy's r e s e a r c h p h i l o s o p h y c o n s i s t s i n understanding the mechanisms of polymerization of conventional monomers, and i t s use i n the design of new polymeric materials (163) has proven very productive. 13

Graft Copolymers. As i n the case of ring-opening polymerization, l a b i l e h a l i d e s can be used i n conjunction with a Lewis acid i n t h i s case to produce carbenium species. I f the i n i t i a t i o n takes place by addition, graft copolymers can be obtained by the grafting from technique when the l a b i l e h a l i d e i s p a r t of a polymer c h a i n (10, 163-165). M P-Cl + E t A l C l > [P Et AlCl ~] > P - poly(M) + E t A l C l +

2

2

2


2

116


A large variety of graft copolymers was prepared by t h i s technique and some are presented i n the box (159). Under s u i t a b l e r e a c t i o n c o n d i t i o n s , g r a f t copolymers free of homopolymers c o u l d be prepared. I n i t i a t i o n of graft copolymerization by radiation-induced c a t i o n i c mechanism was recently reviewed by Stannett (160). This method i s e s p e c i a l l y u s e f u l f o r c a t i o n i c graft copolymerization from i n e r t polymer supports. I n i f e r Technique. I n i f e r s are b i f u n c t i o n a l i n i t i a t o r - c h a i n t r a n s f e r agents t h a t have been used f o r the p r e p a r a t i o n of a,0)d i f u n c t i o n a l p o l y i s o b u t y l e n e c a r r y i n g ^ ~ C H C ( C H q ) C l end groups Q 0 . 164, 166-168). The mechanism of the i n i f e r system based on dicumyl c h l o r i d e BClg and isobutylene i s o u t l i n e d below.


9

9


5.


PERCEC

117

Representative Graft Copolymers Prepared by Carbocationic Techniques I.

Elastomeric backbones A.

Elastomeric branches


a

Poly(butadiene-g-isobutylene) Poly[chloroprene-g-(isobutylene-co-isoprene)] Poly[isobutylene-co-isoprene)-g-chloroprene]

a

B.

Glassy branches a

Poly[(ethylene-co-propylene)-g-styrene] Poly[(isobutylene-co-isoprene)-g-styrene] '" Poly(butadiene-g-a-methylstyrene) a

a

C.

Two branches (bigrafts) 1.

A glassy and an elastomeric branch Poly[ethylene-co-propylene-co-1,4-hexadiene) -£-styrene-g-isobutylene] a , b

2.

Two glassy branches Poly[(ethylene-co-propylene-co-1,4-hexadiene) -g^styrene-g-a-methylstyrene] ' a

II.

b

Glassy backbones A.

Elastomeric branches P o l y ( v i n y l chloride-g-isobutylene) Chloromethylated polystyrene^g-polyisobutylene

B.

Glassy branches P o l y ( v i n y l chloride-^-styrene)

a

L i g h t l y chlorinated backbone used.

b

L i g h t l y brominated backbone used.



118


The f u n c t i o n a l i t y of the obtained t e l e c h e l i c polyisobutylenes i s a f f e c t e d m a i n l y by the i n t r a m o l e c u l a r c y c l o a l k y l a t i o n of the initiator.

temperature and s o l v e n t mixture c o n d i t i o n s (169, 170). By u s i n g tricumyl c h l o r i d e - B C ^ t r i n i f e r system, three-arm star t e l e c h e l i c polyisobutylenes carrying exactly three -C(CH ) C1 end groups have been synthesized by Kennedy et a l . (171, 172). 3

3

Q u a s i - l i v i n g Carbocationic P o l y m e r i z a t i o n . R e c e n t l y , Kennedy et a l . (165, 173) developed p o l y m e r i z a t i o n systems i n which under well-defined conditions (a s p e c i a l manner of continuous mixing of monomer w i t h i n i t i a t i n g systems), c h a i n t e r m i n a t i o n and c h a i n t r a n s f e r to monomer are r e v e r s i b l e or a v o i d a b l e , and f o r a l l p r a c t i c a l purposes the system behaves as i f R and R ^ are equal to zero. Fast R^ was achieved by premixing the ingredients of the i n i t i a t i n g systems. K i n e t i c equations have been d e r i v e d a c c o r d i n g to which the molecular weight of the polymer can be c o n t r o l l e d by the cumulative amount o f monomer a d d e d and i n i t i a l concentrations of i n i t i a t o r : B? = M -./[I] . This equation i s very s i m i l a r to that defining l i v i n g conditions: DP = [ M ] / [ I ] . A few studied systems that polymerize under q u a s i - l i v i n g conditions are HoO-BClo-a-methylstyrene, C ^ H ^ ^ ^ C l - B C ^ - a - m e t h y l s t y r e n e , tert-BuCl-TiCl -isobutylene. t

t Q t a

t x

Q

n

Q

4

Proton Traps. 2 , 6 - D i - t e r t - b u t y l - 4 - m e t h y l pyridine (DBMP) and 2,6d i - t e r t - b u t y l p y r i d i n e (DtBP) are hindered bases i n c a p a b l e of r e a c t i n g w i t h e l e c t r o p h i l e s other than p r o t o n i c a c i d s . Consequently, they can be s u c c e s s f u l l y used for the trapping of protons during t h e i r transfer to monomer (174, 175). At the same time they can disseminate between the two major i n i t i a t i o n mechanisms encountered i n c a t i o n i c p o l y m e r i z a t i o n , t h a t i s , p r o t o n i c i n i t i a t i o n or carbenium i n i t i a t i o n (176). Block Copolymers. Two conventional techniques were applied for the s y n t h e s i s of b l o c k c o p o l y m e r s : i n i t i a t i o n o f monomer polymerization from a preformed polymer containing an i n i t i a t o r as chain end or ends, and the addition of a second monomer to a l i v i n g polymerization of the f i r s t one.


5. PERCEC


Poly(a-methy1 s t y r e n e - ] ) - i s o b u t y l e n e - J ^ - a - m e t h y l s t y r e n e ) was prepared by the i n i t i a t i o n of a-methylstyrene polymerization from a d i t e l e c h e l i c polyisobutylene containing t e r t - c h l o r i n e end groups. A l E t 2 C l was used as c o i n i t i a t o r (177). Poly(isobutylene-b-styrene) and poly(isobutylene-_b-ot-methy 1 s t y r e n e ) were prepared by the i n i t i a t i o n of s t y r e n e and a-methylstyrene polymerization from an asymmetric t e l e c h e l i c p o l y i s o b u t y l e n e , t h a t i s , (CHo^-CsCh-CH?" PiB—CH -C(CH ) C1 and A l E t C l (178). Addition of a second monomer to a l i v i n g polymer chain was used to produce block copolymers from N - v i n y l c a r b a z o l e and v i n y l ethers by u s i n g s t a b l e carbenium s a l t s as i n i t i a t o r s (179). The same avenue was used by Higashimura et a l . (180) to produce a b l o c k copolymer by i n i t i a t i o n of the p o l y m e r i z a t i o n of i s o b u t y l v i n y l e t h e r from a l o n g - l i v e d po 1 y - j ) - m e t h o x y s t y r e n e . Living p o l y m e r i z a t i o n of j)-methoxystyrene | N - v i n y l c a r b a z o l e has s u c c e s s f u l l y been achieved by using iodine as i n i t i a t o r (180, 181). By using a programmed successive a d d i t i o n of monomers, G i u s t i (182) s u c c e s s f u l l y p r e p a r e d b l o c k c o p o l y m e r s from s t y r e n e and isobutylene. A d e t a i l e d d e s c r i p t i o n of the sequential copolymer synthesis by carbocationic polymerization i s presented i n a recent book (3). 2


119

3

2

2

a n (

Heterogeneous G r a f t Copolymerization. P o l y ( v i n y l c h l o r i d e ) f i l m s and powders (183) and chlorinated polypropylene (184) f i b e r s were g r a f t e d w i t h s t y r e n e , i s o b u t y l e n e , and s t y r e n e , r e s p e c t i v e l y . Grafting from techniques were used. By using the same technique a s i l i c a surface f i r s t treated with c h l o r o s i l y l functional groups was grafted with polyisobutylene and b u t y l rubber (185, 186): CHo CHo J "HCl ) -Si-OH + Cl-Si-CH CH2CH Cl2 > -SiO-SiQ^Ch^dCh^Cl + A l E t C l 3

3

2

2

2

^H^ isobutylene

-Si-g-polyisobutylene

Grafting on technique was used to produce a graft copolymer s i l i c a & - p o l y ( J N - t e r t - b u t y l a z i r i d i n e ) (187). The technique used was the c o u p l i n g of a s i l i c a - c o n t a i n i n g amine group w i t h a t e m p o r a r i l y l i v i n g poly(tert-butylaziridine). -0*f -OH

+ (Et 0) Si-(CH ) NH 3

3

2

3

—*

poly (TBJ^+N^J

poly (TBA^ N^J t r

-0^Si(CH ) NH 2

3

2

+

^Si(CH ) N^poly(TBA) 2

3

\

or +

2

-I- N H ( C H ) S i ( O E t ) — ^ poly ( i B ^ - p o l y (TBA)NH(CH > Si(OEt) 2

2

3

3


2

3

3


120

These heterophase methods are very i n t e r e s t i n g academic as w e l l as t e c h n o l o g i c a l point of view.

from both an


Polymers with Functional End Groups Polymers w i t h Two F u n c t i o n a l End G r o u p s : T e l e c h e l i c s . In accordance w i t h t h e i r h i s t o r i c a l appearance, polymers w i t h two functional end groups w i l l be considered f i r s t . A very large range of t e l e c h e l i c p o l y i s o b u t y l e n e s ( P I B ) were s y n t h e s i z e d and characterized by Kennedy and h i s coworkers (165). These data were a l r e a d y reviewed s e v e r a l times (_3, 165, 188). A few avenues f o r PIB t e l e c h e l i c s preparation based on the i n i f e r technique w i l l be presented. The dehydrochlorination of a,u>-di(t:ert-chloro)polyisobutylene led to a,a>-di(isopropenyl)polyisobutylene (189) i n the quantitative y i e l d . The r e g i o s e l e c t i v e hydroboration of a,a)d i ( i s o p r o p e n y l ) p o l y i s o b u t y l e n e f o l l o w e d by a l k a l i n e hydrogen peroxide o x i d a t i o n l e d to a new t e l e c h e l i c polyisobutylene d i o l carrying two terminal primary hydroxyl end groups (190), that i s , a, u)-di( hydroxy) poly isobutylene.

tBuOK THF

The same chemistry was used f o r the three-arm s t a r t e l e c h e l i c polyisobutylene synthesis (171). These two functional groups (that i s , p r o p e n y l and h y d r o x y l ) a f f o r d the p o s s i b i l i t y of almost any k i n d of f u n c t i o n a l groups to be i n t r o d u c e d at the c h a i n ends of polyisobutylene. By d e r i v a t i o n of hydroxyl and propenyl terminated polyisobutylene, t e l e c h e l i c s c o n t a i n i n g c a r b o x y l i c (191, 192), - S i C l and H-SiH (193, 194), a c r y l o y l and m e t h a c r y l o y l (195), o x y c a r b o n y l (196), amine, cyanato (197), e t h y n y l , n i t r i l e (198), p h e n o l , and epoxy (199) groups have been s y n t h e s i z e d . These materials are useful for a large v a r i e t y of a p p l i c a t i o n s such as chain extension, networks, and block copolymers.


5.

PERCEC


121

a , U ) - D i ( a c r y l o y l ) p o l y ( T H F ) and a,o)-di(raethacryloyl)poly(THF) were prepared by the p o l y m e r i z a t i o n of THF i n i t i a t e d by p r o t o n i c acids (HSbF^ or CF3SO3H) i n the presence of a c r y l i c or methacrylic anhydride as a transfer agent (200). On the basis of the fact that t r a c e s of a c r y l i c o r m e t h a c r y l i c a c i d s a c c e l e r a t e the polymerization, the f o l l o w i n g mechanism of reaction was proposed: n — > A ^ A

+ AcOH

&

^ ^

~-0Ac + H-0 2

4

N

2

RfO(CH ) ]4Q-CH-CH 2

4

2

Copolymers w i t h s t y r e n e and a l k y l methacrylates were synthesized a l s o . The coupling method was i n i t i a l l y developed by Asami et a l . (200, 210) who succeeded i n preparing polymerizable oligomers from l i v i n g poly(THF) by coupling with v i n y l phenolate (CHo=CH-C H -0") or w i t h v i n y l b e n z y l a l c o h o l a t e ( C H = C H - C H - C H 0 ~ ) . The homopolymerization and c o p o l y m e r i z a t i o n of these Macromers were s t u d i e d (209). Another Macromer was designed by G o e t h a l s and V l e g e l s (211). The deactivation of the a c t i v e species (aziridinium ions) of l i v i n g 6

2

6

4

2


4

5.

PERCEC


p o l y ( l - t e r t - b u t y l a z i r i d i n e ) by m e t h a c r y l i c a c i d corresponding polyamine-methacrylate ester Macromer. ^ < J

CF S0


3

+

C F

3

S 0

3

C H CH

3

C F

3

led

123 to

the

S O

3

3

^SN^ + "0-5-

-

2

Additional Research Reports The 6th I n t e r n a t i o n a l Symposium on C a t i o n i c P o l y m e r i z a t i o n and Related Processes was held i n Ghent, Belgium (August 30 - September 2, 1983). I t s Proceedings were published as a book (212). Recent review a r t i c l e s on the f o l l o w i n g topics were published: the controversy concerning the c a t i o n i c ring-opening polymerization of c y c l i c a c e t a l s ( 2 1 3 ) , p h o t o i n i t i a t o r s for cationic p o l y m e r i z a t i o n ( 2 1 4 ) , l i v i n g p o l y m e r i z a t i o n and s e l e c t i v e dimerization (215), macromonomers (216), and f u n c t i o n a l polymers and sequential copolymers by carbocationic polymerization (217). Two s p e c i a l i s s u e s c o n t a i n i n g Kennedy's work on the use of s t e r i c a l l y hindered amines i n c a r b o c a t i o n i c polymerization (218) and on q u a s i - l i v i n g carbocationic p o l y m e r i z a t i o n (219) were a l s o published. Acknowledgment The author wishes to e x p r e s s h i s g r a t i t u d e to S. Penczek f o r h i s c a r e f u l r e a d i n g and c r i t i c i s m of t h i s manuscript and to J . P. Kennedy f o r many d i s c u s s i o n s . The f i n a n c i a l support of the N a t i o n a l S c i e n c e Foundation i s g r a t e f u l l y acknowledged. (Grant DMR: 82-13895) L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

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