15 Redox Polymerization i n Emulsion
Downloaded by MONASH UNIV on December 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch015
HENRY WARSON Solihull Chemical Services, 284 Warwick Rd., Solihull, England, B92 7AF
Redox polymerisation, the use of an oxidizing and a reducing agent simultaneously to ensure a rapid free radical polymerisat ion at relatively low temperatures, i.e. ambient or even subambient, was discovered independently in the USA, Great Britain and Germany during the period 1940-5, but not generally published until 1946. Since work prior to 1955 has been reviewed in detail (1), this paper will be restricted to mention of a few newer systems, and to some ideas of the mechanisms of redox systems. THEORETICAL TRENDS The vast number of redox processes which have been disclosed and which will be the subject of a separate publication are not matched by theoretical developments in elucidating the systems. A number of general features are becoming slowly established. One is the pH sensitivity of many systems, particularly where there is a third component. Thus whereas the potassium persulf ate - sodium bisulfite system is accelerated by Fe in acidic media, acceleration of polymerisation occurs with Cu in an alkaline medium. Unusually with Cu , a second peak occurs at alkaline pH if both methyl methacrylate and vinyl acetate are added, one of the few examples where a vinyl acetate copolymer is formed in the presence of a copper compound. (2). Complex formation is indicated, producing active intermediates which form radicals. (3) Vinyl bromide is polymerised in presence of H O and a chelate iron compound at 32°. (4) This has already been mentioned by Bacon(1), whilst coordination compounds and their effect on radical polymerisation have been discussed. (5) Redox systems which have been the subject of recent examinC at ion include potassium permanganate - tartaric acid (6), and potassium persulfate - ascorbic acid. (7) Whilst experiments were with the water soluble acrylamide, they should be adaptable to emulsion conditions. The ascorbic acid reductant is of inter* est as it is not interfered with by air or monomer stabilisers. With KlkiO - tartaric acid, the termination rate seems to be 2+
+
+
2
2
A
228
In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
15.
WARSON
Redox Polymerization
in
Emulsion
229
bimolecular. In the view o f the c u r r e n t w r i t e r i t i s probable that i n most cases the reducing moiety p l a y s the major p a r t i n forming the radT i c a l which i n i t i a t e s p o l y m e r i s a t i o n . T h i s was suggested by S u l l y (8), but evidence, a l b e i t i n d i r e c t , comes from another source. ''Ihen a p e r s u l f a t e - b i s u l f i t e redox i n i t i a t i o n o f v i n y l acetate i s performed, and* the r e s u l t a n t polymer f i l m , o p t i o n a l l y pigmented a p p l i e d to a s t e e l s u b s t r a t e , i t has been observed that t h i s f i l m i s much more e f f i c i e n t i n imparting c o r r o s i o n r e s i s t a n c e than one prepared with i d e n t i c a l s t a b i l i s e r s , but with a p e r s u l f a t e i n i t C i a t o r o n l y . The e f f e c t i s most marked with copolymers o f v i n y l acetate and the v i n y l e s t e r o f a h i g h l y branched C- a c i d , known as "Veova" (Europe) o r "W10" (USA - S h e l l Chemicals). T h i s may be accounted f o r by assuming that the r a d i c a l i n i t i a t i n g polym2 e r i s a t i o n i s such that there i s a s u l f u r to carbon bond, XD^S.CH CH(0C0CH ) as an end group r a t h e r than an oxygen to c a r b l n bond as XD S0CH CH(0C0CE ) The l a t t e r would probably occur w i t h a r a d i c a l *S0 . d e r i v e d from a p e r s u l f a t e . Whilst a s u l f a t e end group c o u l d Be assumed to hydrolyse f a i r l y r e a d i l y , hence g i v i n g r i s e to c o r r o s i o n o f a s u b s t r a t e , hydrolysis i s much l e s s l i k e l y to occur where there i s a s u l f o n a t e end group as would probably r e s u l t from a r a d i c a l d e r i v e d from a s u l f u r reducing a c i d . I t i s also to be noted that i n the hypophosphorous a c i d - diazonium s a l t system, to be mentioned l a t e r , there i s strong evidence that there i s bound phosphorus i n the p o l y ( a c r y l C o n i t r i l e formed. (9) (10) (40) Another i n t e r e s t i n g f a c t o r i n emulsion p o l y m e r i s a t i o n i s v a r y i n g s o l u b i l i t i e s o f the i n i t i a t o r s i n the monomers. One suggestion i s that the most a c t i v e hydroperoxides are those which are the l e a s t s o l u b l e i n the aqueous phase. Reductants are not i n * d i c a t e d , the r e f e r e n c e being to USSR rubber p r o d u c t i o n . An expiant a t i o n might w e l l be that the r a d i c a l - producing r e a c t i o n i s funC damentally a s u r f a c e one, under which c o n d i t i o n s byproducts and secondary r e a c t i o n s are at a minimum. The d i f f e r e n t i a l r e a c t i v i t y i e s o f the hydroperoxides might a l s o be a f a c t o r , and c a r e f u l comparisons would be necessary, o p e r a t i n g a t such temperatures that t h e i r h a l f l i v e s were equal, ( l l ) There seems l i t t l e doubt that the most e f f i c i e n t way o f o p e r a t i n g a redox system i s to add the components i n stages as suggested i n e a r l i e r p u b l i c a t i o n s , by gradual a d d i t i o n , o r by adding 1 component, u s u a l l y the per-compound, a t the s t a r t , and adding the reductant g r a d u a l l y . ( 1 2 ) ( 1 3 ) . Some e m u l s i f i e r s a c t d i r e c t l y as reductants. T h i s a p p l i e s p a r t i c u l a r l y to quaternary ammonium compounds. Thus c e t y l p y r i d i n e ium bromide, with benzoyl eroxide o r hydrogen peroxide w i l l i n i t S i a t e the p o l y m e r i s a t i o n o f chloroprene, methyl methacrylate and styrene i n emulsion, although the mechanism i s not c l e a r f possibly there may be a r a d i c a l formed from the quaternary n i t r o g e n . I t i s i n t e r e s t i n g to note that p o l y s t y r e n e , prepared i n emulsion with c e t y l p y r i d i n i u m bromide and hydrogen peroxide , has a syndiotacC t i c s t r u c t u r e , suggesting that under some c o n d i t i o n s these
Downloaded by MONASH UNIV on December 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch015
0
In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
EMULSION
230
POLYMERIZATION
systems a r e s t e r e o - d i r e c t i v e . ( 1 4 ) ( 1 5 ) I t i s p o s s i b l e that under s p e c i a l c o n d i t i o n s the hydroxyl groups i n many nonionic surfact£ ants may have s u i t a b l e reducing p r o p e r t i e s f o r redox i n i t i a t i o n * P o l y v i n y l a l c o h o l may a l s o f u n c t i o n as a redox component under some circumstances*(see below)
Downloaded by MONASH UNIV on December 8, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch015
Graft
Copolymers
G r a f t copolymerisation on to an e x i s t i n g polymer backbone has proved to be one o f the major i n t e r e s t s with redox polymerisC a t i o n systems. The main f a c t o r f o r e f f i c i e n c y i s the removal o f a hydrogen atom from a C atom i n the polymer backbone to produce a r a d i c a l on which a branch c h a i n w i l l form. The redueià*nt probably f u n c t i o n s by a s s i s t i n g i n the r a p i d formation erf hydroperoxide r a d i c a l s i n the f i r s t p l a c e , e.g., from cumene hydroperoxide, probably the most f r e q u e n t l y encountered o f g r a f t i n i t i a t o r s . I t i s used i n c o n j u n c t i o n with sodium s u l f i t e , sodium formaldehyde s u l f o x y l a t e , dextrose and d e x t r i n , with a range o f polymers and copolymers. (16) ( 1 7 ) ( l 8 ) ( 1 9 ) · The v e r y frequent simultaneous i n c l u s i o n o f t e r t - d o d e c y l o r a s i m i l a r mercaptan, not o n l y with butadiene copolymers, suggests that i t p l a y s a d i r e c t p a r t i n the i n i t i a t i o n , r a t h e r than a c t as only a c h a i n t r a n s f e r agent.(20) P o l y v i n y l a l c o h o l , when used as an e m u l s i f i e r , undoubtedly a c t s as a g r a f t base to some extent, and on to which the monomer, u s u a l l y v i n y l a c e t a t e , forms branches during p o l y m e r i s a t i o n . I t s water s o l u b i l i t y a s s i s t s the e f f c i e n c y o f g r a f t i n g , which may be best with the p a r t i a l l y h y d r o l y s e d p o l y v i n y l acetate normally used as an emulsion s t a b i l i s e r . The type o f polymer, e i t h e r formed on a "seed" o f about 20 - 2 5 $ o f i t s e l f , o r by a "continuous process i s probably so h i g h l y branched as a r e s u l t o f m u l t i p l e g r a f t s that i t i s de f a c t o c r o s s l i n k e d , as shown by the l a c k o f s o l u b i l i t y i n e t h y l a l c o h o l . I n the standard p r e p a r a t i o n , with e i t h e r hydrogen peroxide o r t e r t - b u t y l hydroperoxide together with Na ( o r Zn) formaldehyde s u l f o x y l a t e , these a r e added c o n t i n C uously with monomer. These c o n d i t i o n s produce the maximum g r a f t , and a s t a b l e emulsion, o f p a r t i c l e s i z e about lfrm , although with a r a t h e r wide s c a t t e r o f s i z e s , i s formed, and the l a t e x does not g e l on a d d i t i o n o f e t h y l a l c o h o l . I t i s not p o s s i b l e to h y d r o l y s e these modified p o l y v i n y l acetates completely to p o l y t v i n y l a l c o h o l . The reducing groups i n t h i s case may p r o v i d e charged end groups to the polymer chains, improving l a t e x s t a b i l i t y . A recent paper discusses f u r t h e r d e t a i l s o f g r a f t copolymerisation i n emulsion. ( 2 1 ) 11
Chlproformic E s t e r with a Peroxide o r a P e r s u l f a t e
