16 Specific Interactions of Lithium Chloride in the Anionic
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Polymerization of Lactams GIORGIO BONTÁ, ALBERTO CIFERRI, and SAVERIO RUSSO Centro Studi Chimico-Fisici di Macromolecole Sintetiche e Naturali C.N.R. and Istituto di Chimica Industriale, University of Genoa, 16132 Genoa, Italy
The ranking of simple anions and cations according to their ability to affect the properties of two-and three-component systems began to be reported in the earlier days of colloid chemistry (1). In binary salt-water mixtures, for instance, a well defined order of effectiveness of anions and cations in altering the entropy of dilution, or in shifting the maximum of infrared band to ward the position corresponding to the vapor phase was observed (2-4). In ternary systems such as salt, water,and a polar solute (e.g., an aminoacid), an order of ion effectiveness in altering the solubility of the polar solute was reported (5). Quite generally, the solubility was increased at low salt concentration (salting-in) and decreased at relatively high salt concentration (salting-out). With some ions however, notably SCN , Br , Li , the salting-in effect was always more pronounced, and occurring in a larger salt concentration range, than for the other ions, the so called salting-out agents such as F , S0 , Mg . Corresponding eff e c t s on the a c t i v i t y c o e f f i c i e n t o f poorly s o l u b l e s a l t s i n e l e c t r o l y t i c s o l u t i o n s were, of c o u r s e , observed i n e a r l i e r v e r i f i c a t i o n s o f the Debye-Hückel theory ( 6 , 7 ) . However, no t h e o r e t i c a l e l a b o r a t i o n based on purely e l e c t r o s t a t i c c o n s i d e r a t i o n s , nor a l t e r n a t i v e approaches based on such concepts as ion h y d r a t i o n , wat e r s t r u c t u r e , c o m p r e s s i b i l i t y of s o l u t i o n s or s a l t a c t i v i t y , were r e a l l y s u c c e s f u l i n o f f e r i n g a u n i f i e d d e s c r i p t i o n o f the various s a l t e f f e c t s described above ( 6 , 7 ) . -
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When the r o l e of s a l t s on polymeric substances was c o n s i d e r e d , a s i m i l a r i t y with the e f f e c t s observed with simpler s o l u t e s was g e n e r a l l y f o u n d . For i n s t a n c e , the shrinkage temperature o f collagen tendons swollen i n aqueous s a l t s o l u t i o n i s depressed by i n c r e a s i n g the concentration o f s a l t s such as KSCN o r Li Br and increased by i n c r e a s i n g KF o r concentration (8-10).(Figure la). These e f f e c t s are obviously e q u i v a l e n t to an i n c r e a s e , and to a decrease, o f the s o l u b i l i t y of amorphous c o l l a g e n , r e s p e c t i vely ( 9 ) . Nagy and Jencks (11) i n v e s t i g a t e d the r o l e of s a l t s i n the polymerization o f G-actTTT i n aqueous s o l u t i o n and found that s a l t i n g - i n agents, such as L i B r , i n h i b i t the polymerization which, a g a i n , i s a manifestation o f a decrease o f the a c t i v i t y c o e f f i -
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Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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due to the s a l t .
E a r l i e r i n v e s t i g a t o r s Q2) d e a l i n g with s a l t - p o l y m e r i n t e r a c t i o n s introduced an i n t e r p r e t a t i o n o f the observed e f f e c t s whicha p p a r e n t l y - was not considered by the i n v e s t i g a t o r s who dealt with the behavior o f simpler s o l u t e s . The i n t e r p r e t a t i o n i s the c l a s s i c a l one o f s o l u b i l i z a t i o n with binding o f a s o l v e n t component t o the s o l u t e s p e c i e s . It now appears t h a t t h i s i n t e r p r e t a t i o n may have general v a l i d i t y (13-16). However, the a l t e r n a t i v e i n t e r p r e t a t i o n o f an i n d i r e c t s a l t e f f e c t - mediated by the r o l e o f s a l t on the water s t r u c t u r e (2,3)- stimulated debates i n the e a r l y twenties ( V ) , and i t i s s t i l l not d e f i n i t i v e l y abandoned (17). It was i n order t o obtain a compelling evidence i n favor o f the b i n d i n g model, and against the water s t r u c t u r e r o l e , t h a t we decided to i n v e s t i g a t e the r o l e o f s a l t s on p o l a r polymers i n the complete absence o f water (18-20). The r e s u l t s have demonstrated t h a t water s t r u c t u r e - o r water i t s e l f - has no p r e v a l e n t r o l e i n s a l t - p o l y m e r i n t e r a c t i o n . In f a c t those s a l t s which are able to depress the melting temperature o f polymers i n the presence o f water are a l s o able t o cause a s i m i l a r e f f e c t i n the absence o f water. This i s i l l u s t r a t e d , f o r i n s t a n c e , i n Figure 1 by the comparison o f the r o l e o f s a l t s i n depressing the shrinkage temper a t u r e o f swollen c o l l a g e n tendons and i n depressing the melting temperature o f p o l y p y r r o l i d o n e (nylon 4).
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Figure 1. (a) Effect of various salts on the melting temperature of collagen swollen in water; (b) Effect of Ltd on the melting and decomposition temperatures of polypyrrolidone
Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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While a d i r e c t binding of s a l t i n g - i n agents to swollen c o l l a gen could q u a n t i t a t i v e l y be determined (13), the f i n a l proof of the binding mechanism should be based oriThe corresponding demon s t r a t i o n o f s a l t - polymer adducts in the molten s t a t e , i n the absence o f water. The occurrence o f s p e c i f i c i n t e r a c t i o n s i n b i n a ry system i s demonstrated i n the present i n v e s t i g a t i o n i n the c a se of Li CI and lactams such as ε-caprolactam and p y r r o l i d o n e . The study o f binary p o l y m e r - s a l t systems - i n a d d i t i o n to i t s i n t e r e s t as a t o o l f o r e l u c i d a t i n g the p h y s i c a l chemistry o f the i n t e r a c t i o n - has i n d i c a t e d r e l e v a n t i m p l i c a t i o n s bearing on the processing o f s y n t h e t i c p o l a r polymers. The depression o f the melting temperature f o r polyamides o f the nylon s e r i e s (21) due t o O . O S L i C l mole f r a c t i o n i s e x h i b i t e d i n Figure 2. The i n c r e a -
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Figure 2. Difference between the melting temperature of pure polymer and that of LiCu-polymer mixtures containing 0.05 salt mole fraction as a function of the number of methylene groups per repeating unit
s i n g depression obtained with the more p o l a r members o f the s e r i e s implies the p o s s i b i l i t y o f processing temperatures c o n s i d e r a b l y below the conventional melting temperature of pure polymers. The b e n e f i c i a l e f f e c t o f s a l t s i s a l s o evidenced i n the a l t e r a t i o n o f other p r o p e r t i e s of the pure polymers - such as c r y s t a l l i z a t i o n r a t e ^ 9 ) and melt v i s c o s i t y (20)-which c o n t r o l the proces s i n g behavior. In the p a r t i c u l a r case o f p o l y p y r r o l i d o n e , i t i s known that the proximity o f decomposition and melting temperature has posed formidable d i f f i c u l t i e s to the processing of the pure polymer. The a d d i t i o n o f small amounts o f L i C l does not a f f e c t the decomposition temperature and allows a large depression o f the melting temperature (^40°C at 5% w/w) (21), thus allowing p r o cessing uncomplicated by thermal degradation. The presence of L i C l a l s o allows the obtainment o f nylon 6 f i b e r s with mechanical p r o p e r t i e s s u p e r i o r to those obtained i n the absence o f s a l t (22).
Saegusa and Goethals; Ring-Opening Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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r\eces_
In s p i t e of the above des cri bed b e n e f i c i a l e f f e c t s , the sary step o f a c c u r a t e l y mixing polymer with s a l t adds some d i f f i c u l t i e s , particularly i f a large-scale industrial application is c o n s i d e r e d . The p o s s i b i l i t y of polymerizing the monomer i n the pre sence of s a l t has been attempted i n order to avoid the mixing s t e p . Preliminary attempts to polymerize ε-caprolactam i n the presence of L i C l using the conventional h y d r o l i t i c p o l y m e r i z a t i o n f o r nylon 6 have g i v e n , however, discouraging r e s u l t s . The s a l t i n h i b i t s po l y m e r i z a t i o n and very low molecular weights(
