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Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch028
Present View of the Anionic Polymerization of Methyl Methacrylate and Related Esters in Polar Solvents AXEL H. E. MÜLLER Institut für Physikalische Chemie, Universitat Mainz, D-6500 Mainz, West Germany A review is given on the kinetics of the anionic polymerization of methyl methacrylate and t e r t . - b u t y l methacrylate in tetrahydrofuran and 1,2-dimethoxyethane, including major results of the author's l a boratory. The Arrhenius plots for the propagation reaction are linear and independent of the counterion (i.e. Na , Cs ). The results are discussed assuming the active centre to be a contact ion pair with an enolate-like anion, the counterion thus exhibiting little influence on the reactivity of the carbanion. The kinetics of the termination reaction seem to be very complex and still are not f u l l y understood. From the kinetic results it is very l i k e l y that side reactions occuring in the i n i t i a t i o n process are responsible for chain termination during polymerization. +
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The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l-4) . For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7) . Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8) . On the other hand, the anionic polymerization of polar monomers appears to be very complicated. This is due to the functional groups which give rise to physical interactions (e.g. association), as well as chemical side reactions (e. g. termination 0097-6156/81/0166-0441$05.25/0 © 1981 American Chemical Society In Anionic Polymerization; McGrath, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch028
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ANIONIC POLYMERIZATION
and t r a n s f e r ) , e s p e c i a l l y in non-polar s o l v e n t s . In the case o f the a n i o n i c p o l y m e r i z a t i o n o f methyl methacrylate (MMA) in toluene or toluene/THF mixtures t h i s was demonstrated by broad and m u l t i modal molecular weight d i s t r i b u t i o n s , the e x i s t e n c e o f high amounts o f o l i g o m e r i c side-products and complex k i n e t i c s which still are not f u l l y understood ( f o r reviews see 2/I2/JLLJ . Since the d i s c o v e r y o f Fox e t a l . (12) t h a t in the a n i o n i c p o l y m e r i z a t i o n o f MMA the t a c t i c i t y o f the r e s u l t i n g polymer is f u l l y c o n t r o l l e d by the experimental c o n d i t i o n s , many attempts have been made t o e l u c i d a t e s t r u c t u r e s o f the a c t i v e species r e s p o n s i b l e f o r the t a c t i c i t i e s obtained under d i f f e r e n t c o n d i t i o n s . However, t h i s can o n l y be achieved, if a complete a n a l y s i s o f the p o l y m e r i z a t i o n process is performed by i n v e s t i g a t i n g the k i n e t i c s of p o l y m e r i z a t i o n in d i f f e r e n t systems, as has been done f o r styrene. The s i d e r e a c t i o n s complicating the k i n e t i c s in non-polar s o l v e n t s are decreased t o a h i g h extent, when p o l a r s o l v e n t s a r e used i n s t e a d o f non-polar ones. Wenger (13) reported t h a t PMMA-Li remained l i v i n g in t e t r a h y d r o f u r a n (THF) a t -78°C... f o r more than 4o h, which was proven by the p o l y m e r i z a t i o n o f new monomer added to the l i v i n g s o l u t i o n . By a comparison o f the number-average molecular weights o f the polymer before and a f t e r monomer a d d i t i o n Graham e t a l . (14) showed t h a t under s i m i l a r c o n d i t i o n s no new chains are formed from t r a n s f e r , when new monomer is added, so t h a t the p o l y m e r i z a t i o n is i n i t i a t e d by l i v i n g PMMA-Li. Roig e t a l . (150 i n v e s t i g a t e d the molecular weight d i s t r i b u t i o n s o f PMMAs prepared in THF and 1,2-dimethoxyethane (DME), u s i n g b i p h e n y l sodium as the i n i t i a t o r and Wenger s technique o f slow d e s t i n a t i o n o f the monomer t o the i n i t i a t o r s o l u t i o n . The molecular weight d i s t r i b u t i o n s r e p o r t e d are eery narrow(l.o2 • M /M • 1.ο6). Under the same experimental c o n d i t i o n s Guzman and B e l l o (16) reported bimodal d i s t r i b u t i o n s . T h i s was a t t r i b u t e d t o an e q u i l i brium between two a c t i v e species (anion and i o n p a i r ) with slow i n t e r c o n v e r s i o n . However, from the r a t e s o f d i s s o c i a t i o n known from other systems, t h i s explanation is not probable (
