Ring-Opening Polymerization - ACS Publications - American

14 Cationic Heterocyclic Polymerization E. FRANTA and L. REIBEL

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: August 16, 1985 | doi: 10.1021/bk-1985-0286.ch014

Centre de Recherches sur les Macromolecules (CNRS), 6 rue Boussingault, 67083 Strasbourg Cédex, France

The preparation of well defined polymers and co polymers with low polydispersity in weight, composition and structure has been achieved within the last 20 years thanks to the advent of the living anionic technique (1,2), the number of relevant monomers being however limited. The extension of this approach to cationically polymerizable monomers has been attempted and some success achieved in the last few years concerning olefinic monomers, one giving rise to the so called "quasi living" polymers (3) and another one leading to a truly living system when one uses a particular iodine-based initiation in relation with some stabilized monomers (4). Other interesting candidates are the heterocyclic monomers. It has been shown that some cyclic imines can produce living systems (5) and that tetrahydrofuran polymerization propagates through a tertiary oxonium cation which is stable provided that the counter-ion is suitably chosen (6); the same is true for cyclic acetals such as dioxolane and dioxepane when proper initiators are chosen (7). For THF polymerization, i n order to obtain l i v i n g systems, i n i t i a t i o n has to be performed by appropriate i n i t i a t o r s such as t r i f l i e derivatives (8) or oxocarbenium salts (9). The use of organic halides i n conjunction with various s i l v e r salts has also been advocated (10-12), but c o n f l i c t i n g results have been reported concerning 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 . In order to use this s i l v e r salt based method to prepare copolymers, one has to know with accuracy how the halides carried by a polymer w i l l react with the s i l v e r salt used i f one i s to achieve controlled g r a f t i n g , i . e . determine beforehand the number and length of the grafts and to avoid the formation of homopolymer; therefore we have studied various organic halides that we have considered as modelcompounds . 0097-6156/85/0286-O183S06.00/0 © 1985 American Chemical Society

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

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RING-OPENING POLYMERIZATION


Experimental A l l of the experiments have been carried out under high vacuum. Materials have been p u r i f i e d according to standard procedures (12); k i n e t i c measurements by dilatometry have been described elsewhere (9^) as well as the polymerization procedure. *H NMR was used to characterize the i n i t i a t o r groups attached on oligopoly-THF formed as well as UV spectroscopy whenever possible; a vapor phase chromatograph coupled to a mass spectrometer was used to separate and i d e n t i f y low molecular weight compounds formed during i n i t i a t i o n . Molecular weight have been determined by gel permeation chromatography or osmometry. Mechanism and Kinetics Two d i f f e r e n t steps have to be distinguished: • the f i r s t one i s r e l a t i v e to the metathetic reaction between the organic halide and the s i l v e r s a l t :

RX + AgSbF

6

SbF

6

+ AgX

the carbenium formed i s very reactive and usually cannot be i s o l a t e d , though this has been possible i n a few cases: when chlorodiphenylmethane (13) or 9-bromofluorene (14) are the halides. the second step i s related to the i n i t i a t i o n i t s e l f . competitive ways are operative.

Several

a) I n i t i a t i o n by Addition.

the t e r t i a r y c y c l i c oxonium formed i n this process i s an e f f i c i e n t i n i t i a t o r for the polymerization of THF leading to a l i v i n g system. This i s the kind of i n i t i a t i o n that i s desirable for our purpose since i t produces narrow molecular weight d i s t r i bution for the poly-THF obtained, easy control of the number average degree of polymerization and i f the organic group i s a polymer, we w i l l obtain blocking or grafting - depending where the halide i s located - i n the absence of homo-poly-THF. b) I n i t i a t i o n by Proton Elimination.

This leads to the formation of an o l e f i n and to a secondary c y c l i c oxonium which i s a slow i n i t i a t o r for THF polymerization. Besides i f R i s a polymer, one obtains a mixture of homopolymers and no g r a f t i n g .


14.

Cationic Heterocyclic Polymerization

FRANTA AND REIBEL

c)

I n i t i a t i o n by Hydride Abstraction.

+

R SbF


185

6

+

•

RH + °^^J

S b F

6

This mechanism has been shown to be active when t r i t y l salts are used (15) but i s actually more complicated than proposed (13). It has not been observed for other i n i t i a t o r s so f a r . After the solution of the organic halide and of the s i l v e r s a l t have been mixed, the f i r s t reaction to take place i s the metathetic one, with i t s own k i n e t i c s which varies very much depending on the halide and pn the organic group. Then i n i t i a t i o n per se takes place with various possible mechanisms as we have seen above and also with various k i n e t i c s : fast i n the f i r s t case, ( i n i t i a t i o n by addition) slow i n the two other cases. It i s therefore meaningless and deceptive to try to characterize an i n i t i a t o r by simply observing the quantity of poly-THF formed after a certain time: involving two subsequent reactions this says nothing about the f r a c t i o n of organic halide having reacted and about the mechanism. This global estimation has been sometimes used i n the l i t e r a t u r e but i n our opinion should be avoided. We have, therefore, studied the reaction of various organic halides with a s i l v e r salt ( s i l v e r hexafluoroantimonate mostly but also s i l v e r t r i f l a t e when stated): dilatometric measurements of the k i n e t i c s of polymerization provide us with information r e l a t i v e to the o v e r a l l concentration i n active s i t e s involved i n the propagation. *H NMR determination conducted on polymers after p u r i f i c a t i o n enable us to determine the end groups and to relate them to the structure of the i n i t i a t o r , provided that the molecular weight of the samples are s u f f i c e n t l y low to allow for a suitable accuracy. UV spectroscopy was used for the same purpose when the organic halide carried a chromophore. These measurements enable us to determine the f r a c t i o n of i n i t i a t o r which has i n i t i a t e d the THF polymerization by addition. Whenever necessary we have analyzed the reaction mixture to determine the presence of low molecular weights originating from the i n i t i a t i o n process: a vapor phase chromatograph coupled to a mass spectrometer enabled us to separate and identify the low molecular weight compounds and thus to confirm our hypothesis concerning the mechanism. Molecular weight of the samples were determined usually by membrane osmometry and gel permeation chromatography was used to determine the molecular weight d i s t r i b u t i o n . The values obtained were then compared to the theoretical ones, supposing a l i v i n g system i . e . : each i n i t i a l organic halide gives r i s e to one macromolecule on which i t i s attached at the end and no transfer to monomer and no termination take place. Results and Discussion Influence of the Leaving Halide and the Organic Group. In order to determine the influence of the leaving group we have studied the series of a l l y l iodide, bromide and chloride: a l l y l iodide within minutes produces a fast and nearly quantitative (=£0%) i n i t i a t i o n ;



186

RING-OPENING POLYMERIZATION

less than quantitative i n i t i a t i o n i s probably due i n this case to d i f f i c u l t i e s i n handling the f a i r l y unstable s t a r t i n g material; quantitative s i l v e r iodide p r e c i p i t a t i o n i s immediate. A l l y l bromide i s much slower: only 50% of the s i l v e r bromide has precipitated a f t e r 120 minutes and the same proportion of active s i t e s i s created during this period as compared to the i n i t i a l l y present a l l y l bromide. A l l y l chloride i s s t i l l much slower: a f t e r two hours a small proportion of s i l v e r chloride has precipitated and less than 10% of the i n i t i a l l y present a l l y l chloride has produced an active s i t e . *H NMR determination on low molecular weight material enabled us to determine that for a l l y l iodide, a l l of the poly-THF chains carried an a l l y l group: t h i s i s not surprising since proton elimination i s not possible i n this case. Benzyl bromide and benzyl chloride were also studied and the difference in a c t i v i t y because of the leaving group was observed as well; benzyl bromide has quantitatively reacted after about 30 minutes: that i s , each molecule of organic halide produced one macromolecule and i s attached to i t . The chloride being not so good a leaving group, we observed that a f t e r 10 hours, only 8% of the halide had produced an active s i t e ; the benzyl group i s attached to the polymer which again i s not surprising since proton elimination i s not possible. Terminal iodide, bromide and chloride were also studied in d e t a i l ; one observes vast differences i n the rates of s i l v e r halide p r e c i p i t a t i o n : Although complete within minutes for the iodide, i t takes less than an hour for the bromide and several days for the c h l o r i d e . This i s c l e a r l y the trend that one expects because of the nature of the leaving group, but the proportion of active s i t e s versus time behaves s i m i l a r l y for the 3 halides (Figure 1); the chloride remains the slowest but the iodide and the bromide behave i d e n t i c a l l y , within experimental errors i . e . both are very slow i n i t i a t o r s : after 2 hours only about 12% of the i n i t i a l l y present halide have given r i s e to an active s i t e . Mass spectroscopy enabled us to characterize various compounds l i k e isobutene, two of i t s dimers ((CH3)3C-CH=C(CH3>2 and (CH3>3C-CH2—OCH2) and some trimers. We can conclude that CH i n i t i a t i o n by addition onto the t e r t i a r y b u t y l cation i s p r a c t i c a l l y non existent since i t could not be detected, but that i n i t i a t i o n took place by proton elimination producing isobutene; this l a t t e r , i n an a c i d i c medium, gives r i s e to some dimers and trimers terminated by a double bond after elimination (16). We have also used 2-iodopropane and 3-iodopentane as models for polybutadiene carrying some secondary iodide obtained after reacting HI onto polybutadiene under controlled conditions to avoid c y c l i z a t i o n . The results are presented on F i g . 2. Under our conditions 2-iodopropane i s a f a s t , quantitative i n i t i a t o r proceding by addition (16) through proton elimination i s t h e o r e t i c a l l y possible; 3-iodopropane, a better model for the polymer shows a d r a s t i c a l l y d i f f e r e n t behavior: only 25% of the i n i t i a t o r proceeds by addition, the rest i . e . 75% proceeds by elimination and indeed we determined the presence i n the solution of various compounds such as 1-pentene and 2-pentene, and several of their dimers and trimers 3


Cationic Heterocyclic Polymerization

FRANTA AND REIBEL


14.

lOOk-O

187

Ο—O-

50

-

10 _L

100

50 time (minutes) Figure 2.

E f f i c i e n c y of d i f f e r e n t secondary iodides i n the poly merization of THF: 2-iodopropane(o), 3-iodopentane(Q) , hydroiodated polybutadiene (...-CH -CH=CH-(CH )2-CHI(CH )3-CH=CH) (Δ); |M| = 12.3 M, 1.5 χ 10 M

Ring-Opening Polymerization - ACS Publications - American

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