Cationic polymerization of glycolide in the presence of antimony trifluoride

CATIONIC POLYMERIZATION OF GLYCOLIDE IN THE PRESENCE OF ANTIMONY TRIFLUORIDE* G. S. S~I~A, M. V. FOm-~A, A. K. KHOMYXKOV,V. S. LIVSHITS,V. A. SAvI~ and YE. B. LYUDVIG L. Ya. Karpov Physico-chemical I n s t i t u t e

(Received 3 November 1974)

The polymerization of glycolide in a melt cont~inlng SbFs proceeds to 100% conversion in the temperature interval 160-175 ° and the n u m b e r of active centres increases with time. The polymerization process features reactions of chain propagation and acyl ion binding b y carbonyl groups of the monomer and polymer molecules, as normally occurs with lactones, b u t a n additional feature of this process is the dissociation of ion pairs to free ions. The initial stages of polymerization involve relatively slow initiation with the participation of two initiator molecules. I n the light of data on the viscosity of the polyglyeolide in the melt the "living" character of the growing chain ends is postulated. A formula is derived which relates the viscosity of polyglycolide in the melt to number-average molecular mass.

PROCESSES of cationic polymerization and copolymerization of glycolide 0--CH2 O~ ~ are involved in the preparation of chain polymers which CH2--0

in view of their unique resorption capacity in living organisms are widely used for various surgical purposes. Data on glycolide polymerization are mainly confined to the patent literature; the one paper that has appeared in this connection contains material that is too limited to allow elucidation of the main features of the polymerization mechanism [1]. Our aim in the work described below was to analyse the polymerization of glycolide in a melt containing SbF3 as the initiator, and to determine the main parameters of the process, including the relationship between the viscosity of the polymer in the melt and its molecular mass. Glyeolide was repeated recrystallized from car,~fully purified T H F in a dry argon atmosphere. The end content of glycolic acid was ~ 10 -~ mole/1. A n t i m o n y trifluoride was purified by vacuum distillation and was then measured out into globules on a weight basis without coming into contact with air. T e t r a b u t y l a m m o n i u m tetrafluoroantimonatc (TBA) was synthesized in the m a n n e r described in [2]. TBA was measured out beforehand under vacuum conditions. * Vysokomol. soyed. A17: No. 12, 2726-2732, 1975. 3133

3134

G . S . SA~n~A e¢ al.

The polymerization rates were determined by an ampoule-method, varying the temperature from 150 to 180% Before the start of an experiment the initiator was dissolved in molten glycolide at 100 ° and the solution was decanted into the ampoules in vacuo. The polymer yield in a given time was determined by weighing; the ampoule containing 1 ~ Z ~3 g

~IO

o

I

50

150 Time ~min

IoO.Mo/.

1

4

O.

ely

C

2

3

l'O

80 I00 Time, rain

I

1-o

FI~. 1. Kinetic curves of glyeolide polymerization in the melt in the presence of SbF3~(a), Mo--M the semi-logarithmic anaznorphoses (b) and the c/v ratio vs. - (c) at 170 °. Initial conM centration co)< 103, mole/L: 1--8.40, 2--6.00, 3--4-00. 4--2.90 5--1.50 and 6--0.84. the reaction mixture was broken, and the mixture was kept in boiling ethyl acetate until dissolution of the monomer was complete, after which the solution was filtered and the polymer weighed. I t was found by special tests t h a t at 180 ° and above the polymerization process proceeds with decay of active centres with time. At 150 ° the polymer separates out in the early stages of conversion, which makes it impossible for the process to be observed under homogeneous conditions. In view of this temperatures of 160, 165, 170 and

Cationic polymerization of glycolide

3135

175 ° were selected as the principle working temperatures. I t should be noted t h a t although t h e melting point of the resulting polymers is 240 °, the process is m a i n l y a homogeneous o n e within the temperature interval selected. This is apparently due to the relatively slow

rate at which polyglycolide crystallizes under these conditions. The viscosity of th0 polyglycolide in the melt was determined with the aid of an M-V-2 capillary viscometer.

Kinetics and mechanism of glycolide polymerization.

Figure la shows the kinetic curves for glyeolide polymerization at 170 ° plotted with variation in the initiator concentration. Similar series of curves were plotted at the other temperatures. The shape of the curves plotted on semi-logarithmic coordinates is as seen in Fig. lb. According to the data referred to above the polymerization goes to 100% conversion, the number of active eentres increasing with time. I t was reported in [3] that the cationic polymerization of lactones under conditions of rapid initiation in the absence of kinetic chain termination is characterized b y the chain propagation reaction Jq,

II

_ _ ~ - - C + +- 0 . . . .

II

N+--C+

(I)

and in addition b y reversible binding of aeyl ions b y earbonyl groups of monomer and polymer molecules

oII

o]I k,

0

O-

II

I

_/v~--C+ -:- O=C

oli

/

o]

(ii)

~ _ ~ # - - C + ' " O " C "0

0-

/ _a~--(;* @ O- C-- ~ _.ha,-- C+'"O"'C k,.,

li

(III)

Kinetically this is reflected in the following relationship between the reaction rate and concentrations of the components: dM kpcd'VI v---- -- d--/ = k~M-t-b~(Mo--M ) '

(1)

or

Co v

kl kp

]c2 Mo--M ]% M

where co is the starting initiator concentration, and M0 and M are initial and current concentrations of monomer respectively, i.e. on (%/v)--(Mo--M/M) coordinates this process will be described b y a straight line. As was noted in [4], the above mechanism is kineticMly identical to one whereby reactions (II) and (III) are combined with a propagation reaction consisting in monomoleculax opening of the ion formed in accordance with reaction (II) 0

(.)-- CH..,

1!

/ ---~--C+"'O"--(: -\

0

0

~,

"\C kt)' II II ]l = 0 ~ , __vw--C--O--CIIz--C--O_CH=--C* / C[I..,--O

(IV)

~136

G.S. SA~NA et a/.

I n the latter case equation (I) will be written as

%/v=1/k'+ k2(M0--M) klk'pM As there are no ground for preferring any one of the mechanisms in question we will examine the data obtained and for convenience one of the mechanisms (mechanism (I)-(III)) will be used. The kinetic data on the polymerization of glycolide (Fig. la) were analysed on the above coordinates, i.e. it was assumed that a change in the number of active centres owing to displacement of the equilibrium of reactions (II) and (III) is the sole cause of a change in the rate of the polymerization process. The results 2"g -kl

1

?

2

3

1.0

/

x

x

×x

I

I

I

I

2

a

2

g

I

"~

.rn-I , L,

FIG. 2. E f f e c t i v e r a t e c o n s t a n t s kz a n d kz vs. 1]~/c a t t e m p e r a t u r e s of: 1 - - 1 7 5 , 2 - 170, 3 - - 165, 4 - - 160 ° .

of this analysis for a temperature of 170 ° are shown in Fig. lc (v was found from the slope of the tangents at points selected on the curves on conversion-time coordinates). As can be seen from Fig. 10, the curves are rectifiable on the given coordinates throughout the polymerization process, apart from the very early stages of the reaction which apparently involve a relatively slow process of initiation which ends only when 5 - 1 0 ~ conversion has been reached. According to formula (1) the straight lines ought to make intercepts of kl/kp on the ordinate axis, and the slope of the lines ought to give k2/kp. The fact that these values are dependent on the initiator concentration means that there must be a further reaction involving the participation of active centres in the system, apart from the processes already mentioned. In ionic polymerization processes relationships between propagation rate constants and the initiator concentration are most p r o b a b l y due to the dissociation of ion pairs to free ions, provided that there is a

Cationic polymerization of glyoolide

3137

considerable difference in t h e i r reactivities [5]. L e t us consider t h e s y s t e m u n d e r review f r o m this s t a n d p o i n t . T h e dielectric constants of some lactones in melts are as follows:* Lactone T, °C e

fl-propiolactone 20 42-8

8-eaprolactone 20 38.3

glyeolide 90 9.8

I t can be seen f r o m these d a t a t h a t a glycolide melt a t 9 0 ° is a relatively low polar m e d i u m (at 160-170 ° the p o l a r i t y m u s t be still lower). I n f o r m a t i o n given in [6] on t h e dissociation o f t r i a l k y l o x o n i u m salts to free ions result in a dissociation c o n s t a n t of ~ 10 -6 in the case of e ~ 10. I t seems probable t h a t despite t h e m a r k e d delocalization o n t h e charge on the ions P a n d Q c o m p a r e d w i t h t h e t r i a l k y l o x o n i u m ions, t h e r a t e of dissociation of the l a t t e r t o free ions m a y be quite low in view o f the low dielectric c o n s t a n t o f the medium. T h e r e a c t i o n s m o s t p r o b a b l y occurring in t h e s y s t e m u n d e r review are (X)

(p+~

IP) t)

O LI

~'~ ' 0 ....

~

i: ~

C+=:-O'"C

() ]

.d

I <

--/~/Y~--- (]+

SbF[

+

o (O+)

i

;bF;--

OH

Z-~Z~ (Y'"O'"C ',

__m~- C+ SbF(-

o (Q) ~d I~ . . . . ~ C+"-O'"C st,r~I

o ~.

SbF~

~) ~

T h e a b o v e set of reactions was considered from a kinetic s t a n d p o i n t u n d e r t h e following conditions: 1) c o = P + Q , i.e. the free ion concentrations are low c o m p a r e d with the conc e n t r a t i o n s o f P a n d Q. 2) P a n d Q ion pairs dissociate in equal degrees to free ions. T a k i n g a c c o u n t o f the b a l a n c e / e q u a t i o n co=P++-Q a n d A = P + + Q + , we h a v e X=

A

ca

k~M+k2 (Mo--M)

k~*[k~M+k~(Mo--M)]

(C=Co/2). Now, since v=kr~XM, we have c

k~

k2

.

Mo--M

v -- kp k a t c * + - -kp - k ~ ' c * - - "iV1

(2)

I n the l a t t e r case i n t e r c e p t s m a d e on (c/v)-[(Mo--M)/M)] coordinates on t h o o r d i n a t e axis, a n d t h e slopes of these lines will d e p e n d on initiator c o n c e n t r a t i o n to the power o f ½, or the reciprocal values to t h e power of c-*. F i g u r e 2 shows plots o f (kp/kx)exp a n d (kp/k2)exp vs. c-* for different t e m p e r a t u r e s . T h e l i n e a r i t y o f * The dielectric constants of the lactones were kindly measured by E. S. Petrov.

3138

G.S. SA~n~A 6t a~.

the curves in question evidences the validity of the assumption made above. In this case #p kp K1---- ( - ~ ) e x = ~ - k~c-';

K~= (~)e.p=

kp -k2 #tdc-÷

The values of kpkd~#l and kplCda/lcl can be found for different temperatures from the tangents of the angles of slope of the lines. B y plotting these values on Arrhenius coordinates we obtain effective activation energies of 6.3 and 7.0 kcal/ /mole respectively. I t should be noted that apart from the reason given above the fact that the constants are functions of c -a could also be due to other factors. For instance, it could be the result of dimeric association of growing chain ends. Although association of the latter type seems improbable it cannot be altogether ruled out

5"

/x

Z

/

// .

5g

,

150

Time, rain FIG. 3. Polymerization of glyoolide without TBA (1) and with a TBA addition (2): 1--co=6.00× 10-3 mole/1., 2--C0=CTBA=6"00× 10-8 mole/1. at temperatures of 160-170 °. Further confirmation was therefore needed in regard to the question of dissociation to free ions. The polymerization of glycolido was accordingly carried out in the presence of (n-Bu)aN+SbF4 - salt, which is a source of SbF4- ions capable of suppressing dissociation during the polymerization process. I t can be seen from Fig. 3 that the addition of TBA leads to a marked reduction in the polymerization rate, which can be taken as confirmation of the proposed mechanism. In equation (2) Co~2replaces c o owing to the proposed mechanism of initiation of the polymerization in presence of SbF 3 2SbF 3 --> SbF+SbF~ ~ X I f this type of initiation does take place (if it is reasonably slow, and is reflected kinetically) it means that the reaction will be second order in respect to initiator concentration in the absence of dissociation to free ions. Should dissociation of

Cationic polymerization of glycolide

3139

ion pairs to free ions as considered-above occur simultaneously, the order of t h e reaction in respect to initiator concentration must be changed to first order. The kinetic data relating to the early stages of conversion show t h a t here we do, in fact, have a first order reaction (Fig. 4). 1

/.

uo~lO mole.l.4.sec-/

i

/

.

J

8

q

-

co.lg~ mole/l. FIG. 4. Initial rates of glycolide polymerization v0 vs. iaitiator concentration at different temperature: 1 -- 175, 2-- 170, 3 -- 165, 4-- 160°; v0determined as a ratio o f c o n v e r s i o n (in 300 sec)/time (300 see).

Molecular mass of polyglycolide.

I t is extremely difficult to determine polyglycolide molecular masses because no solvent has yet been found t h a t would be suitable for work over the required range of molecular mass. In the present investigation an a t t e m p t was made to determine molecular masses, starting with the mechanism proposed for the reaction. I t is known t h a t molecular masses of polymers prepared by lactone polymerization in accordance with a cationic mechanism are lower t h a n the theoretical values which could have been anticipated where the polymerization is of the type of "living" polymers. This is accounted for by the presence of processes of the type of o o o=c

II

I

,

It

0

II

(CH2)2--, tW'-- CH~--CH+.--C--0--C--CH=CH~ ~, H+

/W~-- CH2--CH2--C+ @ 0

The structure of the polyclycolide molecule would rule out a reaction of this sort, and one can therefore say t h a t the process in the case in question will be of the type of "living" polymers. Now, provided t h a t the MMD does not vary with changes in the concentration of the components or with temperature, it is to be expected t h a t log ~/(0 being the viscosity of the polyglycolide in the melt) will be a linear function of AM/c (AM being the amount of monomer polymerized) (Fig. 5). We plotted points corresponding to varying degrees of conversion, catalyst concentration and temperature. The fact t h a t all the points plot into a single

G . S . SA-~INAet al.

3140

straight line bears out the proposition regarding glycolide polymerization being of the type of "living" polymers. The line in Fig. 5 has a slope of 3-4, which is typical for the slope of lines relating viscosity values in a melt to average degrees of polymerization for the great majority of polymers.

xS

log G

~'0 o,

/x

3"0

/ I

I

I

3"Z5

3"50

3"75

AM log -~--

FIG. 5. Logarithmic dependence of viscosity (poise, 240 ° ) of the melt versus average degree of polymerization, varying: / - - t e m p e r a t u r e and conversion; 2--co and conversion.

I n view of these results we obtain the following formula relating the viscosity of polyglycolide in the melt with number average molecular weight log ~/= 3.4 log Mn --4.75 (at 240°). The latter formula is not a universal one, and is valid only in the case of the given MMD. In accordance with the mechanism discussed above one can assume t h a t the coefficient 4.75 either relates to the most probable MMD (if there is enough time for it to be established in the course of the process) or else t h a t it is close to unity (if the rate at which the most probable distribution is established in relatively slow). Deviations from these relations m a y appear as a result of the influence of the early stages of the process involving incomplete initiation. Translated by R. J. A. I-IENDRY REFERENCES 1. K. CHUJO, H. KOBAYASHI, J. SUZUKI, S. TOKUHARA and M. TANABE, Makromolek. Chem. 100: 262, 1967 2. {3. J. ADAMS and A. J. DOWNS, J. Chem. Soc. A: 1534, 1971 3. Ye. B. LYUDVlG and A. K. KItOMYAKOV, Dold. AN SSSR 2Ol: 877, 1971; Ye. B. LYUDVIG, A. K. KHOMYAKOV and G. S. SANINA, J. Polymer Sci., Polymer Symposia, No. 42, 289, 1973 4. Ye. B. LYUDVIG and B. G. BELEN'KAYA, J. Maeromolec. Sci. AS: 819, 1974 5. M. SCHWARTZ, Anionic Polymerization, p. 401, Izd. "Nauka", 1971 6. J. M. SANGSTER and D. J. WORSFOLD, J. Macromolec. Sci. 7A: 1415, 1973