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Polymerization in highly viscous media and three-dimensional polymerization—II. The initial stage of polymerization of polyester-acrylates
POLYMERIZATION IN HIGHLY VISCOUS MEDIA AND THREEDIMENSIONAL POLYMERIZATION--H. THE INITIAL STAGE OF POLYMERIZATION OF POLYESTER-ACRYLATES* G. V. KOROLEV, A. A. BERLIN and T. YA. KEFELI Institute of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 19 June 1961) W E HAVE previously studied the kinetics of polymerization of three polyester-
acrylates at 20-25 ° in bulk and in solution in the presence of the initiator system benzoyl peroxide/dimethylaniline, with and without initiator additions [1, 2]. The rate of polymerization was measured b y the adiabatic-isothermal variant of the thermometric method [3]. It was sho~a t h a t the specific features of the kinetics of polymerization of polyester-acrylates (auto-acceleration beginning at a low degree of conversion, the specific effect of solvent and inhibitor additions) are associated with the formation and accumulation in the system of a threedimensional polymeric product. On the basis of these preliminary results the kinetics of the three dimensional polymerization of polyester-acrylates were formulated qualitatively. Our subsequent work has been directed toward a detailed, quantitative study of this process at various stages of conversion in order to establish the mechanism of three-dimensional polymerization. I t was found that it was first necessary to solve the following problems: (1) Refinement of the thermometric method of measuring the rate of threedimeimional polymerization. The calorimetric apparatus that we constructed, with a reaction cell of special design [4] enabled the uncertainty in determining the initial moment of reaction to be reduced to 15-30 sec, and the sensitivity to be increased so that it was possible to measure the rate of polymerization in the very earliest stages of conversion when the process is not yet complicated by erosslinking. 2. Measurement of the concentration of free radicals accumulating during polymerization. This problem was solved by the use of the ESR method. 3. The synthesis of highly viscous, ideal solvents, identical in properties with the oligomeric polyesters-acrylates being studied, but incapable of polymerization. Oligomerie products were synthesized by condensation telomerization of the same polyfunctional acids and alcohols and in the same ratios as those used for synthesis of the polyester-acrylates, but with the difference that the regulator * Vysokomol. soyed. 4: No. lO, 1520-1527, 1962. 482
Polymerization of polyester-acrylatee
483
of the chain length of the oligomers was not methacrylic acid b u t the saturated, isobutyric acid, which is closely related to the former in chemical structure. Consequently the products do not contain unsaturated end groups and are therefore not capable of free-radical polymerization. 4. The kinetic study of a large number of polyester-acrylates differing in nature and in chain length. The present communication presents only results relating to the initial stage of polymerization of polyester-acrylates.
The effect of the viscosity of the reaction medium on the rate of polymerization of polyester-acrylates During the preliminary s t u d y [1, 2] it was found that the bulk polymerizability increases in the order TGM-3~MDF-1
TGM-3 MGPh-9 MDPh- 1 MDPh-2 MBPh-1 MDA- 1 MMA
Viscosity at 25° (cS)
in bulk
5o% IDPh-2*
75% IDPh-2
k~for polymerization in b u l k
(1/mole/see) 10 95
60 1000 115 55
3 7 4 17.5 5 3 0"75
5
8
2-3 × 3.6 X 1.9 × 1.4 × 6.4)<
7.5 1.2"*
2.3 X 106 4.0 × l0 T***
8
9.8
4-3 14 6
8
10 e l0 s 106 l0 s 105
* Vtzco6ity of IDPh-2 is 800-900 cS. ** 80% IDPh-2. *** Published values, 3 x 10' at 50*
of the methacrylic groups, hence with increasing distance between the two methacrylic groups on passing from the esters of mono-ethylene glycol to those of tri-ethylene glycol the reactivity of the dimethacrylic esters increases.
484
G. V. KOROLEV et al.
I n order to examine the possible causes of the different'reactivity of polyester-acrylates, differing in the nature and length of the oligomeric chain, t h e initial rates of polymerization, w0 were measured for a large number of oligomers, differing in viscosity (see Table). Polymerization was carried out at 50 ° in the presence of 0 . 5 ~ by weight of dicyclohexylperoxydicarbonate (I) [6] as initiator, in bulk and also in solution in benzene and in a highly viscous solvent consisting of the telomer obtained b y condensation of diethylene glycol, phthalic acid and isobutyric acid, with a degree of polymerization n--~2 (arbitrarily called IDPh-2). The polymerization rate, w0 was measured by an improved thermometric method (calorimetric variant [4]). The synthesis [1,7], purification and nomenclature of the oligomers have been described previously. TGM-3 is triethylene glycol dimethacrylate and MGPh-9 is the dimethacrylate of (bis-triethylene glycol)phthalate. The other polyester-acrylates are polyesters of the type L(AB)nAL, where L and B are mono- and dibasic acid radicals respectively, and A is a dihydric alcohol radical. The figures after the abbreviated name in all cases except TGM-3 and MGPh-9 (these are industrial codes) denote the value of n, and the letters denote the following radicals: D, diethylene glycol; B, butanediol; A, Ph, I and M, adipic, phthalic, isobutyric and methacrylic acids respectively; MMA is methyl methacrylate. The results show that when the system is diluted with a solvent of higher viscosity than that of the polyester-acrylate present, in all cases w0 falls, whereas when the solvent is of lower viscosity than the polyester-acrylate wo increases. For example the addition of 2 0 ~ (by weight) of benzene to TGM-3 reduces wo b y 15%, and when the same quantity of benzene is added to the much more viscous MGPh-9, w0 falls by ~ 100%. It is seen from the Table that the addition of the highly viscous, ideal solvent IDPh-2, with a viscosity of 800-900 cS, to MDPh-2, with a viscosity of 1000 cS, reduces wo a little, but the addition of IDPh-2 to all the other polyester-acrylates, none of which has a viscosity greater than 115 cS, increases w0 considerably. The sensitivity of the initial polymerization rate, w0, to the viscosity of the medium indicates that almost at the very lowest degree of conversion in the polymerization of all these polyester-acrylates, including even the comparatively low viscosity material TGM-3, the termination reaction is diffusion controlled. The value of w0 for the bulk polymerization of the various polyester-acrylates (see Table) increases in the order T G M - 3 ~ M D A - I < M D P h - I ~ M B P h - I ~ ~ M G P h - 9 < M D P h - 2 . The viscosities of these materials increase in the order T G M - 3 < M I ) A - I - ~ M D P h - I < M G P h - 9 < M B P h - I < M D P h - 2 , i.e. in all eases except MGPh-9 the value of wo varies in the same direction as the variation in the original viscosity of the polyester-acrylates. When these compounds were polymerized under conditions of approximately equal viscosity (the addition of about 75% of the highly viscous, ideal solvent IDPh-2) (see Table) the differences in w0 levelled out and the values of w0 practically coincided.
Polymerization of polyester-acrylates
485
We suggest t h a t the equalization of the values of w0 when the viscosity of the me.dium is equalized, and the concomitant variation of w0 for the various polyester-acrylates with the variation of their initial viscosities can be interpreted within the framework of the foUowing suppositions: (a) at degrees of conversion close to zero the propagation constant, kp, is practically the same for all the polyester-acrylates, i.e. the chemical nature and length of the oligomer chain have only a weak effect on the reactivity of the terminal methacrylate groups; (b) the termination rate constant, kt, falls steeply with increasing viscosity of the medium; (c) the value of kt under conditions of equal viscosity is weakly dependent on the nature and length of the oligomer chain. I t is evident t h a t the intramolecular association of the methacrylate groups observed b y one of us [5] in the polymerization of the dimethaerylates of mono-, di- and triethylene glycol is possible only in esters in which the double bonds are sufficiently close to one another. Evidently this does not arise in the above oligomers. The conclusion m a y therefore be drawn that the difference in the apparent reactivity of the above polyester-acrylates in bulk polymerization is not associated with the different chemical nature of the polyhydric alcohol and dibasic acid radicals making up the oligomer chain, but is mainly the result of a difference in the physical properties (viscosity) of the medium. The fact that kp for these polyester-acrylates is only weakly dependent on the chain length of the olig0mer conforms with published information [8, 9] on the reactivity of the double bond in methacrylate esters. For the polymerization of CH2=C(CH)aCOOR at 30 °, kp is 350 l/mole/sec for R=CH3; 467 for R = n - - C a H v and 362 for R----C4H9, i.e. kp varies little with increasing length of R. I f one regards the polyester-acrylates as methacrylate esters in which R is an oligomeric chain it is reasonable to suppose that the value of k~ for the polyester-acrylates will be little different from ]cp for the above methacrylate esters. Then taking the published propagation rate constant for MMA, k~ ~ 400 1./ /mole/sec (at 50°), and the value of the rate constant for the decomposition of I as k0=8.46 × 10-5 sec-1 (at 50 ° in benzene) [6], and assuming that the efficiency of initiation (f is usually ~ 0.5) and kp are considerably more weakly dependent on viscosity t h a n k t, it is possible to estimate the values of/¢t (for degrees of conversion close to zero) from wo for the various cases given in the Table. The last column of the Table gives the values of k t for polymerization of the polyester-acrylates in bulk. These estimated values of kt show t h a t even for polyester-acrylates of relatively low viscosity (TGM-3), k t is less t h a n t h a t of MMA by a factor of ~ 15. For the more highly viscous materials (M_DPh-2), kt differs from that of MMA by about two orders of magnitude. With an increase in the viscosity of the polyester-acrylate by a factor of ~ 100 the value of kt falls b y a factor of ~ 10. I n a solution containing 750/o of IDPh-2, ]¢c for all the polyester acrylates is (3-4) × 105 1./mole/sec, and for MMA with 80% of IDPh-2 k t ~, 10L This dif-
486
G . V . KOROLEV e t a / .
ference between the values of kt for M2J_A and the polyester-acrylates i s not unexpected, because it is obvious t h a t the coefficients of diffusion of the ,macroradicals -CHIC(CHs) -CHsC(CH3) -CHsC(CHs) I
COOR
I
COOR
i
(II)
COOR
in the highly viscous medium of IDPh-2, will be considerably lower when R is an oligomeric polyester chain than when R is CH 3, particularly when it is remembered that the maeroradicals of type II, containing a polyester chain with one unreacted terminal double bond in each unit, can be branched even at degrees of conversion close to zero. Finally it should be noted that from the point of view of steric hindrance it would be expected that k~ would fall with increasing length of the polyester chain. However in the case of our polyester-acrylates the length and flexibility of the oligomeric chain are sufficient to prevent the presence of the oligomeric "tail" having any substantial effect on the mobility of the reactive end groups when the oligomer molecules move in media in which t h e y are not fixed (i.e. in media possessing flow). This is specific to polyester-acrylates in which the length of the oligomeric chain is sufficient to give rise to properties close to those of polymer chains and, as will be shown in subsequent communications, is associated with a number of kinetic peculiarities of the polymerization of polyesteracrylates. The limitation of the idea of a weak effect of the long, oligomeric "tail" on the mobility of the end g~oups to only flowing media is very necessary, because (as further study showed) differences in the flexibility of the chains of these polyester-acrylates which approximately level out in flowing media, in erosslinked media where the molecules of the oligomer are fixed (suspended in a three-dimensional framework), lead to substantial differences in the mobility of the end groups. In certain cases the value of kp can be dependent on the shape of the oligomer molecule. For example if the chain is so long and flexible that it winds into a coil, kp can fall as a result of steric hindrance. We have observed such steric hindrance in a number of MDA-n polyester-acrylates when n ~ 2 . Evidently the o]igomer molecules containing the long hydrocarbon chain of the adipic acid radical are coiled. I f the chain is very rigid k~ must fall with increasing length of the molecule. From stereochemical considerations it follows that when the diethylene glycol radical is replaced by the butanediol radical the rigidity of the oligomeric chain must increase. Consequently polyester-acrylates of the MBPh-n type were synthesized and studied. I n spite of the increase in viscosity of the reaction medium on passing from MBPh-1 to MBPh-2, w0 fell to some extent, evidently indicating that kp falls considerably on passing along the series of homologous telomers from MBPh-1 to MBPh-2 with increasing chain length, as a result of an increase in steric hindrance. I t is thus a general feature of the oligomers t h a t the reactivity of the end groups in polymerization in media possessing flow is independent of the length
Polymerization of polyester-acrylates
487
and nature of the oligomeric chain, though in certain cases in addition to this general fact it is necessary to take into account specific features of the structure of the oligomer molecules. Mechanism of chain termination in the initial stage of conversion Since the radical R, formed b y decomposition of the initiator I, and the macro-radical R 1 of type II, formed b y the reaction R-bM-*R1, where M is an oligomer molecule, differ considerably in mobility in a viscous medium, two mechanisms for the polymerization of the polyester-acrylates in the initial stag e of conversion can be p u t forward, differing in the nature, of the termination steps: A.
W¢
I--~R R+M
W~
B.
I--~R
kp
k~
R+M ---* RI
---* R 1
k~
k~
RI+M--~ R1
RI+M---> Rl
kt
kt
Rl+R1 --~dead products
R+R1 ~ dead products
It has been found [10] that even in polymerization in media of low viscosity a considerable proportion of the radicals R take part directly in termination of the type R z ~-R. It would be expected that in highly viscous media termination of the type R I ÷ R 1 does not occur at all because of the low mobility of R1, and that termination occurs mainly b y interaction of the small (and therefore mobile) radical R with the macro-radical, R r Chain termination of the type R - ~ R need not be considered because the stationary concentration of R 1 must be considerably higher than that of R. From mechanism A it follows that w0-----k~[M]~/wdkt (1) (where w~ is the rate of initiation), and from B, Wo=k~/k~[M] ~ (2). Our measurements of w 0 at various initiator concentrations show that in all cases w 0 ~[I] °'5. A typical curve is shown in Fig. 1. w~10-a
(,,i,, -')
6 4
L//
2 l_.
0
0.2
]
I
I
(}4
0.6
fl'8
fl] (wt. ~ ) FIG. 1 Dependence of. the initial rate of polymerization of polyester-acrylates on initiator concentration; 50°, reaction system MGPh-9÷50~/o IDPh-2. Thus the experimental results are in agreement with equation (1). Consequently, in spite of the fact that the macro-radicals R 1 have comparatively low mobility, as is shown b y the results presented in the Table, the fraction of
488
G.V. KoRo1.I~v et al.
radicals R taking part in termination of the type R q-R x is very small and the polyester-acrylates polymerize only b y mechanism A. Determination of the rate of initiation in the initial stage of polymerization I n investigations involving ~he modelling of any given physical property of a reaction system it is often necessary to carry out the process in various model media, i.e. in various solvents. I t is therefore necessary tO have information on the effect of different media on the rate of decomposition of the initiator. We have observed t h a t peroxides decompose much more rapidly in glycols than in benzene, toluene or similar solvents. For example the decomposition rate constants for benzoyl peroxide and dicyclohexylperoxydicarbon~te in benzene at 50 ° are 1.1 x 10-6 sec -1 and 8.46 × 10-5 sec -1 respectively, and in triethylene glycol 4.5 × 10-s sec -~ and 2 × 10-4 sec -1 respectively. Since all the above polyester-acrylates (except MBPh-1) and the ideal, highviscosity solvent IDPh-2 contain di- and triethylene glycol radicals in the oligomeric chain it would be expected t h a t the accelerating action of the latter would be preserved to some extent in these. Moreover, since the different polyesteracrylates contain different proportions of glycol radicals the initiator should decompose at different rates in each of the polyester-acrylates, and this should be taken into account when comparing the values of wo (see Table). Consequently the necessity arises of measuring the rates of decomposition of initiators (peroxides) in the oligomers. The usual methSds for the determination of peroxides are very unsuitable in such media as polyester-acrylates, IDPh-2 and the like, because of the insolubility of the oligomers (and especially of the crosslinked polymers formed when the decomposition of peroxides is being studied) in the reagents used for the chemical determination of peroxides. The necessity of extraction leads to complications and a prolonged procedure for the determination of peroxides in these cases, and at the same time does not give sufficiently accurate results. We have developed the following kinetic method for determination of the rate constant k 0 for the decomposition of peroxides (applicable to initiators of any type). We derived equations [11] for an instantaneous degree of conversion C t and a rate of initiation wt, taking into account the loss of initiator. Analysis of these equations shows that in the case of initiators for which k0 satisfies the condition k 0 >4k~ [I]o/kt (where [I]0 is the initial concentration of initiator) polymerization does not go to completion (the final degree of conversion Coo is less than 100%). In this case k o =2wo/C~ (wo is the initial rate of polymerization, Coo is the limiting degree of conversion.) I f however k0<4/c~ [I]o/kt, Coo is always ~ 100% and equation (1) cannot be used for the determination of k 0.
Polymerization of polyester-acrylates
489
I n this case k 0 can be determined with a little more complication from the relationship: log [M]t=log [M]° +0.217 kot Wt
W0
(where [M]0 and [M]t are the initial and instantaneous monomer concentrations), from the slope of the graph of log [M]t/w t against t.
0.55 0
I
I
Io 2o r ,e(m@ Ko=g.S.lO- (sec-9
FIG. 2 Determination of k o, the decomposition rate constant of dicyclohexylperoxydicarbonate, in 80~ IDPh-2+20yo MMA at 50° by the kinetic method. This method enables k o for the given conditions to be determined from a single rate curve. Figure 2 shows the relationship log [M]dwt = f ( t ) , from which the rate constant for the decomposition of dicyclohexylperoxydicarbonate (1) in IDPh-2 at 50 ° was calculated (k0=-9.8 × 10-5 sec-Z). The experiment were carried out as follows. A mixture of the solvent (IDPh-2), the indicator monomer (we usually took 20% b y weight of MMA) and the initiator were placed in the thermometric apparatus and the rate curve was plotted. Then b y graphical differentiation of the latter the relationship of the type shown in Fig. 2 was obtained. Since the value of k 0 found is close to the published value for the decomposition of I in benzene (k0=8.46 × 10-5 sec -1 at 50°), it m a y be considered that the di- and triethylene glycol radicals in the polyester-acrylates have very little effect on the rate of decomposition of I. Consequently for the calculation of k t it is justifiable to regard the rate of initiation as being the same for all the reaction systems in question. CONCLUSIONS
(1) The initial rate, w0 of polymerization (initiated b y dicyclohexylperoxydicarbonate) of polyester-acrylate oligomers has been measured b y the thermometric method at 50 °. The oligomers differed in the length and nature of the chain. Polymerization was carried out in bulk and in solution in a highly viscous, ideal solvent, consisting of an oligomer sim;lar in chemical structure to the polyester-acrylates b u t incapable of free-radical polymerization. (2) It was found that in the bulk polymerization of polyester-acrylates differing in viscosity the value of w0 increases with increasing initial viscosity of the oligomer. When the polyester-acrylates are polymerized under conditions of equal viscosity (dilution with a highly viscous solvent) w 0 is practically the same in all cases.
490
Yu. S. LIPATOV
(3) The results are i n t e r p r e t e d on ~he a s s u m p t i o n t h a t t h e p r o p a g a t i o n rate c o n s t a n t is t h e same for all t h e polyester-acrylates studied a n d t h a t chain t e r m i n a t i o n is diffusion controlled f r o m t h e v e r y beginning o f polymerization. The t e r m i n a t i o n rate c o n s t a n t s for the various polyester-acrylates are estimated. (4) The decomposition rate c o n s t a n t for t h e p o l y m e r i z a t i o n initiator, dicycloh e x y l p o r o x y d i c a r b o n a t e , has been d e t e r m i n e d directly in the reaction s y s t e m b y a kinetic m e t h o d . Translated by E. O. PHILLIPS REFERENCES
1. T. YA. KEFELI, G. V. KOROLEV and YU. M. FILIPOVSKAYA, International Symposium on Macromolecular Chemistry. Section I, p. 47, Moscow, June 1960 2. G. V. KOROLEV, L. I. MAKHONINA and A. A. BERLIN, Vysokomol. soyed. 3: 198, 1961 3. G. V. KOROLEV, B. V. PAVLOV and A. A. BERLIN, Vysokomol. soyed. 1: 1936, 1959 4. G. V. KOROLEV, Rus. Pat. No. 137, 304, Class 421, 335; Byull. izobret. No. 7, 1961 5. A. A. BERLIN et at., Sb. statei po obshch, khimii, 2: 1554, 1953 6. A. G. RAZUVAYEV, L. M. TERMAN, V. R. LIKHTEROV and V. S. ETLIS, International Symposium on Maeromoleeular Chemistry. Section II, p. 53, Moscow, June 1960 7. A. A. BERLIN, G. L. POPOVA and YE. F. ISAYEVA, Dokl. Akad. Nauk SSSR, 123: 282, 1958; Vysokomol. soyed. 1: 7, 951, 1959; A. A. BERLIN, T. YA. KEFELI, YU. M. FILIPOVSKAYA and YU. M. SIVERGIN, Vysokomol. soyed. 2: 411, 1960 8. C. WALLING, Svobodnye radlkaly v rastvorakh. (Free Radicals in Solution.) Foreign Literature Publishing House, Moscow (Russian translation,) 1960 9. KH. S. BAGDASAR'YAN, Teoriya radikal'noi polimerizatsii. (Theory of Radical Polymerfzation.) Izd. Akad. Nauk SSSR, Moscow, 1959 10. G. HENRICHI-0LIVE and S. OLIVE, Makromol. Chem. 37: 71, 1960 11. B. V. PAVLOV and G. V. KOROLEV, Vysokomol. soyed. 1: 869, 1959
STUDIES OF POLYMER-FILLER INTERACTIONmVI. SOME RHEOLOGICAL PROPERTIES OF POLYMER SOLUTIONS IN THE PRESENCE OF FILLERS* YU. S. LIPATOVt Institute of General and Inorganic Chemistry, Beloruss. S.S.R. Academy of Sciences (Received 19 June 1961) T H E s t u d y o f t h e properties of p o l y m e r solutions in t h e presence o f fillers is o f g r e a t practical interest for the p a i n t i n d u s t r y , t h e reinforced plastics i n d u s t r y , * Vysokomol. soyed. 4: No. 10, 1528-1536, 1962, t The experimental work was carried out by M. G. Barkovskaya.
(Received 19 June 1961) W E HAVE previously studied the kinetics of polymerization of three polyester-
acrylates at 20-25 ° in bulk and in solution in the presence of the initiator system benzoyl peroxide/dimethylaniline, with and without initiator additions [1, 2]. The rate of polymerization was measured b y the adiabatic-isothermal variant of the thermometric method [3]. It was sho~a t h a t the specific features of the kinetics of polymerization of polyester-acrylates (auto-acceleration beginning at a low degree of conversion, the specific effect of solvent and inhibitor additions) are associated with the formation and accumulation in the system of a threedimensional polymeric product. On the basis of these preliminary results the kinetics of the three dimensional polymerization of polyester-acrylates were formulated qualitatively. Our subsequent work has been directed toward a detailed, quantitative study of this process at various stages of conversion in order to establish the mechanism of three-dimensional polymerization. I t was found that it was first necessary to solve the following problems: (1) Refinement of the thermometric method of measuring the rate of threedimeimional polymerization. The calorimetric apparatus that we constructed, with a reaction cell of special design [4] enabled the uncertainty in determining the initial moment of reaction to be reduced to 15-30 sec, and the sensitivity to be increased so that it was possible to measure the rate of polymerization in the very earliest stages of conversion when the process is not yet complicated by erosslinking. 2. Measurement of the concentration of free radicals accumulating during polymerization. This problem was solved by the use of the ESR method. 3. The synthesis of highly viscous, ideal solvents, identical in properties with the oligomeric polyesters-acrylates being studied, but incapable of polymerization. Oligomerie products were synthesized by condensation telomerization of the same polyfunctional acids and alcohols and in the same ratios as those used for synthesis of the polyester-acrylates, but with the difference that the regulator * Vysokomol. soyed. 4: No. lO, 1520-1527, 1962. 482
Polymerization of polyester-acrylatee
483
of the chain length of the oligomers was not methacrylic acid b u t the saturated, isobutyric acid, which is closely related to the former in chemical structure. Consequently the products do not contain unsaturated end groups and are therefore not capable of free-radical polymerization. 4. The kinetic study of a large number of polyester-acrylates differing in nature and in chain length. The present communication presents only results relating to the initial stage of polymerization of polyester-acrylates.
The effect of the viscosity of the reaction medium on the rate of polymerization of polyester-acrylates During the preliminary s t u d y [1, 2] it was found that the bulk polymerizability increases in the order TGM-3~MDF-1
TGM-3 MGPh-9 MDPh- 1 MDPh-2 MBPh-1 MDA- 1 MMA
Viscosity at 25° (cS)
in bulk
5o% IDPh-2*
75% IDPh-2
k~for polymerization in b u l k
(1/mole/see) 10 95
60 1000 115 55
3 7 4 17.5 5 3 0"75
5
8
2-3 × 3.6 X 1.9 × 1.4 × 6.4)<
7.5 1.2"*
2.3 X 106 4.0 × l0 T***
8
9.8
4-3 14 6
8
10 e l0 s 106 l0 s 105
* Vtzco6ity of IDPh-2 is 800-900 cS. ** 80% IDPh-2. *** Published values, 3 x 10' at 50*
of the methacrylic groups, hence with increasing distance between the two methacrylic groups on passing from the esters of mono-ethylene glycol to those of tri-ethylene glycol the reactivity of the dimethacrylic esters increases.
484
G. V. KOROLEV et al.
I n order to examine the possible causes of the different'reactivity of polyester-acrylates, differing in the nature and length of the oligomeric chain, t h e initial rates of polymerization, w0 were measured for a large number of oligomers, differing in viscosity (see Table). Polymerization was carried out at 50 ° in the presence of 0 . 5 ~ by weight of dicyclohexylperoxydicarbonate (I) [6] as initiator, in bulk and also in solution in benzene and in a highly viscous solvent consisting of the telomer obtained b y condensation of diethylene glycol, phthalic acid and isobutyric acid, with a degree of polymerization n--~2 (arbitrarily called IDPh-2). The polymerization rate, w0 was measured by an improved thermometric method (calorimetric variant [4]). The synthesis [1,7], purification and nomenclature of the oligomers have been described previously. TGM-3 is triethylene glycol dimethacrylate and MGPh-9 is the dimethacrylate of (bis-triethylene glycol)phthalate. The other polyester-acrylates are polyesters of the type L(AB)nAL, where L and B are mono- and dibasic acid radicals respectively, and A is a dihydric alcohol radical. The figures after the abbreviated name in all cases except TGM-3 and MGPh-9 (these are industrial codes) denote the value of n, and the letters denote the following radicals: D, diethylene glycol; B, butanediol; A, Ph, I and M, adipic, phthalic, isobutyric and methacrylic acids respectively; MMA is methyl methacrylate. The results show that when the system is diluted with a solvent of higher viscosity than that of the polyester-acrylate present, in all cases w0 falls, whereas when the solvent is of lower viscosity than the polyester-acrylate wo increases. For example the addition of 2 0 ~ (by weight) of benzene to TGM-3 reduces wo b y 15%, and when the same quantity of benzene is added to the much more viscous MGPh-9, w0 falls by ~ 100%. It is seen from the Table that the addition of the highly viscous, ideal solvent IDPh-2, with a viscosity of 800-900 cS, to MDPh-2, with a viscosity of 1000 cS, reduces wo a little, but the addition of IDPh-2 to all the other polyester-acrylates, none of which has a viscosity greater than 115 cS, increases w0 considerably. The sensitivity of the initial polymerization rate, w0, to the viscosity of the medium indicates that almost at the very lowest degree of conversion in the polymerization of all these polyester-acrylates, including even the comparatively low viscosity material TGM-3, the termination reaction is diffusion controlled. The value of w0 for the bulk polymerization of the various polyester-acrylates (see Table) increases in the order T G M - 3 ~ M D A - I < M D P h - I ~ M B P h - I ~ ~ M G P h - 9 < M D P h - 2 . The viscosities of these materials increase in the order T G M - 3 < M I ) A - I - ~ M D P h - I < M G P h - 9 < M B P h - I < M D P h - 2 , i.e. in all eases except MGPh-9 the value of wo varies in the same direction as the variation in the original viscosity of the polyester-acrylates. When these compounds were polymerized under conditions of approximately equal viscosity (the addition of about 75% of the highly viscous, ideal solvent IDPh-2) (see Table) the differences in w0 levelled out and the values of w0 practically coincided.
Polymerization of polyester-acrylates
485
We suggest t h a t the equalization of the values of w0 when the viscosity of the me.dium is equalized, and the concomitant variation of w0 for the various polyester-acrylates with the variation of their initial viscosities can be interpreted within the framework of the foUowing suppositions: (a) at degrees of conversion close to zero the propagation constant, kp, is practically the same for all the polyester-acrylates, i.e. the chemical nature and length of the oligomer chain have only a weak effect on the reactivity of the terminal methacrylate groups; (b) the termination rate constant, kt, falls steeply with increasing viscosity of the medium; (c) the value of kt under conditions of equal viscosity is weakly dependent on the nature and length of the oligomer chain. I t is evident t h a t the intramolecular association of the methacrylate groups observed b y one of us [5] in the polymerization of the dimethaerylates of mono-, di- and triethylene glycol is possible only in esters in which the double bonds are sufficiently close to one another. Evidently this does not arise in the above oligomers. The conclusion m a y therefore be drawn that the difference in the apparent reactivity of the above polyester-acrylates in bulk polymerization is not associated with the different chemical nature of the polyhydric alcohol and dibasic acid radicals making up the oligomer chain, but is mainly the result of a difference in the physical properties (viscosity) of the medium. The fact that kp for these polyester-acrylates is only weakly dependent on the chain length of the olig0mer conforms with published information [8, 9] on the reactivity of the double bond in methacrylate esters. For the polymerization of CH2=C(CH)aCOOR at 30 °, kp is 350 l/mole/sec for R=CH3; 467 for R = n - - C a H v and 362 for R----C4H9, i.e. kp varies little with increasing length of R. I f one regards the polyester-acrylates as methacrylate esters in which R is an oligomeric chain it is reasonable to suppose that the value of k~ for the polyester-acrylates will be little different from ]cp for the above methacrylate esters. Then taking the published propagation rate constant for MMA, k~ ~ 400 1./ /mole/sec (at 50°), and the value of the rate constant for the decomposition of I as k0=8.46 × 10-5 sec-1 (at 50 ° in benzene) [6], and assuming that the efficiency of initiation (f is usually ~ 0.5) and kp are considerably more weakly dependent on viscosity t h a n k t, it is possible to estimate the values of/¢t (for degrees of conversion close to zero) from wo for the various cases given in the Table. The last column of the Table gives the values of k t for polymerization of the polyester-acrylates in bulk. These estimated values of kt show t h a t even for polyester-acrylates of relatively low viscosity (TGM-3), k t is less t h a n t h a t of MMA by a factor of ~ 15. For the more highly viscous materials (M_DPh-2), kt differs from that of MMA by about two orders of magnitude. With an increase in the viscosity of the polyester-acrylate by a factor of ~ 100 the value of kt falls b y a factor of ~ 10. I n a solution containing 750/o of IDPh-2, ]¢c for all the polyester acrylates is (3-4) × 105 1./mole/sec, and for MMA with 80% of IDPh-2 k t ~, 10L This dif-
486
G . V . KOROLEV e t a / .
ference between the values of kt for M2J_A and the polyester-acrylates i s not unexpected, because it is obvious t h a t the coefficients of diffusion of the ,macroradicals -CHIC(CHs) -CHsC(CH3) -CHsC(CHs) I
COOR
I
COOR
i
(II)
COOR
in the highly viscous medium of IDPh-2, will be considerably lower when R is an oligomeric polyester chain than when R is CH 3, particularly when it is remembered that the maeroradicals of type II, containing a polyester chain with one unreacted terminal double bond in each unit, can be branched even at degrees of conversion close to zero. Finally it should be noted that from the point of view of steric hindrance it would be expected that k~ would fall with increasing length of the polyester chain. However in the case of our polyester-acrylates the length and flexibility of the oligomeric chain are sufficient to prevent the presence of the oligomeric "tail" having any substantial effect on the mobility of the reactive end groups when the oligomer molecules move in media in which t h e y are not fixed (i.e. in media possessing flow). This is specific to polyester-acrylates in which the length of the oligomeric chain is sufficient to give rise to properties close to those of polymer chains and, as will be shown in subsequent communications, is associated with a number of kinetic peculiarities of the polymerization of polyesteracrylates. The limitation of the idea of a weak effect of the long, oligomeric "tail" on the mobility of the end g~oups to only flowing media is very necessary, because (as further study showed) differences in the flexibility of the chains of these polyester-acrylates which approximately level out in flowing media, in erosslinked media where the molecules of the oligomer are fixed (suspended in a three-dimensional framework), lead to substantial differences in the mobility of the end groups. In certain cases the value of kp can be dependent on the shape of the oligomer molecule. For example if the chain is so long and flexible that it winds into a coil, kp can fall as a result of steric hindrance. We have observed such steric hindrance in a number of MDA-n polyester-acrylates when n ~ 2 . Evidently the o]igomer molecules containing the long hydrocarbon chain of the adipic acid radical are coiled. I f the chain is very rigid k~ must fall with increasing length of the molecule. From stereochemical considerations it follows that when the diethylene glycol radical is replaced by the butanediol radical the rigidity of the oligomeric chain must increase. Consequently polyester-acrylates of the MBPh-n type were synthesized and studied. I n spite of the increase in viscosity of the reaction medium on passing from MBPh-1 to MBPh-2, w0 fell to some extent, evidently indicating that kp falls considerably on passing along the series of homologous telomers from MBPh-1 to MBPh-2 with increasing chain length, as a result of an increase in steric hindrance. I t is thus a general feature of the oligomers t h a t the reactivity of the end groups in polymerization in media possessing flow is independent of the length
Polymerization of polyester-acrylates
487
and nature of the oligomeric chain, though in certain cases in addition to this general fact it is necessary to take into account specific features of the structure of the oligomer molecules. Mechanism of chain termination in the initial stage of conversion Since the radical R, formed b y decomposition of the initiator I, and the macro-radical R 1 of type II, formed b y the reaction R-bM-*R1, where M is an oligomer molecule, differ considerably in mobility in a viscous medium, two mechanisms for the polymerization of the polyester-acrylates in the initial stag e of conversion can be p u t forward, differing in the nature, of the termination steps: A.
W¢
I--~R R+M
W~
B.
I--~R
kp
k~
R+M ---* RI
---* R 1
k~
k~
RI+M--~ R1
RI+M---> Rl
kt
kt
Rl+R1 --~dead products
R+R1 ~ dead products
It has been found [10] that even in polymerization in media of low viscosity a considerable proportion of the radicals R take part directly in termination of the type R z ~-R. It would be expected that in highly viscous media termination of the type R I ÷ R 1 does not occur at all because of the low mobility of R1, and that termination occurs mainly b y interaction of the small (and therefore mobile) radical R with the macro-radical, R r Chain termination of the type R - ~ R need not be considered because the stationary concentration of R 1 must be considerably higher than that of R. From mechanism A it follows that w0-----k~[M]~/wdkt (1) (where w~ is the rate of initiation), and from B, Wo=k~/k~[M] ~ (2). Our measurements of w 0 at various initiator concentrations show that in all cases w 0 ~[I] °'5. A typical curve is shown in Fig. 1. w~10-a
(,,i,, -')
6 4
L//
2 l_.
0
0.2
]
I
I
(}4
0.6
fl'8
fl] (wt. ~ ) FIG. 1 Dependence of. the initial rate of polymerization of polyester-acrylates on initiator concentration; 50°, reaction system MGPh-9÷50~/o IDPh-2. Thus the experimental results are in agreement with equation (1). Consequently, in spite of the fact that the macro-radicals R 1 have comparatively low mobility, as is shown b y the results presented in the Table, the fraction of
488
G.V. KoRo1.I~v et al.
radicals R taking part in termination of the type R q-R x is very small and the polyester-acrylates polymerize only b y mechanism A. Determination of the rate of initiation in the initial stage of polymerization I n investigations involving ~he modelling of any given physical property of a reaction system it is often necessary to carry out the process in various model media, i.e. in various solvents. I t is therefore necessary tO have information on the effect of different media on the rate of decomposition of the initiator. We have observed t h a t peroxides decompose much more rapidly in glycols than in benzene, toluene or similar solvents. For example the decomposition rate constants for benzoyl peroxide and dicyclohexylperoxydicarbon~te in benzene at 50 ° are 1.1 x 10-6 sec -1 and 8.46 × 10-5 sec -1 respectively, and in triethylene glycol 4.5 × 10-s sec -~ and 2 × 10-4 sec -1 respectively. Since all the above polyester-acrylates (except MBPh-1) and the ideal, highviscosity solvent IDPh-2 contain di- and triethylene glycol radicals in the oligomeric chain it would be expected t h a t the accelerating action of the latter would be preserved to some extent in these. Moreover, since the different polyesteracrylates contain different proportions of glycol radicals the initiator should decompose at different rates in each of the polyester-acrylates, and this should be taken into account when comparing the values of wo (see Table). Consequently the necessity arises of measuring the rates of decomposition of initiators (peroxides) in the oligomers. The usual methSds for the determination of peroxides are very unsuitable in such media as polyester-acrylates, IDPh-2 and the like, because of the insolubility of the oligomers (and especially of the crosslinked polymers formed when the decomposition of peroxides is being studied) in the reagents used for the chemical determination of peroxides. The necessity of extraction leads to complications and a prolonged procedure for the determination of peroxides in these cases, and at the same time does not give sufficiently accurate results. We have developed the following kinetic method for determination of the rate constant k 0 for the decomposition of peroxides (applicable to initiators of any type). We derived equations [11] for an instantaneous degree of conversion C t and a rate of initiation wt, taking into account the loss of initiator. Analysis of these equations shows that in the case of initiators for which k0 satisfies the condition k 0 >4k~ [I]o/kt (where [I]0 is the initial concentration of initiator) polymerization does not go to completion (the final degree of conversion Coo is less than 100%). In this case k o =2wo/C~ (wo is the initial rate of polymerization, Coo is the limiting degree of conversion.) I f however k0<4/c~ [I]o/kt, Coo is always ~ 100% and equation (1) cannot be used for the determination of k 0.
Polymerization of polyester-acrylates
489
I n this case k 0 can be determined with a little more complication from the relationship: log [M]t=log [M]° +0.217 kot Wt
W0
(where [M]0 and [M]t are the initial and instantaneous monomer concentrations), from the slope of the graph of log [M]t/w t against t.
0.55 0
I
I
Io 2o r ,e(m@ Ko=g.S.lO- (sec-9
FIG. 2 Determination of k o, the decomposition rate constant of dicyclohexylperoxydicarbonate, in 80~ IDPh-2+20yo MMA at 50° by the kinetic method. This method enables k o for the given conditions to be determined from a single rate curve. Figure 2 shows the relationship log [M]dwt = f ( t ) , from which the rate constant for the decomposition of dicyclohexylperoxydicarbonate (1) in IDPh-2 at 50 ° was calculated (k0=-9.8 × 10-5 sec-Z). The experiment were carried out as follows. A mixture of the solvent (IDPh-2), the indicator monomer (we usually took 20% b y weight of MMA) and the initiator were placed in the thermometric apparatus and the rate curve was plotted. Then b y graphical differentiation of the latter the relationship of the type shown in Fig. 2 was obtained. Since the value of k 0 found is close to the published value for the decomposition of I in benzene (k0=8.46 × 10-5 sec -1 at 50°), it m a y be considered that the di- and triethylene glycol radicals in the polyester-acrylates have very little effect on the rate of decomposition of I. Consequently for the calculation of k t it is justifiable to regard the rate of initiation as being the same for all the reaction systems in question. CONCLUSIONS
(1) The initial rate, w0 of polymerization (initiated b y dicyclohexylperoxydicarbonate) of polyester-acrylate oligomers has been measured b y the thermometric method at 50 °. The oligomers differed in the length and nature of the chain. Polymerization was carried out in bulk and in solution in a highly viscous, ideal solvent, consisting of an oligomer sim;lar in chemical structure to the polyester-acrylates b u t incapable of free-radical polymerization. (2) It was found that in the bulk polymerization of polyester-acrylates differing in viscosity the value of w0 increases with increasing initial viscosity of the oligomer. When the polyester-acrylates are polymerized under conditions of equal viscosity (dilution with a highly viscous solvent) w 0 is practically the same in all cases.
490
Yu. S. LIPATOV
(3) The results are i n t e r p r e t e d on ~he a s s u m p t i o n t h a t t h e p r o p a g a t i o n rate c o n s t a n t is t h e same for all t h e polyester-acrylates studied a n d t h a t chain t e r m i n a t i o n is diffusion controlled f r o m t h e v e r y beginning o f polymerization. The t e r m i n a t i o n rate c o n s t a n t s for the various polyester-acrylates are estimated. (4) The decomposition rate c o n s t a n t for t h e p o l y m e r i z a t i o n initiator, dicycloh e x y l p o r o x y d i c a r b o n a t e , has been d e t e r m i n e d directly in the reaction s y s t e m b y a kinetic m e t h o d . Translated by E. O. PHILLIPS REFERENCES
1. T. YA. KEFELI, G. V. KOROLEV and YU. M. FILIPOVSKAYA, International Symposium on Macromolecular Chemistry. Section I, p. 47, Moscow, June 1960 2. G. V. KOROLEV, L. I. MAKHONINA and A. A. BERLIN, Vysokomol. soyed. 3: 198, 1961 3. G. V. KOROLEV, B. V. PAVLOV and A. A. BERLIN, Vysokomol. soyed. 1: 1936, 1959 4. G. V. KOROLEV, Rus. Pat. No. 137, 304, Class 421, 335; Byull. izobret. No. 7, 1961 5. A. A. BERLIN et at., Sb. statei po obshch, khimii, 2: 1554, 1953 6. A. G. RAZUVAYEV, L. M. TERMAN, V. R. LIKHTEROV and V. S. ETLIS, International Symposium on Maeromoleeular Chemistry. Section II, p. 53, Moscow, June 1960 7. A. A. BERLIN, G. L. POPOVA and YE. F. ISAYEVA, Dokl. Akad. Nauk SSSR, 123: 282, 1958; Vysokomol. soyed. 1: 7, 951, 1959; A. A. BERLIN, T. YA. KEFELI, YU. M. FILIPOVSKAYA and YU. M. SIVERGIN, Vysokomol. soyed. 2: 411, 1960 8. C. WALLING, Svobodnye radlkaly v rastvorakh. (Free Radicals in Solution.) Foreign Literature Publishing House, Moscow (Russian translation,) 1960 9. KH. S. BAGDASAR'YAN, Teoriya radikal'noi polimerizatsii. (Theory of Radical Polymerfzation.) Izd. Akad. Nauk SSSR, Moscow, 1959 10. G. HENRICHI-0LIVE and S. OLIVE, Makromol. Chem. 37: 71, 1960 11. B. V. PAVLOV and G. V. KOROLEV, Vysokomol. soyed. 1: 869, 1959
STUDIES OF POLYMER-FILLER INTERACTIONmVI. SOME RHEOLOGICAL PROPERTIES OF POLYMER SOLUTIONS IN THE PRESENCE OF FILLERS* YU. S. LIPATOVt Institute of General and Inorganic Chemistry, Beloruss. S.S.R. Academy of Sciences (Received 19 June 1961) T H E s t u d y o f t h e properties of p o l y m e r solutions in t h e presence o f fillers is o f g r e a t practical interest for the p a i n t i n d u s t r y , t h e reinforced plastics i n d u s t r y , * Vysokomol. soyed. 4: No. 10, 1528-1536, 1962, t The experimental work was carried out by M. G. Barkovskaya.