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The kinetics and mechanism of polymerization of methyl methacrylate radical in the presence of ortho-phosphoric acid
Kinetics and mechanism of polymerization of methyl methacrylate radical 37. 38. 39. 40. 41. 42. 43. 44.
1195
R. S. MOORE, J. Polymer Sci. 5, A-2: 711, 1967 A. PETERLIN and K. SAKAOKU, J. Appl. Phys. 38: 4154, 1967 R. ROBERTSON, J. Polymer Sci. 10, A - I : 2437, 1972 S. NOMURA, M. MATSUO and H. KAWAI, J. Polymer Sci., Polymer Phys. Ed. 12: 1371, 1974 Yu. A. ZUBOV, V. I. SELIKHOVA and V. A. KARGIN, Vysokomol. soyed. A9: 353, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 2, 394, 1967) V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. A12: 2136, 1970 (Translated in Polymer 'Sei. U.S.S.R. 12: 9, 2422, 1970) J. L. HAY and A. KELLER, J. Mater. Sci. h 41, 1966; 2: 538, 1967 V. I. GERASIMOV a n d D. Ya. TSVANKIN, Vysokomol. soyed. A l l : 2652, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 12, 3013, 1969)
THE KINETICS AND MECHANISM OF POLYMERIZATION OF METHYL METHACRYLATE RADICAL IN THE PRESENCE OF O R T t l O - PHOSPHORIC ACID* YE. S. GARI~A, A. V. OLENIN, V. I:). ZUBOV and V. A. KABANOV M. V. Lomonosov State University, Moscow
(Received 30 May 1975) The photo-initiated homo-polymerization of methyl methacrylate (MMA) has been studied at 25°C in the presence of ortho-phosphorie acid as complex former. Post-polymerization accompanied by a mol.wt, increase has been found to occur in addition to an increase in reaction rate after ceasing photo-initiation iu the system MMA-H3POa. The factor responsible for the continuation of the post-polymerization in such a system has been found to be the macro-radicals whose active life is greatly prolonged in the presence of the complex former. The effects of various factors in the kinetics of the post-illnmination process have been studied. A rapid drop of the bimoleeular termination rate of the polymethylmeehaerylate (PMMA) radicals has been found to be linked with a change of the eonfigurational properties and the associations of the growing chains as a result of the eomplexing with laI3POd. A mechanism is suggested for the polymerization in the described system.
TEn photo-initiated polymerization of methyl methacrylate (MMA) dissolved in ortho-phosphoric acid had been reported in earlier work [1, 2] to be characterized by a higher initial rate t h a n the bulk polymerization carried out under identical conditions. The mol.wt, was also found to increase. Long life reactive centres also formed in the polymerizing system and propagate the chain even after the source of UV source had been switched off. * Vysokomol. soyed. A18: No. 5, 1040-1046, 1976.
1196
Y~,. S. GAWU~Aet a/.
The work described here is a detailed s t u d y of the kinetics and mechanism of the polymerization of MMA in the presence of H3PO 4. Kinetics of polymerization of MMA in the presence of H3PO4. The reaction kinetics were investigated at a molar ratio [HsPO4] : [MMA] of 0.3 to 6. The polymerization was initiated by UV of wavelength 4--365 nm at 25°C in the presence of benzyl as sensitizer ( c : l . 9 X 10 -4 mole/1.). Dilatometry was used to record the reaction kinetics and the mol.wt, of the polymers (PM~IA) were determined by viscometry in dichloroethane; equation [~]=1.7×10-3M °'6* [3] was used in the calculations.
1"8 -
~
-2
3
0.8 0
lO0
300
Time, rain FIG. 1
500
0
20
40
60
q,%
FIG. 2
FIG. 1. The kinetic polymerization curves of." /--pure MMA; 2--system (H3PO4]/[MMA~ =0.3; 3--[HsPO4]/[MMA]=6. (U-Y'-light; the process temperature is 25°C here and in Figs. 2-6). FIG. 2. The reaction rate as a function of the conversion for: /--pure MMA; 2--system [HsPO4]/[MMA]-----0-3; 3-- [H3PO4]/[MMA]= 6. (UV-light). Addition of HsPO 4 to MMA resulted in a rapid increase in the initial rate and also in an abrupt change in the progress of the kinetic curves in the 0-70~/o conversion range. Figure 1 shows the kinetic curves at high conversion using various tI3PO 4 quantities, while l~ig. 2 shows the rate as a function of conversion. One can see t h a t the change from the pure monomer to the MMA-H3PO , system reduced the time required to reach a specific conversion and of the peak rate in the gel stage, and also of the peak conversion rate; auto-acceleration of the. reaction starts at a lower conversions. The peak conversion rate begins almost immediately on starting the photo-initiation and remains unchanged up to 300/o conversion when the ratio of the components is 6. Systems with a large acid content give rise to an instantaneous reaction ;as is normally applicable from the gel stage onwards. The initial rate of MMA photo-polymerization as a function of the [H3PO4]/ /[MMA] ratio for the 0-5% conversion range is shown in Fig. 3. One can see t h e very rapid increase in the rate with acid/monomer ratios between 0.5 and 1,
Kinetics and mechanism of polymerization of methyl methacrylate radical
1197
A detailed kinetic study of the initial stage of the MMA polymerization in H ~ P 0 , solution ([HaPO4]/[MMA]----6) showed the order of the reaction to be close to 0.6 with respect to UV intensity. This means t h a t the majority of the chains decay by a bimolecular mechanism when the radicals are initiated by a continuous light source. The order of the reaction with respect to monomer, determined by varying the ratio of MMA to the non-polymerizing diluent, i.e. ethyl acetate (EA) with [HaPO4]/[MMAq-EA]=6 , will be slightly above unity and will almost agree with t h a t for the monomer during the MMA photo-polymerization in EA without o-phosphoric acid. We also examined the polymerization of the M ~ A - H s P O 4 system in the presence of n-butyl mercaptan (BM) or lauryl mercaptan (LM) as chain transfer agents with [HaPO4]/[MM_A] ratios of 0.5, 1, or 6. As in normal radical reactions, there was a steady decrease in the mol.wt, of the polymerizate when the chain transfer agent concentration was increased. The transfer constants Ca for the mercaptans with PMM_A radicals given below were calculated from equation (1):
1
1
[s]
>= Yo
(i)
in which/50 is the average degree of polymerization of PMMA without a transfer agent; [S], [M], transfer agent and monomer concentration respectively, mole/1. Transfer agent [CH,PO,]/[MMA]
BM 0 0.45
Ca
LM 0.5 0.10
BM 1.0 0.07
BM 6.0 0.02
Some reduction in Ca in the HsP04 containing systems compared with pure M_IV[A, could be due to a decrease in the activity of the transfer agent as a result of protonization of the mercaptan by the acid. The photo-polymerization of the MMA-H3P04 system is inhibited by benzoquinone (BQ). The order of reaction with respect to light intensity will be u n i t y in the presence of BQ, i.e. practically all the reactive centres decay on the inhibitor molecules. The rates and the viscosity average mol.wt, given below are for various BQ concentrations and [HaPO4]/[MMA]----1, T = 2 5 ° 0 , ~=365 nm, and b e n z y l = 1.9 × 10 -4 mole/1. [BQ] × 10a, mole/1. Wp× 104, mole/1..
0 3.4
0-6 2.6
0.8 2.1
1.0 2.8
1.5 0.76
2.0 0.4
11.4 0.04
3-24
2.57
2.45
1-8
1.26
0.26
114.0 0
"see
M v× l0 s
45
--
The dependence of the average degree of polymerization /5 on the inhibitor concentration can be satisfactorily made linear according to equation (2) [4]: 1
1
]~in
3= & + -[BQ], kpt l
(2}
1198
YE. S. G ~ I N A
et at.
in which -Po is the average degree of polymerization of PMMA without inhibitor under otherwise identical conditions; loin and/~p, rate constants of inhibition and chain propagation respectively; [M], [BQ], monomer and benzoquinone concentrations respectively, molell. 5
-#
~illum x 101 SeC
I
0
I
2
f
I
#
I
I
#
4
[HsP04]/ [MMA]
12
ZO
EBQ] = I
Fro. 3
otelL
Fro. 4
:FIG. 3. T h e initial r a t e of t h e M M A p h o t o - p o l y m e r i z a t i o n as a f u n c t i o n of t h e [HaPO4]/ /[MY/k] m o l a r ratio. (UV-light, ),= 365 rim. T h e benzyl c o n c e n t r a t i o n used here a n d in Figs. 4-6 is 1.9 × 10 -4 mole/l.). :FIG. 4. The inverse degree o£ p o l y m e r i z a t i o n of MM.A for s y s t e m M M A - H s P O 4 as a f u n c t i o n of B Q c o n c e n t r a t i o n . ([H3PO4]/[MMA] = 1, l_J-V-light, ), = 365 rim).
1/_P is plotted is a function of [BQ] in Fig. 4. The slope of the plot was used to find the ratio kin/lop=2.3 (it was 5.5 in a polymerization of pure MMA at 44.1°C [5]). The polymerization of the MMA-HsP04 system stopped rapidly in the presence of the inhibitor when the UV was switched off. In post-polymerization conditions, where there is no generation of primary radicals, the macro-radicals decayed predominantly by reacting with the B Q molecules and the reaction describing this is:
log
wt
=
,n[B Q] t,
(3)
in which w t , is the rate when the UV was switched off; w t, rate of post-polymerization at time, t. The processing of the kinetic extinction curve in the eqn. (3) coordinates, assuming t h a t the inhibitor consumption during post-polymerization can be neglected, made it possible to determine the rate constant for inhibition as being kin----1.4x l0 S 1./mole.sec (Fig. 5). From this we get kp-----102 1./mole.see which is close to the order of the rate constant for the M:IVIAphoto-polymerization a t 25°C without o-phosphoric acid (kp ~m~=200 1./mole. sec [5]). The kinetic results obtained in the presence of both the chain transfer agent a n d the inhibitor led to the conclusion t h a t MMA polymerization is a typical radical process in which the activity of the PMMA radicals in the
Kinetics and mechanism of polymerization of methyl methacrylate radical
1199
propagation step is similar to that during pure MSMApolymerization. It is therefore natural to assume that the considerable acceleration found in the presence of o-phosphoric acid must be due to a reduction in the bimolecular termination reaction. An evaluation of the termination rate constant, using [HaPO4]/[MM_A]= 1 and equation
1
kowp
in which wp is the rate of photo-polymerization, mole/1..sec, showed ko=104 1./mole.see, while that for pure MM_A, was approximately 107 1./mole.sec [5]. The presence of H s P 0 4 thus reduced the k0 b y 3 powers of ten.
109(, 2.¢
/-2
0
12
2¢
T/me, rnin I~IG. 5. The low (w~o/wc)as a function of time for the post-polymerization of system MMAH3P04 in the presence of 1.1 × 10 .3 mole/l. BQ. ([H3P0,]/[MMA]~ 1, UV-light, )t~365 am).
The photo-iditiated post-polymerization of the M M A - H 3 P O 4 system. The rate constant for mutual termination makes it clear that M]FtA polymerization in the presence of HsPO~ ought to s}ow down gradually when photo-initiation stops and virtually to stop within a few minutes. However, it does not stop after switching off the UV source and continues at a slightly lower rate for 10-30 hr until a high conversion of the monomer is reached. The lowest conversion after which post-polymerization can be observed is 0.5%. The system appears externally homogeneous during this process; Fig. 6 shows a typical kinetic curve; the course of polymerization of pure M:M_Ais shown for comparison under identical conditions, the process naturally stopping rapidly after the U V source had been switched off. The mol.wt, increase during the post-polymerization is a linear function of conversion. (The mol.wt, of these polymers is usually greater b y a factor of 10 than that of thePM1VEA produced under identical conditions without H3P04). I t was found that the concentration of macromolecules [N], calculated from
[-M]oq
Y~.. S. GA~I~A et al.
1200
in which q is the degree of conversion; PT--current degree of polymerization, determined during post-polymerization, which remained almost constant (see Table). The constancy of IN] indicates the absence of a n y initiation after the U V source had been switched off, and also that the effect of the chain transfer reaction under our conditions is only slight.
Wpo.~'+[[ltu'n 0"+ 2
Mu ~lO's
Conversion~%
.~ 38 ~ 8"
-0¢
q
z/[-1~
0.t
0 ~i'x'l 0
20
O0
Time, m in F~(~. 6
180
180
f
I
I
1
3
5
1 7
[H3P04] / [-MMA] FIG. 7
FIG. 6. The photo- and post-polymerization kinetics for system MMA/HsPO4." a--I : 6; b--pure MMA (UV-light, 2----365 nm. The UV extinction is indicated by an arrow). FIG. 7. The rate ratio of post- to photo-polymerization (1) and the mol.wt, of the post-polymerizate (2) as functions of the [HsPO~]/[MMA] molar ratio. A 1"6~o conversion during the photo-reaction; total conversion= 3~o. Cessation initiation is followed in a homogeneous liquid phase system b y a termination reaction in which only those PMMA radicals participate which are present in the system when the UV is switched off. The other part is stable and propagates the reaction b y a "living-chains" mechanism in post-polymerization conditions. We investigated the effects of some factors on the kinetics of the postillumination polymerization. We mentioned already that addition of inhibitor (10 -= mole/1. BQ) results in a rapid extinction of the post-polymerization and that a further increase in the B Q concentration prevents a post-illumination reaction altogether. The presence of a chain transfer agent, i.e. BM, greatly reduces the rate of the post-polymerization as a function of its concentration, or as a function of the mol.wt, decrease of the polymerizate, which is the same thing. In other words the number of the PMMA radicals remaining in the system after the photo-initiation, which can be estimated from the rate ratio of the postphoto- to the photo-reaction, will drop as a function of the decrease in average chain length. This would mean that the capacity for non-terminated propagation under post-polymerization conditions is possessed only b y macro-radicals having a specific length. The number of the macro-radicals responsible for the post-photo-process depends on the conversion reached during the continuous photo-initiation period.
Kinetics and mechanism of polymerization of methyl methacrylate radical
1201
The WposdWmumratio will initially show a rapid increase as a function of photoconversion a t a fixed a c i d : m o n o m e r ratio ([HsPO4]/[MMA]=6); it reaches i t s limit already near 15%. Using a fixed photo-conversion (qnlum= 1"6%) will give a number of long-life macro-radicals depending on the phosphoric acid content of the system. Figure 7 illustrates the dependence of the wpo~dWinumratio and of the mol.wt, of the post-polymerizate on the acid/monomer molar ratio. One can see t h a t the relative rate of the post-polymerization increases as a function of the H3PO~ content and also reaches the (upper) limit at [HsPO4]/[MMA]= 3. The mol.wt, of the PMMA increases at the same time. CORRELATION OF THE ~o CONVERSION WITH THE DEGREE OF I'OLY~ERIZATIOI~OF THE ~CROMOLECULESDURII~GTHE MM.AHsPO4 POST-POLYMERIZATIOlq {25°C, 1"9 × 10-4 mole/1, of benzyl, 1.6~/oconversion in the photopolymerization under U'V-light, )l=365 nm) /[MMA]
[M]0, mole/1,
q, ~o
6
1.9
3.3
[H,PO,]/
5.7
PwX 10-' [N]molefl. × 10',
5"8 9.7 10.7
10.5 15-5 20.1 23.8
60 71 88
2.0 2.8 4.8 7.2
3.5 5.0 6.2 9.8
3.3 3.2
85
4.4 4.2
The characteristics of iMMA radical polymerization in the presence of ttsPO ~ cannot be understood on the basis of the usual theories about the chemical reactions which take place in homogeneous liquid media. One must take into account here the specific effect of the structural and physical properties of the medium and those of the macromolecules on the kinetics of the diffusion controlled bimolecular decay reaction of the macro-radicals. A distinct example of how internal physical factors affect the homogeneous polymerization kinetics, i.e. the gel stage, given by some authors [6-8], is an association with macromolecular aggregation and the formation of a steric fluctuating network. The strueturation study carried out during the bulk polymerization of MMA by a radical mechanism, using light scattering [8], gave rise to the theory t h a t micro-associates of macromolecules form with the macro-radicals and are fluctuating in nature; this already happens in the early phase of the monomer conversion and the steric network starts to form at polymer concentrations at which gel formation starts. The period of existence of the associates in such a network is much longer t h a n during t h e initial conversion period. I t was natural assume t h a t o-phosphoric acid would act as a polyfunctional
1202
YE. S, G~LRI~£ e~ al.
complex former, and that this function is primarily due to a greater probability of fluctuating associates forming which are stabilized during an early conversion stage as a result of intra- and inter-molecular H bond formation between the carbonyl groups contained in the complex and the 3 functional groups present in phosphoric acid [2]. The probability of the macro-radical, present in the associates, participating in the decay will rapidly diminish due to a decrease of the diffusion rate of their reactive ends towards each other. The rate of the photo-polymerization consequently increases in the partly structurated system, i.e. under gelling conditions. Nevertheless, the macro-radicals will participate during continuing initiation in the bimolecular termination reaction b y coming in contact with the continuously generated "short" radicals of propagation which retain a relatively large diffusion capacity. The constant availability of such radicals ensures a pseudo-stationary process, i.e. the order of the reaction with respect to the initiation rate will be close to 0.5. Two types of propagation chains thus exist during photo-initiation in associates with the macro-radicals, namely those incapable of reacting with each other, and the "short" radicals capable of participating in the decay and in reactions with each other, as well as with the stabilized macro-radicals. The availability of the low tool.we, radicals stops immediately after the U V source is switched on. The number of radicals which exists in the system at this moment will also depend on the competition between decay and propagation reactions with their incorporation in the associates. A proportion of the radicals will decay b y the bimolecular mechanism while another proportion will succeed in reaching a considerable chain length and will form associates; they will also remain capable of reacting further and will be responsible for the progress of the post-polymerization b y a "living-chain" mechanism. The modification of the structural and physical properties of the polymerization system b y means of poly-functional complex formers such as ortho-phosphoric acid thus enables us to control the bimolecular chain termination efficiently in the liquid phase radical polymerization of 1VIMA, even to the degree of completely eliminating macro-radical decay. Translated by K. A. AX~E~ REFERENCES
I. Ye. S. (~ARINA, Ye. G. LAGUTKINA, V. P. ZUBOV and V. A. KABANOV, Vysokomol. soyed. B14: 563, 1972 (Not translated in Polymer Sei. U.S.S.R.) 2. Ye. S. GARINA, T. lYl. KUZNETSOVA, ¥. P. ZUBOV and V. A. KABANOV, Dokl. Akad. Nauk SSSR 209: 380, 1973 3. F. BILLMEYER and @. D. THAN, J. Am. Chem. Soc. 74: 4763, 1955 4. Kh. S. BAGDASAR'YAN, Zh. fiz. khim. 32: 2614, 1958 5: J. KICE, J. Polymer Sci. 19: 123, 1956 6. S. Ya. FRENKEL', Vvedenie v statisticheskuyu teoriyu polimerizatsii (Introduction to the Statistical Polymerization Theory). Izd. "Nauka", 1965
Isothermal compressibility of PMMA
1203
7. D. A. EDEL'SHTEIN, B. R. SMIRNOV, V. P. GRACHEV and G. V. KOROLEV, Khimiya aromatieheskikh i nepredel'nykh soyedinenii (The Chemistry of Aromatic and Unsaturated Compounds). Irkutsk, 1971 8. R. A. SIMONYAN, V. A. KASANKIN, M. B. LACHINOV, V. P. ZUBOV and V. A. KABANOV, Dok]. Akad. Nauk SSSI~ 217: 631, 1974
T H E I S O T H E R M A L C O M P R E S S I B I L I T Y OF P O L Y M E T H Y L METHACRYLATE B. P. S H $ A ~ ,
IN V A R I O U S
PHYSICAL
STATES*
I. M. ~/~ONICH, S. A. A~Z~[AXOV and N. Y r . AVERBAKH
(Received 2 July 1975) The instrument used to determine the specific equilibrium volumes of polymers in the 1-9000 kg/em ~ pressure range and at temperatures up to 220°C is described. The specific volumes and isothermal compressibility factors are tabulated for PMMA; the glass temperatures and free volumes of PMMA have been determined as functions of pressure. The type of functions obtained is thought to be due to structural irregularities of the PMMA.
THE connections between t he pressure, t e m p e r a t u r e and t he specific volume of a polymer is a f unda m ent al characteristic which can be used to get a v a r i e t y o f information a bout the t h e r m o d y n a m i c parameters and t he structure, and some other features of technological importance. The s t u d y of these correlations has been the subject of a series of investigations carried out on various polymer types. Th e classical object, belonging to the group of typical amorphous, polar polymers, is PMMA. The dat a about the P - V - T correlations are of scientific as well as practical interest because of the m arked increase in the production a nd processing of this polymer. Especially noticeable is the lack of dat a for t he lower range of pressures (below 500 kg/cm z) and high ones (above 2000 kg/cm2). This made it quite difficult to get a general picture of the behaviour of PMMA over a wide range of temperatures and pressures which embrace various physical states of the polymer. The literature dat a are not easily compared due to the considerable experimental differences as well as differences in the preparation methods used. Our aim was here to determine the equilibrium specific volume of PMMA within a wide range of t e m per a t ur es and pressures; we used for this purpose a single experimental technique and sample preparation method; we also intended t o fiud the b o u n d a r y separating the glass-like from the highly elastic state within t h e pressure limits used. * Vysokomol. soyed. A18: No. 5, 1047-1052, 1976.
1195
R. S. MOORE, J. Polymer Sci. 5, A-2: 711, 1967 A. PETERLIN and K. SAKAOKU, J. Appl. Phys. 38: 4154, 1967 R. ROBERTSON, J. Polymer Sci. 10, A - I : 2437, 1972 S. NOMURA, M. MATSUO and H. KAWAI, J. Polymer Sci., Polymer Phys. Ed. 12: 1371, 1974 Yu. A. ZUBOV, V. I. SELIKHOVA and V. A. KARGIN, Vysokomol. soyed. A9: 353, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 2, 394, 1967) V. I. GERASIMOV and D. Ya. TSVANKIN, Vysokomol. soyed. A12: 2136, 1970 (Translated in Polymer 'Sei. U.S.S.R. 12: 9, 2422, 1970) J. L. HAY and A. KELLER, J. Mater. Sci. h 41, 1966; 2: 538, 1967 V. I. GERASIMOV a n d D. Ya. TSVANKIN, Vysokomol. soyed. A l l : 2652, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 12, 3013, 1969)
THE KINETICS AND MECHANISM OF POLYMERIZATION OF METHYL METHACRYLATE RADICAL IN THE PRESENCE OF O R T t l O - PHOSPHORIC ACID* YE. S. GARI~A, A. V. OLENIN, V. I:). ZUBOV and V. A. KABANOV M. V. Lomonosov State University, Moscow
(Received 30 May 1975) The photo-initiated homo-polymerization of methyl methacrylate (MMA) has been studied at 25°C in the presence of ortho-phosphorie acid as complex former. Post-polymerization accompanied by a mol.wt, increase has been found to occur in addition to an increase in reaction rate after ceasing photo-initiation iu the system MMA-H3POa. The factor responsible for the continuation of the post-polymerization in such a system has been found to be the macro-radicals whose active life is greatly prolonged in the presence of the complex former. The effects of various factors in the kinetics of the post-illnmination process have been studied. A rapid drop of the bimoleeular termination rate of the polymethylmeehaerylate (PMMA) radicals has been found to be linked with a change of the eonfigurational properties and the associations of the growing chains as a result of the eomplexing with laI3POd. A mechanism is suggested for the polymerization in the described system.
TEn photo-initiated polymerization of methyl methacrylate (MMA) dissolved in ortho-phosphoric acid had been reported in earlier work [1, 2] to be characterized by a higher initial rate t h a n the bulk polymerization carried out under identical conditions. The mol.wt, was also found to increase. Long life reactive centres also formed in the polymerizing system and propagate the chain even after the source of UV source had been switched off. * Vysokomol. soyed. A18: No. 5, 1040-1046, 1976.
1196
Y~,. S. GAWU~Aet a/.
The work described here is a detailed s t u d y of the kinetics and mechanism of the polymerization of MMA in the presence of H3PO 4. Kinetics of polymerization of MMA in the presence of H3PO4. The reaction kinetics were investigated at a molar ratio [HsPO4] : [MMA] of 0.3 to 6. The polymerization was initiated by UV of wavelength 4--365 nm at 25°C in the presence of benzyl as sensitizer ( c : l . 9 X 10 -4 mole/1.). Dilatometry was used to record the reaction kinetics and the mol.wt, of the polymers (PM~IA) were determined by viscometry in dichloroethane; equation [~]=1.7×10-3M °'6* [3] was used in the calculations.
1"8 -
~
-2
3
0.8 0
lO0
300
Time, rain FIG. 1
500
0
20
40
60
q,%
FIG. 2
FIG. 1. The kinetic polymerization curves of." /--pure MMA; 2--system (H3PO4]/[MMA~ =0.3; 3--[HsPO4]/[MMA]=6. (U-Y'-light; the process temperature is 25°C here and in Figs. 2-6). FIG. 2. The reaction rate as a function of the conversion for: /--pure MMA; 2--system [HsPO4]/[MMA]-----0-3; 3-- [H3PO4]/[MMA]= 6. (UV-light). Addition of HsPO 4 to MMA resulted in a rapid increase in the initial rate and also in an abrupt change in the progress of the kinetic curves in the 0-70~/o conversion range. Figure 1 shows the kinetic curves at high conversion using various tI3PO 4 quantities, while l~ig. 2 shows the rate as a function of conversion. One can see t h a t the change from the pure monomer to the MMA-H3PO , system reduced the time required to reach a specific conversion and of the peak rate in the gel stage, and also of the peak conversion rate; auto-acceleration of the. reaction starts at a lower conversions. The peak conversion rate begins almost immediately on starting the photo-initiation and remains unchanged up to 300/o conversion when the ratio of the components is 6. Systems with a large acid content give rise to an instantaneous reaction ;as is normally applicable from the gel stage onwards. The initial rate of MMA photo-polymerization as a function of the [H3PO4]/ /[MMA] ratio for the 0-5% conversion range is shown in Fig. 3. One can see t h e very rapid increase in the rate with acid/monomer ratios between 0.5 and 1,
Kinetics and mechanism of polymerization of methyl methacrylate radical
1197
A detailed kinetic study of the initial stage of the MMA polymerization in H ~ P 0 , solution ([HaPO4]/[MMA]----6) showed the order of the reaction to be close to 0.6 with respect to UV intensity. This means t h a t the majority of the chains decay by a bimolecular mechanism when the radicals are initiated by a continuous light source. The order of the reaction with respect to monomer, determined by varying the ratio of MMA to the non-polymerizing diluent, i.e. ethyl acetate (EA) with [HaPO4]/[MMAq-EA]=6 , will be slightly above unity and will almost agree with t h a t for the monomer during the MMA photo-polymerization in EA without o-phosphoric acid. We also examined the polymerization of the M ~ A - H s P O 4 system in the presence of n-butyl mercaptan (BM) or lauryl mercaptan (LM) as chain transfer agents with [HaPO4]/[MM_A] ratios of 0.5, 1, or 6. As in normal radical reactions, there was a steady decrease in the mol.wt, of the polymerizate when the chain transfer agent concentration was increased. The transfer constants Ca for the mercaptans with PMM_A radicals given below were calculated from equation (1):
1
1
[s]
>= Yo
(i)
in which/50 is the average degree of polymerization of PMMA without a transfer agent; [S], [M], transfer agent and monomer concentration respectively, mole/1. Transfer agent [CH,PO,]/[MMA]
BM 0 0.45
Ca
LM 0.5 0.10
BM 1.0 0.07
BM 6.0 0.02
Some reduction in Ca in the HsP04 containing systems compared with pure M_IV[A, could be due to a decrease in the activity of the transfer agent as a result of protonization of the mercaptan by the acid. The photo-polymerization of the MMA-H3P04 system is inhibited by benzoquinone (BQ). The order of reaction with respect to light intensity will be u n i t y in the presence of BQ, i.e. practically all the reactive centres decay on the inhibitor molecules. The rates and the viscosity average mol.wt, given below are for various BQ concentrations and [HaPO4]/[MMA]----1, T = 2 5 ° 0 , ~=365 nm, and b e n z y l = 1.9 × 10 -4 mole/1. [BQ] × 10a, mole/1. Wp× 104, mole/1..
0 3.4
0-6 2.6
0.8 2.1
1.0 2.8
1.5 0.76
2.0 0.4
11.4 0.04
3-24
2.57
2.45
1-8
1.26
0.26
114.0 0
"see
M v× l0 s
45
--
The dependence of the average degree of polymerization /5 on the inhibitor concentration can be satisfactorily made linear according to equation (2) [4]: 1
1
]~in
3= & + -[BQ], kpt l
(2}
1198
YE. S. G ~ I N A
et at.
in which -Po is the average degree of polymerization of PMMA without inhibitor under otherwise identical conditions; loin and/~p, rate constants of inhibition and chain propagation respectively; [M], [BQ], monomer and benzoquinone concentrations respectively, molell. 5
-#
~illum x 101 SeC
I
0
I
2
f
I
#
I
I
#
4
[HsP04]/ [MMA]
12
ZO
EBQ] = I
Fro. 3
otelL
Fro. 4
:FIG. 3. T h e initial r a t e of t h e M M A p h o t o - p o l y m e r i z a t i o n as a f u n c t i o n of t h e [HaPO4]/ /[MY/k] m o l a r ratio. (UV-light, ),= 365 rim. T h e benzyl c o n c e n t r a t i o n used here a n d in Figs. 4-6 is 1.9 × 10 -4 mole/l.). :FIG. 4. The inverse degree o£ p o l y m e r i z a t i o n of MM.A for s y s t e m M M A - H s P O 4 as a f u n c t i o n of B Q c o n c e n t r a t i o n . ([H3PO4]/[MMA] = 1, l_J-V-light, ), = 365 rim).
1/_P is plotted is a function of [BQ] in Fig. 4. The slope of the plot was used to find the ratio kin/lop=2.3 (it was 5.5 in a polymerization of pure MMA at 44.1°C [5]). The polymerization of the MMA-HsP04 system stopped rapidly in the presence of the inhibitor when the UV was switched off. In post-polymerization conditions, where there is no generation of primary radicals, the macro-radicals decayed predominantly by reacting with the B Q molecules and the reaction describing this is:
log
wt
=
,n[B Q] t,
(3)
in which w t , is the rate when the UV was switched off; w t, rate of post-polymerization at time, t. The processing of the kinetic extinction curve in the eqn. (3) coordinates, assuming t h a t the inhibitor consumption during post-polymerization can be neglected, made it possible to determine the rate constant for inhibition as being kin----1.4x l0 S 1./mole.sec (Fig. 5). From this we get kp-----102 1./mole.see which is close to the order of the rate constant for the M:IVIAphoto-polymerization a t 25°C without o-phosphoric acid (kp ~m~=200 1./mole. sec [5]). The kinetic results obtained in the presence of both the chain transfer agent a n d the inhibitor led to the conclusion t h a t MMA polymerization is a typical radical process in which the activity of the PMMA radicals in the
Kinetics and mechanism of polymerization of methyl methacrylate radical
1199
propagation step is similar to that during pure MSMApolymerization. It is therefore natural to assume that the considerable acceleration found in the presence of o-phosphoric acid must be due to a reduction in the bimolecular termination reaction. An evaluation of the termination rate constant, using [HaPO4]/[MM_A]= 1 and equation
1
kowp
in which wp is the rate of photo-polymerization, mole/1..sec, showed ko=104 1./mole.see, while that for pure MM_A, was approximately 107 1./mole.sec [5]. The presence of H s P 0 4 thus reduced the k0 b y 3 powers of ten.
109(, 2.¢
/-2
0
12
2¢
T/me, rnin I~IG. 5. The low (w~o/wc)as a function of time for the post-polymerization of system MMAH3P04 in the presence of 1.1 × 10 .3 mole/l. BQ. ([H3P0,]/[MMA]~ 1, UV-light, )t~365 am).
The photo-iditiated post-polymerization of the M M A - H 3 P O 4 system. The rate constant for mutual termination makes it clear that M]FtA polymerization in the presence of HsPO~ ought to s}ow down gradually when photo-initiation stops and virtually to stop within a few minutes. However, it does not stop after switching off the UV source and continues at a slightly lower rate for 10-30 hr until a high conversion of the monomer is reached. The lowest conversion after which post-polymerization can be observed is 0.5%. The system appears externally homogeneous during this process; Fig. 6 shows a typical kinetic curve; the course of polymerization of pure M:M_Ais shown for comparison under identical conditions, the process naturally stopping rapidly after the U V source had been switched off. The mol.wt, increase during the post-polymerization is a linear function of conversion. (The mol.wt, of these polymers is usually greater b y a factor of 10 than that of thePM1VEA produced under identical conditions without H3P04). I t was found that the concentration of macromolecules [N], calculated from
[-M]oq
Y~.. S. GA~I~A et al.
1200
in which q is the degree of conversion; PT--current degree of polymerization, determined during post-polymerization, which remained almost constant (see Table). The constancy of IN] indicates the absence of a n y initiation after the U V source had been switched off, and also that the effect of the chain transfer reaction under our conditions is only slight.
Wpo.~'+[[ltu'n 0"+ 2
Mu ~lO's
Conversion~%
.~ 38 ~ 8"
-0¢
q
z/[-1~
0.t
0 ~i'x'l 0
20
O0
Time, m in F~(~. 6
180
180
f
I
I
1
3
5
1 7
[H3P04] / [-MMA] FIG. 7
FIG. 6. The photo- and post-polymerization kinetics for system MMA/HsPO4." a--I : 6; b--pure MMA (UV-light, 2----365 nm. The UV extinction is indicated by an arrow). FIG. 7. The rate ratio of post- to photo-polymerization (1) and the mol.wt, of the post-polymerizate (2) as functions of the [HsPO~]/[MMA] molar ratio. A 1"6~o conversion during the photo-reaction; total conversion= 3~o. Cessation initiation is followed in a homogeneous liquid phase system b y a termination reaction in which only those PMMA radicals participate which are present in the system when the UV is switched off. The other part is stable and propagates the reaction b y a "living-chains" mechanism in post-polymerization conditions. We investigated the effects of some factors on the kinetics of the postillumination polymerization. We mentioned already that addition of inhibitor (10 -= mole/1. BQ) results in a rapid extinction of the post-polymerization and that a further increase in the B Q concentration prevents a post-illumination reaction altogether. The presence of a chain transfer agent, i.e. BM, greatly reduces the rate of the post-polymerization as a function of its concentration, or as a function of the mol.wt, decrease of the polymerizate, which is the same thing. In other words the number of the PMMA radicals remaining in the system after the photo-initiation, which can be estimated from the rate ratio of the postphoto- to the photo-reaction, will drop as a function of the decrease in average chain length. This would mean that the capacity for non-terminated propagation under post-polymerization conditions is possessed only b y macro-radicals having a specific length. The number of the macro-radicals responsible for the post-photo-process depends on the conversion reached during the continuous photo-initiation period.
Kinetics and mechanism of polymerization of methyl methacrylate radical
1201
The WposdWmumratio will initially show a rapid increase as a function of photoconversion a t a fixed a c i d : m o n o m e r ratio ([HsPO4]/[MMA]=6); it reaches i t s limit already near 15%. Using a fixed photo-conversion (qnlum= 1"6%) will give a number of long-life macro-radicals depending on the phosphoric acid content of the system. Figure 7 illustrates the dependence of the wpo~dWinumratio and of the mol.wt, of the post-polymerizate on the acid/monomer molar ratio. One can see t h a t the relative rate of the post-polymerization increases as a function of the H3PO~ content and also reaches the (upper) limit at [HsPO4]/[MMA]= 3. The mol.wt, of the PMMA increases at the same time. CORRELATION OF THE ~o CONVERSION WITH THE DEGREE OF I'OLY~ERIZATIOI~OF THE ~CROMOLECULESDURII~GTHE MM.AHsPO4 POST-POLYMERIZATIOlq {25°C, 1"9 × 10-4 mole/1, of benzyl, 1.6~/oconversion in the photopolymerization under U'V-light, )l=365 nm) /[MMA]
[M]0, mole/1,
q, ~o
6
1.9
3.3
[H,PO,]/
5.7
PwX 10-' [N]molefl. × 10',
5"8 9.7 10.7
10.5 15-5 20.1 23.8
60 71 88
2.0 2.8 4.8 7.2
3.5 5.0 6.2 9.8
3.3 3.2
85
4.4 4.2
The characteristics of iMMA radical polymerization in the presence of ttsPO ~ cannot be understood on the basis of the usual theories about the chemical reactions which take place in homogeneous liquid media. One must take into account here the specific effect of the structural and physical properties of the medium and those of the macromolecules on the kinetics of the diffusion controlled bimolecular decay reaction of the macro-radicals. A distinct example of how internal physical factors affect the homogeneous polymerization kinetics, i.e. the gel stage, given by some authors [6-8], is an association with macromolecular aggregation and the formation of a steric fluctuating network. The strueturation study carried out during the bulk polymerization of MMA by a radical mechanism, using light scattering [8], gave rise to the theory t h a t micro-associates of macromolecules form with the macro-radicals and are fluctuating in nature; this already happens in the early phase of the monomer conversion and the steric network starts to form at polymer concentrations at which gel formation starts. The period of existence of the associates in such a network is much longer t h a n during t h e initial conversion period. I t was natural assume t h a t o-phosphoric acid would act as a polyfunctional
1202
YE. S, G~LRI~£ e~ al.
complex former, and that this function is primarily due to a greater probability of fluctuating associates forming which are stabilized during an early conversion stage as a result of intra- and inter-molecular H bond formation between the carbonyl groups contained in the complex and the 3 functional groups present in phosphoric acid [2]. The probability of the macro-radical, present in the associates, participating in the decay will rapidly diminish due to a decrease of the diffusion rate of their reactive ends towards each other. The rate of the photo-polymerization consequently increases in the partly structurated system, i.e. under gelling conditions. Nevertheless, the macro-radicals will participate during continuing initiation in the bimolecular termination reaction b y coming in contact with the continuously generated "short" radicals of propagation which retain a relatively large diffusion capacity. The constant availability of such radicals ensures a pseudo-stationary process, i.e. the order of the reaction with respect to the initiation rate will be close to 0.5. Two types of propagation chains thus exist during photo-initiation in associates with the macro-radicals, namely those incapable of reacting with each other, and the "short" radicals capable of participating in the decay and in reactions with each other, as well as with the stabilized macro-radicals. The availability of the low tool.we, radicals stops immediately after the U V source is switched on. The number of radicals which exists in the system at this moment will also depend on the competition between decay and propagation reactions with their incorporation in the associates. A proportion of the radicals will decay b y the bimolecular mechanism while another proportion will succeed in reaching a considerable chain length and will form associates; they will also remain capable of reacting further and will be responsible for the progress of the post-polymerization b y a "living-chain" mechanism. The modification of the structural and physical properties of the polymerization system b y means of poly-functional complex formers such as ortho-phosphoric acid thus enables us to control the bimolecular chain termination efficiently in the liquid phase radical polymerization of 1VIMA, even to the degree of completely eliminating macro-radical decay. Translated by K. A. AX~E~ REFERENCES
I. Ye. S. (~ARINA, Ye. G. LAGUTKINA, V. P. ZUBOV and V. A. KABANOV, Vysokomol. soyed. B14: 563, 1972 (Not translated in Polymer Sei. U.S.S.R.) 2. Ye. S. GARINA, T. lYl. KUZNETSOVA, ¥. P. ZUBOV and V. A. KABANOV, Dokl. Akad. Nauk SSSR 209: 380, 1973 3. F. BILLMEYER and @. D. THAN, J. Am. Chem. Soc. 74: 4763, 1955 4. Kh. S. BAGDASAR'YAN, Zh. fiz. khim. 32: 2614, 1958 5: J. KICE, J. Polymer Sci. 19: 123, 1956 6. S. Ya. FRENKEL', Vvedenie v statisticheskuyu teoriyu polimerizatsii (Introduction to the Statistical Polymerization Theory). Izd. "Nauka", 1965
Isothermal compressibility of PMMA
1203
7. D. A. EDEL'SHTEIN, B. R. SMIRNOV, V. P. GRACHEV and G. V. KOROLEV, Khimiya aromatieheskikh i nepredel'nykh soyedinenii (The Chemistry of Aromatic and Unsaturated Compounds). Irkutsk, 1971 8. R. A. SIMONYAN, V. A. KASANKIN, M. B. LACHINOV, V. P. ZUBOV and V. A. KABANOV, Dok]. Akad. Nauk SSSI~ 217: 631, 1974
T H E I S O T H E R M A L C O M P R E S S I B I L I T Y OF P O L Y M E T H Y L METHACRYLATE B. P. S H $ A ~ ,
IN V A R I O U S
PHYSICAL
STATES*
I. M. ~/~ONICH, S. A. A~Z~[AXOV and N. Y r . AVERBAKH
(Received 2 July 1975) The instrument used to determine the specific equilibrium volumes of polymers in the 1-9000 kg/em ~ pressure range and at temperatures up to 220°C is described. The specific volumes and isothermal compressibility factors are tabulated for PMMA; the glass temperatures and free volumes of PMMA have been determined as functions of pressure. The type of functions obtained is thought to be due to structural irregularities of the PMMA.
THE connections between t he pressure, t e m p e r a t u r e and t he specific volume of a polymer is a f unda m ent al characteristic which can be used to get a v a r i e t y o f information a bout the t h e r m o d y n a m i c parameters and t he structure, and some other features of technological importance. The s t u d y of these correlations has been the subject of a series of investigations carried out on various polymer types. Th e classical object, belonging to the group of typical amorphous, polar polymers, is PMMA. The dat a about the P - V - T correlations are of scientific as well as practical interest because of the m arked increase in the production a nd processing of this polymer. Especially noticeable is the lack of dat a for t he lower range of pressures (below 500 kg/cm z) and high ones (above 2000 kg/cm2). This made it quite difficult to get a general picture of the behaviour of PMMA over a wide range of temperatures and pressures which embrace various physical states of the polymer. The literature dat a are not easily compared due to the considerable experimental differences as well as differences in the preparation methods used. Our aim was here to determine the equilibrium specific volume of PMMA within a wide range of t e m per a t ur es and pressures; we used for this purpose a single experimental technique and sample preparation method; we also intended t o fiud the b o u n d a r y separating the glass-like from the highly elastic state within t h e pressure limits used. * Vysokomol. soyed. A18: No. 5, 1047-1052, 1976.