The connection between structure formation and the kinetics of the methyl methacrylate radical polymerization over ZnCl2 at high conversions

'THE CONNECTION BETWEEN STRUCTURE FORMATION AND KINETICS OF THE METHYL .METHACRYLATE RADICAL POLYMERIZATION OVER ZnC12 AT HIGH CONVERSIONS* M. B. LACHI~OV, V. YE. DREVAL', V. A. KAS~XI~, R. A. SIMO~CYAN, N. I. Sg~ULI~A, V. P. ZUBOV and V. A. KABA~OV M. V. Lomonosov State University, Moscow A. M. Gor'ki Urals State University

(Received 8 July 1976) The structural-rheological features of the polymer and of model systems were compared in this s t u d y with the kinetic parameters of the methyl methacrylate (MMA) radical polymerization over ZnCI~ at high conversion and in the presence of a chain transfer agent. F r o m the intensity of the light scattered b y the reaction systems during polymerization it was established t h a t the characteristic conversion ql, associated with the change from a dilute polymethylmethacrylate (PMMA) solution in its monomer to a moderately concentrated solution, in which a fluctuating structural network starts to form, q~, which coincides with the onset of spontaneous acceleration of the polymerization, qa, will shift towards smaller polymer concentrations in the reaction system when the ZnCI~ content is increased. The rheological measurements carried out on model systems containing the hydrogenated monomer analogue, namely methyl isobutyrate (MIB) indicated a considerable increase in viscosity of the PMMA concent r a t e as a result of increase in ZnCl~ concentration. The change from a slightly s t r u c t u r a t e d system to a planar, steric, fluctuating network, observed during these rheological determinations as a kink on the log ~ = / ( l o g c~) plot and the effect of the ZnC1 z additions on the position of this kink, correlated well with the q~ and qa va],uos determined from light scattering and the polymerization kinetics. The results axe explained b y the structuring effect of the ZnCI~ on the PMMA macromolecules. The two coordination bonds of ZnCls are saturated b y reaction with the caxbonyl groups in a single or in two adjacent macromolecules, which increases the PMMA chain rigidity, or t h e y form ligands between the chains. Both these factors will give rise to the formation of supermolecular structures and of a bulky steric network in the polymerizing a n d in model systems. : E ~ I ~ E R i n v e s t i g a t i o n s d e a l t w i t h t h e e f f e c t s o f ZnC12 o n t h e k i n e t i c s o f t h e r a d i c a l polymerization of methyl methacrylate (MMA) over a wide range of conversion [1, 2]. T h e p r e s e n c e o f ZnC12 d u r i n g t h i s p r o c e s s c a u s e d a c h a n g e o f t h e r e a c t i o n rate and of the molecular weight (mol.wt.) of the PMMA, and also reduced t h e c o n v e r s i o n qa a t w h i c h s p o n t a n e o u s a c c e l e r a t i o n s t a r t s (gelling), a s w e l l a s t h e f i n a l c o n v e r s i o n . T o e x p l a i n t h e s e e f f e c t s w e h a d a s s u m e d [1, 2] t h a t s u p p l e mentary structuration occurs due to the complexing of the forming maeromole•e u l e s w i t h ZnC12 . * Vysokomol. soyed. AI9: No. 4, 741-748, 1977. 859

860

M. B. LAOHINOV ¢$ a/.

We investigated in this work some of the structural-rheological features o f polymerization and model systems and compared them with the kinetic parameters of the MMA polymerization in the presence of ZnCl=. This process was carried out in the presence of lauryl mercaptan (LM) as chain transfer agent; the latter ensured the constancy of the mol.wt, at various conversions. The rheo-

qo

20

20O

qO0

I

f

800

800

Time, rain Fzo. 1. Conversion as a function of reaction time in the polymerizations of: 1--MMA; 2 - - M M A + Z n C l = + L M and lauryl peroxide (LP) as initiator; [ L M ] = I × 1 0 -z mole/1, N c = - - 2 - - 0 ; 3--0.04; 4--0.08 mole/L; [ L P ] = 5 × 10 -3 mole]l.; 60°C.

logical properties of the ZnCI= containing PMMA were examined on model systems in which methyl isobutyrate (MIB) was used as the non-polymerizing hydrogenated analogue of the actual monomer, MMA. The equivalence of the rheological properties of the PMMA solutions in MIB and MMA had been established in earlier work [3]. EXPERIMENTAL The preparation of the MMA-ZnC1, solutions and the kinetic measurements method had been described before [ 1, 2]. The intensity of the light scattered by the reaction systems during polymerization w as measured at angles 9=45% 90 ° and 135 ° in a " S h i m a d z u " CPG-21 automatic nephelometer using non-polarized light of k ~ 4 3 6 0 A wavelength. The instrument was standardized with benzene of U V spectroscopic purity which had been dried over Na, and assuming t h a t Rg0, 48.5 × 10 -e. The thin, optical glass reaction chamber ( d = 30 mm} was placed in the standard cylindrical cuvette with benzene as the immersion fluid. The reaction mixtures were freed of dust by repeated filtration through G-5 filters using pressure. The solutions were regarded as dust free when the asyrmnetry of scatter, zt =I~o.]I18o., was less than 1.04. The PMMA used in the theological measurements was the product of suspension polymerization; ~ - - - - 7 × 104. M I P was synthesized by esterifying isobutyrie acid with methanol, followed by a purification method" which was given by Nesmeyanov [4]. The PMMA was dissolved in MIB-ZNCl= mixtures {using analytical grade ZnCI= which had been dehydrated by heating in a HC1 stream [5]), the systems were heated to 90°C until homogeneous. Control,

Kinetics of MMA radical polymerization

861

tests showed no marked decomposition of the PMMA to occur during heating. Ostwald capillary viscometers were used to determine the solution viscosities; depending on the value o f the latter, we also used a pipette viscometer and the Shvedov rotary viscometer. These determinations, the processing of the results and their precision, had been described earlier by Tager and co-workers [6]. Low shear stresses of the order 10-100 dyne/cIll~ were used a n d up to 7-10 fold variations did not affect the measurement accuracy of viscosity, which is evidence for a l~ewtonian flow of the studied solutions. The ZnCI, concentration, given as 2Vc, is the ratio of the number of moles of ZnC1, to t h a t of MMA, or to the sum of moles of M I B and PMMA (base-moles).

RESULTS

A series of the kinetic curves of ~I:MA polymerization, recorded at constant LM concentration and various ZnCl~ contents, is reproduced in Fig. 1. Similar curves shown for comparison are those for a process without any of the additives. Judging from the literature data [7] Fig. 1 shows that the LM (chain transfer agent) greatly reduces the gel effect (compare curve 1 with 2). However, the curves for identical LM concentration, but with a ZnC12 addition, make it quite clear that the latter produces a spontaneous acceleration. The larger the ZnC1,

Rg0~lGV

(0gw

3 0"8 0"7 0"6

1

,t

,

0"5

FIG. 2. The dependence of: / - - t h e light-scattering intensity Rg0; 2 - - t h e reduced process rate w/[M], on conversion during the radical polymerization of MMA; [PL] = 5 × 10 -3 mole/h, 60°C.

content, the further is the start of conversion at spontaneous acceleration displaced towards lower conversions. Addition of ZnCI~ to systems containing sufficient LM has hardly any effect on the mol.wt, of the polymer produced; the latter is in the range 10,000-15,000 at all conversions and ZnCl~ contents. One gathers from the kinetic data that ZnCI~ causes a spontaneous acceleration of the process at a lower polymer content in the reaction system which evidently is due to a specific reactions with the PMMA macromolecules. Confirmation of the above theory was sought from a light scattering study on the systems at various conversions. Figure 2 shows a typical dependence of the light scattering intensity at 90° (R00) on conversion q during the bulk polymeriza-

862

M. B. L A c m ~ o v el aL

tion of pure MMA [8]. Three typical parts can be discerned on this curve: part I (0<:q~ql), in which the system is a dilute polymer solution in its monomer: R~o then increases to a maximum as conversion increases, and this coincides with conversion ql. The polymer concentration in the system at ql is, within experimental error, t h e same as the inverse intrinsic viscosity in the same solvent. There is an increase of the polymer content in part I I (ql ql and results in a reduction of the concentration fluctuations in the system [9, 10]. Typical at this stage is that structuration still does not affect the polymerization kinetics of MMA. The processes take place On the so-called initial stationary part Of the kinetic curve. The rate of change of Rgo decreases in the subsequent increase in polymer concentration in the system, and a change from a non-linear to a linear drop of Rg 0 occurs at some conversion q~ which can be explained b y a completion of the structural elements which form and are responsible for the light scattered b y the system (polymer/monomer) as it changes to a single com= ponent system (polymer). The range in which the change occurs from the non-linear to the linear part of curve Rgo=f(q) concides with the onset of spontaneous acceleration of the process, qa (within experimental error). The agreement of q~ with qa seems to indicate that the spontaneous acceleration which starts in part II results in the formation of a fairly large, live, fluctuating, steric network at q=q~. The trapping b y this network of the propagation radicals reduces their mobility and therefore produces a reduction of the rate constant for termination. The peak on the scattered light curve, and the range in which the non-linear part goes over into the linear Rg0 drop, as well as qa, will steadily shift towards larger ~/o conversions as the mol.wt, decreases; the changes themselves become more indistinct. Systems which give rise to a low mol.wt, polymer and a slight gel effect (Wmax/Wo~-2, qa--45--50%) are indicated b y a Rgo-----f(q) plot in the shape of a cupola (Fig. 3, curve 1). Such composition dependences of Rg0 are known to be typical of binary, low mol.wt, solutions. One can say that the mol. wt. decrease of the PMMA results in an increase of the critical polymer concentration at which the fluctuating" network will form and spontaneous acceleration of the reaction will start. Similar changes in the polymerization kinetics and of the t~9o=f(q ) curves are produced b y additions of solvents to the polymerization systems which have a larger thermodynamic affinity for the polymer than for MMA. This result can be interpreted as being the consequence of an obstacle to sterie network formation b y the appearance of spontaneous acceleration of the reaction. The MMA polymerization in benzene solutions in particular will cause a shift of the critical conversion q~ and the shift of auto-acceleration q, towards larger polymer concentrations in t h e system when the benzene content is increased (see Table).

Kinetics of MMA radical polymerization

863

A LM content large enough to stop the gelling, and the ZnCI~ addition, will: cause the R g o = f ( q ) curves to be steadily transformed and gradually to take on the shape typical for systems containing large mol.wt. PMMA with increasing ZnC12 concentration (Fig. 3). The peaks on the curves will markedly shift towards smaller conversions while the change from part I I to I I I will become more distinct and will shift towards smaller conversions with increasing ZnCI~ con-

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FIG. 3. The dependence of: 1,2,3--the light-scattering intensity Rg0; 1' 2' 3'--the reduced reaction rate w/[M], on conversion during the radical polymerization of MMAq-ZnCI,A-LM. [LM]=I×10 -1 mole/1, [PL]=SX10 -~ mole/l.; No: 1,1'--0; 2,2'-- 0.04; 3,3'-- 0.08, 60°C. tent, although it was said earlier t h a t ZnC12 does not affect the mol.wt, of thepolymer in the presence of a strong chain transfer agent. I t must be pointed out t h a t there is no scattering asymmetry by ZnCl~ containing systems (within experimental error). On this basis it is reckoned t h a t ZnCl~ does not disturb th@ S O M E OF T H E C H A R A C T E R I S T I C S OF R E A C T I O N SYSTEMS AS F O U N D BY LIGHT SCATTERING IN THE MMA I N B E N Z E N E AS S O L V E N T ;

RADICAL POLYMERIZATION

[LP]= 5 ×

10 -3 M O L E / L . , 6 0 ° C

Benzene content, %

~ r × lO s

qa, %

q~, vol. %

0 20 33 40

4.95 3.6 3.4 3.1

12 27-29 33-35 45-47

12 28 37.5 49

homogeneity of the reaction mixture. Furthermore, ZnCI~ was found to be selectively absorbed b y the PMMA macromolecules [11], and t h a t its addition even improves the solvent quality. The ZnCl~ thus plays the part of a structuration~ agent in the systems, and causes the formation of supermolecular associates im the initial and in the advanced stages of conversion.

M. B. LAcm~ov ~ a/.

~64

Independent information about the structuring action of ZnC] s was found
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FIG. 5

Fia. 4. The viscosities of PMMA solutions as functions of polymer concentration c, in MIBZnCI, binary solutions and of ZnC1, contents off No=l--0; 2--0.08; 3--0.17; 4--0.25; 25°C. FIo. 5. The viscosity (Poise) of PMMA solutions as a function of polymer content (vol. ~) in MIB-ZnCI~ binary solutions plotted on log-log coordinates (numbers as in Fig. 4). According to existing theories [12] the viscosity of polymer concentrates depends on the thermodynamic affinity of the latter for the solvent which affects macromolecular conformations and the inter-chain reactions in the solution. An improvement in solvent quality will result first, in the uncoiling of the macromolecules, thus increasing the solution viscosity, and secondly will prevent chain interactions which normally reduce the viscosity. In the case of the PMMA solutions the latter factor is the decisive one [13]. Addition of ZnCl~ t o dilute PMMA solutions in MMA is accompanied b y an 'improvement of solvent quality, as was said before. The same effect must also

Kinetics of MMA radical polymerization

865

be expected in the case of PMMA solutions in MIB. Nevertheless, ZnC]= additions to such solutions were accompanied b y a viscosity increase and not a decrease. This apparent contradiction is easily explained if one remembers the specific reactions in the studied system. The ZnC12 can become adsorbed on the macromolecules in dilute solutions and increase their rigidity. An increase of the PMMA content must be expected to result also in a formation of linkages between the macromolecules owing to the ZnC12 reaction with them by means of its two free coordination bonds; this involves ~he carbonyl groups of the polymer and results in a viscosity increase. Such an increase in concentrates due to specific reactions can also happen in the case of single component, polyfunctional solvents which are good ones in the range of low polymer concentrations. An example of the latter are the solutions of polyvinyl acetate and of its esterification products in diethylenetriamine [14], which acts as a fluctuating ]inl~age between the chains; in this case these are the hydrogen bonds between the amino groups of the solvent and the C - - 0 - and OH-groupe of the polymer. ,I/'Iv, k cel/mole I6

14 12 Czcr, voL%

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FIe. 6. The critical PMMA concentration as a function of the ZnCI= content in solutions at 25°C. FIG. 7. The heat of viscous flow activation for PMMA solutions in: 1 - - M I B ; 2 - - 1 ~ B - Z n C I I binary mixtures, at N o = 0.25.

The structuration in the presence of ZnCI= greatly depends on the polymer concentration of the PMMA solutions, as can be seen from the viscosity dependence on the solution concentration illustrated in log-log coordinates in Fig. 5. At moderate concentrations up to 200/o the viscosity can be approximated by two straight lines which intersect at the "critical" concentration c20r. The steeper

866

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LAcKrNov et o2.

of the two lines, which belongs to c2~C~cr, has a slope of 5, which is a value found for many polymer solutions [15]. The presence of a kink on the log ~ f ( l o g c2) line in the case of PMMA solutions and those of other polymers is evidence for a change from a slightly structured system in a concentration increase above C2cr to a compact, steric, fluctuating network. The ZnCI 2 addition results in a systematic C2cr decrease (Fig. 6). I m p o r t a n t is that the concentration range of t h e structural changes of P I ~ I A solutions in mixtures of SMIB with ZnC]~ ap~ proximately coincides with the same ranges in systems of MMA-PMMA-ZnCI~ when determined rheologically in the first ease and in kinetic studies and b y light scattering in the second. A further concentration increase caused an abrupt deviation from the function log ~ ~ 5 log c~, as is typical for rigid-chain polymer solutions. This deviation is due to the system approaching the glass-like state when the polymer content becomes larger; a larger ZnCl~ content will cause a larger deviation from the log ~ ~ 5 log c~ function. MMA polymerization to a large mol.wt, product reduced the upper limit of conversion of the monomer mixture [2]. The latter is associated with vitrification of the reaction systems [16]; kinetic and rheological measurements point to the tendency of the systems to change to the glass-like state when the ZnCl~ content becomes larger in them. As t h e main rheological determinations were made at lower temperatures than those of the process and the light scattering measurements, we thought it worth finding out how the temperature affects the PMMA solutions in the MIB-ZnCl~ systems. The heat of activation viscous flow, ztHv, from the temperature dependence of the viscosity, calculated from the Frenkel-Eyring exponential formula, are reproduced in Fig. 7. One can see that the ziH v difference for solutions with Nc~--0.25 and 0 is around 20 kcal/mole even at moderate (30%) polymer concentrations, which is typical of structurated systems. A temperature rise from 25 to 55°C slightly levels out the difference in rheologica! properties of PMMA solutions with or without the ZnC12 addition. This is evident as some C2cr increase of the ZnC12 containing solutions and a reduction in the difference between the viscosities. Such a behaviour is obviously connected with a preferential destruction b y thermal vibrations of the structure in the ZnCl~ containing solution. The temperature rise, at least in the studied range, is however unable t o eliminate the structural differences between PMMA solutions with and without ZnC12. This was especially confirmed b y the viscosity differences between systems a t 55°C which reached a factor of 40. T h e ZnCl~ containing PMMA solutions are'therefoie more structured systems at elevated temperatures and have a denser fluctuating network due to the coordination bonds between the zinc and the carbonyl groups of the polymer. Our overall results are thus evidence that ZnCl~ produced stronger molecular interactions in PMMA concentrates due to linkages forming between the macromolecules. The result is a higher tendency for the formation of associates in ZnCI~ containing solutions while t h e r e is a simultaneous increase in the persistence of

Kinetics of MMA radical polymerization

867

s u c h s u p e r m o l e c u l a r f l u c t u a t i n g structures. A steric n e t w o r k will f o r m a t s o m e " c r i t i c a l " p o l y m e r c o n c e n t r a t i o n in t h e solution. I n c r e a s i n g t h e ZnCl~ c o n t e n t will r e d u c e t h i s " c r i t i c a l " c o n c e n t r a t i o n a n d will a t t h e s a m e t i m e increase t h e t e n d e n c y o f t h e s y s t e m to c h a n g e t o t h e glass-like state. T h e effect o f t h e ZnC12 on s t r u c t u r a t i o n o f t h e P M M A m a c r o m o l e c u l e s in its m o n o m e r solution m a k e s c o m p r e h e n s i b l e t h e shift i n t o t h e lower p o l y m e r c o n c e n t r a t i o n r a n g e o f n o t only t h e °re c o n v e r s i o n a t w h i c h s p o n t a n e o u s acceleration starts, b u t of o t h e r conversion characteristics d u r i n g t h e MMA radical p o l y m e r i z a t i o n t o large % conversions in t h e presenc~ of ZnC12. I n conclusion we t h a n k A. A. T a g e r for t h e i n t e r e s t shown in t h i s w o r k a n d for this v a l u a b l e criticism. Translated by K . A. ALLEN

REFERENCES 1. V. P. ZUBOV, L. I. YEFIMOV, V. I. ARUIJN, M. B. LACHINOV et al., Vysokomol. soyed. B15: 588, 1973 (Not translated in Polymer Sci. U.S.S.R.) 2. M. B. LACHINOV, R. A. SIMONYAN, V. P. ZUBOV and V. A. KABANOV, Vysokomol. soyed. A18: 1563, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 7, 1788, 1976) 3. D. M. YEMEL'YANOV, L. I. MYASNIKOVA, A. V. RYABOV, O. V. SHATSKII and V. A. MYACHEV, Khim. i khim. Teldmol. 17: 749, 1974 4. A. N. NESMEYANOV and N. A. NESMEYANOV, Naehala organicheskoi khimii, t. 1 (The Beginnings of Organic Chemistry, Vol. 1). "Khimiya", 1974 5. Yu. A. KARYAKINA and I. I. ANILOV, Chistye khimicheskie veshehestva (Pure Chemical Compounds). "Khimiya", 1974 6. A. A. TAGER, A. I. SUVOROVA, V. Ye. DREVAL, N. P. GAKOVA and S. P. LUTSKAYA, Vysokomol. soyed. A1O: 2278, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 10, 2649, 1968) 7. G. P. GLADYSHEV and V. A. POPOV, Radikal'naya po!imerizatsiya pri glubokikh stepeniyakh prevrasheheniya (Radical Polymerizations with I-Iigh Conversion Effieiencies). "Nauka", 1974 8. R.A. SIMONYAN, V. A. KASAIKIN, M. B. LACHINOV, V. P. ZUBOV and V. A. KABANOV, Dokl. Akad. l~auk SSSR 217: 631, 1974 9. P. DEBYE and A. M. BUCHE, J. Chem. Phys. 18: 1423, 1950 10. A. A. TAGER and V. M. ANDREYEVA, J. Polymer Sei. C16: 1145, 1967 11. R. A. S1MONYAN, Dissertation, 1975 12. A. A. TAGER, V. Ye. DREVAL, G. O. BOTVINNIK, S. B. KENINA et al., Vysokomol. soyed. AI4: 1381, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 6, 1551, 1972) 13. V. Ye. DREVAL, A. Ya. MALKIN, A. A. TAGER and G. V. VII~OGRADOV, Mekhanika Polimerov, 729, 1973 14. Yu. P. SMIRNOVA, V. Ye. ]}REVAL, A. D. AZANOVA and A. A. TAGER, Vysokomol. soyed. AI3: 2397, 1971 (Translated in Polymer Sci. U.S.S.R. 13: ll, 2691, 1971) 15. V. Ye. DREVAL, Dissertation, 1974 16. K. HORIE, I. M1TA and H. KAMBE, J. Polymer Sci. 6; A-I: 2663, 1968 \