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Comparative kinetics of photopolymerization of methyl methacrylate using chloro-derivatives of acetic acid in combination with dimethylaniline as photoinitiators
Eur. PoOm. J. Vol. 25. No. 10, pp. 1049 1053, 1989 Printed in Great Britain. All rights reserved
0014-3057/89 $3.00+0.00 Copyright c Pergamon Press plc
COMPARATIVE KINETICS OF PHOTOPOLYMERIZATION OF METHYL METHACRYLATE USING CHLORO-DERIVATIVES OF ACETIC ACID IN COMBINATION WITH DIMETHYLANILINE AS PHOTOINITIATORS PREMAMOY GHOSH and GAUR! SANKAR MUKHERJEE Department of Plastics and Rubber Technology, Calcutta University, 92 Acharya Prafulla Chandra Road, Calcutta-700 009, India (Received 19 September 1988; in revisedJbrm 23 November 1988)
Abstract--Kinetics of the photopolymerization of methyl methacrylate at 40' using photoinitiator systems containing a chloro-derivative of acetic acid in combination with dimethylaniline (DMA) were studied. Polymerization takes place radically via initial complexation between the acid and DMA in each case. Differences in the structure of the acid, in terms of different degrees of substitution with chlorine, bring about notable differences in the kinetic and related features of the photopolymerization.
INTRODUCTION
A preliminary report on the use of acetic acid and its three chloro-derivatives viz. trichloroacetic acid (TCAA), dichloroacetic acid ( D C A A ) and monochloroacetic acid ( M C A A ) in inducing polymerization of methyl methacrylate ( M M A ) in the presence of an additive, such as dimethylaniline (DMA), or in its absence under photo-conditions has been given [I]. Results of studies on the kinetics of polymerization of M M A in bulk and in diluted systems using the T C A A / D M A combination as photoinitiator have been published [2]. The present paper reports a comparison of kinetic and related features of the photopolymerization of M M A induced separately by the three chloro-derivatives of acetic acid (AA), each in combination with D M A . EXPERIMENTAL PROCEDURES
Monomeric MMA (Western Chemical Corporation) was freed of inhibitors and then purified by distillation in vacuum following the usual procedures [3,4]. DMA (E. Merck) was kept over KOH pellets and purified by distillation. AA and the three chloroacids (TCAA, DCAA and MCAA) were of analytical grade; each was purified by recrystaIIization by cooling and then stored in a desiccator at 5. Polymerization of MMA to low conversions ( < 10%) was studied dilatometrically under the influence of light at 4 0 using a 125 W high pressure Hg-vapour lamp (Philips India Ltd.) without any filter [1] and under conditions of uniform intensity. The dilatometers of borosilicate (Corning) glass were filled with the requisite quantities of monomer containing known quantities of the selected acid and DMA. The acid was used in acetone solution so that a fixed and very low volume proportion of acetone was maintained in each experiment. After a specific time of polymerization, polymers were isolated by precipitation with excess petroleum ether. They were then washed with the non-solvent and dried at 50 in vacuum. Molecular weights ('£n) of the polymers (PMMA) and hence degrees of polymerization (P,,) were determined visco-
metrically using benzene solutions at 30: following standard procedures [3, 4] using the following equation [5] for intrinsic viscosity It/]: [q] = 8.69 × 10 ~ AS/~,>76. ~
(I)
RESULTS In the preliminary studies, it was observed that there was no polymerization of M M A when one of the chloroacetic acids was used as a lone initiator in the dark. U n d e r photo conclition, T C A A and D C A A produced polymers at very low rates (Rp) and after long inhibition periods (IP > 200 rain). The other two acids viz. M C A A and A A and also D M A failed to produce any polymer within 300 rain. Photopolymerization of M M A took place readily when one of the acids was used in combination with D M A [1]. Photopolymerization is normally associated with variable inhibition periods (IP) depending on the concentrations of the initiator components or [Acid] [DMA] in the sense of shorter IP with higher [Acid] [DMA]. Inhibition is presumably caused by adventitious impurities such as traces of oxygen in the N~-flushed system. Much longer IPs result when photopolymerization is done in air (without flushing the system with N2) and in the presence of benzoquinone (0.01%). Spectrophotometric studies reported earlier [1, 2] indicated equilibrium complexation between the initiator components (Acid and D M A ) , K~ being the equilibrium constant, and the complex, I (I = K~ [Acid] [DMA]) was reported to be the actual initiating species. Molecular weight distribution
For each A c i d - D M A system, a polymer obtained in bulk (Table 1) under comparable conditions at 40 (concentration of each initiator component in each system being 5 x 10 ~'tool. 1 ~) was purified by re-
1049
1050
PREMAMOY GHOSH and GAURI SANKAR MUKHERJEE 11.2 -
11.2 -
(I) /14n =0.161 x 106
9.6
£O x 3= c o
Mw = 0 . 3 6 5 x 1 0 6
Z" 0
_ ~- ~
M n = 0.117 x 106
9.6
Mw = 0.478 x 106 8D
(II)
(fw l /~l } =2.97
8.0
Mw/Mn) = 3.12
3=
,..-
6.4
6.4
.9 o
o
4.8
4.8
3.2
3.2
1.6
1.6
I 0
I
]
I
I
0.8
1.6
2.4
3.2
~'----d---~+ 4.0
J
4.8
5.6
I
6.4
0
0.6
MoLecuLar weight (Mx 10-6)
~
4.8
~
3.2
~
1.6 -
._~
MoLecuLar
3.2
weight
4.0 (MxlO
4.8
5.6
-s)
Mn=0.069 X 106
8.0
x: 3= 6.4 E;
2.4
(HI)
9.6
O
1.6
k
M w : 0 . 2 4 3 x 106
\
I
0
]
I
0.4
0.8
~
f
~
i
1.6
2.4
MoLecuLar
I
I
3.2
=-J
4.0
weightlMxl0-el
Fig. 1. G P C d a t a and m o l e c u l a r weight d i s t r i b u t i o n curves for p o l y m e r s ( P M M ~ t ) initiated by the A c i d - D M A c o m b i n a t i o n s as p h o t o i n i t i a t o r s at 4 0 : (i) M C A A - D M A system (II) D C A A - D M A system a n d (III) T C A A - D M A system.
peated precipitation [5] and then examined for polydispersity or heterogeneity index (M,~/M°) using gel permeation chromatography (GPC). The GPC data were obtained at 25: on equipment from Waters Associate using a combination of styragel columns (105, 104 and 103A); the mobile phase was tetrahydrofuran (THF) with a flow rate of 1.5 ml/min. The heterogeneity index for the polymer was within the range of 2.9 and 3.6. The GPC data and the distribution curves for the polymers, initiated by the A c i d - D M A combination as photoinitiator, are shown in Fig. I and Table 1. The observed M~/M, values, higher than expected for ideal systems, apparently arise as a consequence of notable deviation of
the kinetics of the present photopolymerization from ideal behaviour, as detailed in the following sections.
Initiator exponent, k ~/k,-l,alue and apparent activation energy Values of Rp in m o l . l - t-see ~ were determined [2] from the slopes of the initial linear parts of plots of conversion vs time (not shown). In each case, the slope of the plot of the log Rp vs log [Acid] [DMA] gives the value of the initiator exponent. All the initiator systems ( T C A A - D M A , D C A A - D M A and M C A A - D M A ) produced non-ideal kinetics, in each case the initiator exponent being <0.5.
Table I. Photopolymerization of MMA in bulk at 40 using TCAA, DCAA and MCAA, each in combination with DMA as photoinitiator Acid I. TCAA 2. DCAA 3. MCAA
IP (min)
Rp × 105 (mol.I i. sec i )
Heterogeneity index, ~ / ~ o
7 14 20
19.79 12.16 7.53
3.52 3.12 2.97
[ A c i d ] = 5 × 10 -'mobl i { D M A ] = 5 x 10 :mol-I '
Kinetics of the photopolymerization of MMA
1051
Table 2. Photopolymerization of M M A in bulk at 40 using Acid D M A combination as photoinitiator: s u m m a r y of kinetic and other parameters [I, 2] Parameters I. 2.
3. 4. 5. 6. 7 8. 9. 10.
Inhibition period, minutes (for concentration of initiator components ranging from 10 3 10 -'mol. I ~). Initiator exponent k~-r,'k~x 10-'l.mol ~sec ~ E~ (k J/tool) C~K~ :,~ 10~ (1" mol ~) C M x 105 Kinetic parameter for chain initiation, ([kdK~) x 106 (1.mol ~.sec t) Primary radical termination parameter. (k~./k,kp) x 10 ~ (mol ~'l-sec t) Direct initiator termination parameter, (k(K~/k r) (I.mol ~) End groups
The values of the kinetic parameter k~/k, for different systems, calculated from the initial slopes of plots [2] of 1/Pn vs Rp,/[M]2, were close to each other and showed good agreement with the reported value for the free radical polymerization of MMA. The values of k~/k, and initiator exponent also determined in the presence of solvents were similar to those for the bulk system; therefore, the solvents seem not to affect the propagation and termination reactions. The apparent activation energy (E,) for the bulk photopolymerization, (E~ = Ep-EU2) determined in the usual manner [6] for the three photopolymerization systems, are close to 20 kJ/mol, in good agreement with those reported for the radical photopolymerization of MMA [7, 8].
Initiator transfer parameter The Mayo equation expressed in the following form was used to determine the initiator transfer parameter CIK~ at 40 in the bulk polymerization of MMA, where C~ is the chain transfer constant of the initiating complex, I, and K~ is the equilibrium constant for complexation between the initiator components [2] in each case.
MCAA-DMA
DCAA-DMA
TCAA DMA
100-125
50 80
10 40
0,25 1.27 18.5 4.76 1.50 4.00
0.28 1.29 20.8 9.00 0.80 20.0
0.23 1.25 20.9 475 1.40 400
8.47
5,2a
4.20
0.74
3.5z
40.3
Carboxyl, DMA-residue
Carboxyl, DMA-residue
Carboxyl, DMA-residue
The values of the monomer transfer constant (c~) for the three systems obtained from the intercepts of the plots were close and of the order of 10 -5 (Table 2).
Monomer order and role of solvents Photopolymerization of MMA was also carried out at 40 '~in diluted systems using various solvents at different concentrations and at fixed concentrations of the initiator components for each Acid-DMA combination. The monomer exponent in each case was determined from the slope of the linear plot of log Rp vs log [M]; results are summarized in Table 3. A monomer exponent of 1.0 is indicative of the inert nature of the solvent or diluent (normal behaviour). Departure of the monomer exponent from unity indicates non-ideal behaviour; a value less than unity implies that the diluent acts as a rate enhancing additive. T C A A - D M A system produced data indicating normal kinetics with respect to monomer exponent in the presence of the selected solvents [2]. The other two initiator systems showed different extents of kinetic non-ideality in this respect (monomer order < 1) depending upon the nature of the solvent.
(1 "Pn - 1.85 k~Rr,/k~[M]:) = CM + CIK~'[Acid][DMA]/[M]. (2) The value of C~K~ for each system was calculated from the slope of the linear plot of the left hand side of equation (2) against ([Acid] [DMA])/[M] (plots not shown). The C~K~values for the three systems, shown in Table 2, are in the following order: (CIKL,)3CAA DMA > (CIKc)DCAA OMA > (CIKc)MCAA DMA.
DISCUSSION
End-group analyses of polymers produced by using Acid DMA initiator systems indicated that initiation took place by radicals derived from both the acid and DMA parts of the Acid-DMA complex and generated by photodecomposition of the complex in each case [1, 2].
Table 3. Photopolymerization of M M A in diluted systems at 40 using A c i d - D M A combinations as photoinitiators: m o n o m e r exponent and k, value (1.tool ~'sec J) M o n o m e r exponent, k: x 10~' Solvent
TCAA-DMA
1. 2. 3. 4. 5. 6.
1.0 1.0
Benzene Cyclohexanone AA Methyl ethyl ketone Chloroform Carbon tetrachloride
I.I 1.0
0 0
0 0
DCAA DMA 1.0 0.80
0.65 0.46
k, - Solvent modified initiation kinetic parameter [see equation {7)].
0 2.00
4.85 9.96
MCAA DMA 1.0
0
0.58 0.55 0.48 0.35
0.5 1.4 1.9 3.5
1052
PREMAMOYGHOSH and GAURI SANKARMUKHERJEE
Kinetic data, inhibitory effects of air (02) and benzoquinone, and the results of end-group analyses indicate a radical mechanism for the photopolymerizations. The reaction steps covering complexation between initiator components and photodecomposition of the complex into radicals may be represented thus [1]:
The value of the degradative initiator transfer parameter, k(Kc/kp for the three systems, obtained from the slope of the corresponding plots (not shown) based on equation (6) and given in Table 2, are in the reverse order, i.e. (TCAA DMA) > ( D C A A - D M A ) > (MCAA-DMA).
X
O
CH 3
I
If
I
I
I
I
K
X
O-
CH 3
I
I
I
I
I
I
X - - C - - C + N - - P h . ~'X---C--C.---+N--Ph X OH CH 3 (Acid) (DMA)
X OH CH 3 (Acid-DMA) complex, I O
I h,. , X - - - C - -
I
X
(3)
CH2 + N - - P h + HX
I
(4)
I
OH CH 3
where X = CI or H. The non-ideality (initiator exponent <0.5) in each of the A c i d - D M A systems is attributed to significant initiator dependent termination of the following kinds in addition to the usual bimolecular mode of termination: (a) (b)
primary radical termination (characterized by the rate constant, kprt) and termination by the initiating complex via degradative chain transfer (characterized by the rate constant, k~) without reinitiation.
Equation (5) may be derived for the present system [2] for analysis of the effect of primary radical termination [10, 11]: log
R 2p [Acid] [DMA] [M] 2
The highest value for the parameter k~Kc/k p was given by the ( T C A A - D M A ) system. Higher polydispersity (Mw/Mn) for polymers from an initiator system corresponding to higher chlorine substitution of acetic acid presumably arises as a consequence of higher incidences of initiator transfer and direct initiator termination by the degradative process (as reflected from the values of k(Kc/kp). The kinetic features of the three systems may be appreciated on a comparative basis from data in Table 2. The value of the chain initiation parameter ( f k dK¢) obtained for each system from the intercepts of the plots based on equations (5) and (6) follows the order: (TCAA-DMA) > (DCAA-DMA) > (MCAA-DMA). This fits with the observed relative order of rates of polymerization for the three systems (Table 1).
Analysis of the role of solvents In order to understand more clearly whether the selected solvents, some showing prominent rate enhancing effects, have any significant role in influencing the process of initiation, the radical generation steps in the presence of a solvent S are described [2] by the following equations:
log(fk d Kc) k2
I kprt Rp
x "P - 0.8684 k~ kikp[M]:"
(5)
The initiator dependent termination process through degradative initiator transfer may be analysed with the help of an approximate equation [9] of the following form [equation (6)] for the present photopolymerizations [2]: 2k~ Rg =2flCdKc k~ [Acid] [DMA] [M] 2
k~K~ Rp kp [M]'
> (TCAA-DMA).
kds
, pair of radical , pair of radical.
Based on this scheme of radical generation and considering significant termination by degradative initiator transfer along with the usual bimolecular termination, equation (7) may be derived [2] to analyse the role of solvents in influencing initiation in the polymerizations.
(6)
In equations (5) and (6), [M] stands for monomer concentration and all other terms have their usual significances. For analysis of kinetic data with equation (5), the l.h.s, was plotted against Rp/[M]2; for equation (6), the left hand side was plotted against Rp/[M]. In each case, the plot had negative slope, giving evidence for the respective initiator dependent termination process. The value of the primary radical termination parameter kp~t/kikp, obtained from the slope of the appropriate plots [2] (not shown) based on equation (5) for the three polymerization systems and given in Table 2, are in the order: (MCAA-DMA) > (DCAA-DMA)
I+ S
kd
2*'k~
[Acid] [DMA] [M] 2 +
k£K¢ R, ko [M]
kl + k2[S] (7)
where k l = 2 f k d K ~ and k:=2J:kd~K ~ (solventmodified initiation kinetic parameter) A plot of the l.h.s, of equation (7) vs [S], the solvent concentration, should produce a straight line with zero slope (k 2 = 0) for inert solvents. However, a straight line with a positive slope will be obtained for a solvent showing rate enhancement. Inert behaviour giving k:-~ 0 was observed with benzene in each of the three A c i d - D M A systems. For T C A A - D M A system, other solvents also produced inert behaviour for all practical purposes. Solvents other than benzene (Table 3) exhibited rate enhancement in the
Kinetics of the photopolymerization of MMA D C A A - D M A and M C A A - D M A systems to different extents, producing finite and positive k2 values (0.5 x 10 6 to 3.5 x 10-61.mol-~.sec-~ for M C A A D M A a n d 2 . 0 x 1 0 6 t o 1 0 x 10 61.tool ~.sec -l for D C A A - D M A in the order of a higher k: value for a solvent giving a lower fractional m o n o m e r exponent (Table 3). CONCLUSION Comparison of the kinetics of polymerization of M M A using the chloro-acetic acids separately (each in combination with D M A ) as photoinitiators, by means of data given in Tables 1-3, reveals that in each case polymerization proceeds by a radical mechanism via equilibrium complexation between the initiator components. In terms of inhibition periods and rates of polymerization, Rp (Table 1) and kinetic parameter for chain initiation, f k d K ~ (Table 2), the effectiveness of the three initiator systems are in the order: TCAA-DMA > DCAA-DMA > MCAA-DMA. Each system is characterized by kinetic non-ideality with respect to initiator (initiator exponent <0.5). The kinetic non-ideality may be understood and interpreted on the basis of both primary radical termination and degradative initiator transfer. The kinetic non-ideality is in the order TCAA-DMA > DCAA DMA > MCAA-DMA and the reason for kinetic non-ideality appears to be degradative initiator transfer (direct initiator termination) rather than primary radical termination, in view of the m o n o m e r exponents being < 1.0. Low monomer exponents ( < 1.0) for most solvents in the
1053
D C A A - D M A and M C A A - D M A systems are explained on the basis of solvent participation in chain initiation and consequent enhancement of Rp. As the initiator transfer process including the degradative effect causing major kinetic non-ideality becomes more pronounced with increasing chlorine substitution in the chloro acid, the product polymers tend to become more polydisperse in that order and this is clearly indicated by the results of G P C analysis (&lw/M n values, Table 1 and Fig. 1). Acknowledgement--Thanks are due to Alchemie Research Centre, Bombay (Dr D. N. Bhattacharyya) for co-operation and help in GPC analyses of some polymer samples.
REFERENCES
1. P. Ghosh and G. S. Mukherjee. Eur. Polym. J. 22, 103 (1986). 2. P. Ghosh and G. S. Mukherjee. J. Po(vm. Mater. 4, 77 (1987). 3. P. Ghosh, P. S. Mitra and A. N. Banerjee. J. Polym. Sci.: Polym. Chem. Edn 11, 2021 (1973). 4. P. Ghosh and A. N. Banerjee. J. Polym. Sei.; Polvm. Chem. Edn 12, 375 (1974). 5. T. G. Fox, J. B. Kinsinger, H. F. Mason and E. M. Schuele. Polymer 3, 71 (1962). 6. P. Ghosh and S. Chakraborty. J. Polym. Sci.: Polym. Chem. Edn 13, 1531 (1975). 7. P. Ghosh and H. Banerjee. J. Polym. Sci.; Polym. Chem. Edn 16, 633 (1978). 8. P. Ghosh and R. Ghosh. Eur. Polym. J. 17, 545 (1981). 9. P. Ghosh and N. Mukherji. Eur. Polym. J. 17, 541 (1981). 10. P. C. Deb. Eur. Polym. J. 11, 31 (1975). 1I. P. C. Deb and G. Meyerhoff. Eur. PoO~m. J. 10, 709 (1974).
0014-3057/89 $3.00+0.00 Copyright c Pergamon Press plc
COMPARATIVE KINETICS OF PHOTOPOLYMERIZATION OF METHYL METHACRYLATE USING CHLORO-DERIVATIVES OF ACETIC ACID IN COMBINATION WITH DIMETHYLANILINE AS PHOTOINITIATORS PREMAMOY GHOSH and GAUR! SANKAR MUKHERJEE Department of Plastics and Rubber Technology, Calcutta University, 92 Acharya Prafulla Chandra Road, Calcutta-700 009, India (Received 19 September 1988; in revisedJbrm 23 November 1988)
Abstract--Kinetics of the photopolymerization of methyl methacrylate at 40' using photoinitiator systems containing a chloro-derivative of acetic acid in combination with dimethylaniline (DMA) were studied. Polymerization takes place radically via initial complexation between the acid and DMA in each case. Differences in the structure of the acid, in terms of different degrees of substitution with chlorine, bring about notable differences in the kinetic and related features of the photopolymerization.
INTRODUCTION
A preliminary report on the use of acetic acid and its three chloro-derivatives viz. trichloroacetic acid (TCAA), dichloroacetic acid ( D C A A ) and monochloroacetic acid ( M C A A ) in inducing polymerization of methyl methacrylate ( M M A ) in the presence of an additive, such as dimethylaniline (DMA), or in its absence under photo-conditions has been given [I]. Results of studies on the kinetics of polymerization of M M A in bulk and in diluted systems using the T C A A / D M A combination as photoinitiator have been published [2]. The present paper reports a comparison of kinetic and related features of the photopolymerization of M M A induced separately by the three chloro-derivatives of acetic acid (AA), each in combination with D M A . EXPERIMENTAL PROCEDURES
Monomeric MMA (Western Chemical Corporation) was freed of inhibitors and then purified by distillation in vacuum following the usual procedures [3,4]. DMA (E. Merck) was kept over KOH pellets and purified by distillation. AA and the three chloroacids (TCAA, DCAA and MCAA) were of analytical grade; each was purified by recrystaIIization by cooling and then stored in a desiccator at 5. Polymerization of MMA to low conversions ( < 10%) was studied dilatometrically under the influence of light at 4 0 using a 125 W high pressure Hg-vapour lamp (Philips India Ltd.) without any filter [1] and under conditions of uniform intensity. The dilatometers of borosilicate (Corning) glass were filled with the requisite quantities of monomer containing known quantities of the selected acid and DMA. The acid was used in acetone solution so that a fixed and very low volume proportion of acetone was maintained in each experiment. After a specific time of polymerization, polymers were isolated by precipitation with excess petroleum ether. They were then washed with the non-solvent and dried at 50 in vacuum. Molecular weights ('£n) of the polymers (PMMA) and hence degrees of polymerization (P,,) were determined visco-
metrically using benzene solutions at 30: following standard procedures [3, 4] using the following equation [5] for intrinsic viscosity It/]: [q] = 8.69 × 10 ~ AS/~,>76. ~
(I)
RESULTS In the preliminary studies, it was observed that there was no polymerization of M M A when one of the chloroacetic acids was used as a lone initiator in the dark. U n d e r photo conclition, T C A A and D C A A produced polymers at very low rates (Rp) and after long inhibition periods (IP > 200 rain). The other two acids viz. M C A A and A A and also D M A failed to produce any polymer within 300 rain. Photopolymerization of M M A took place readily when one of the acids was used in combination with D M A [1]. Photopolymerization is normally associated with variable inhibition periods (IP) depending on the concentrations of the initiator components or [Acid] [DMA] in the sense of shorter IP with higher [Acid] [DMA]. Inhibition is presumably caused by adventitious impurities such as traces of oxygen in the N~-flushed system. Much longer IPs result when photopolymerization is done in air (without flushing the system with N2) and in the presence of benzoquinone (0.01%). Spectrophotometric studies reported earlier [1, 2] indicated equilibrium complexation between the initiator components (Acid and D M A ) , K~ being the equilibrium constant, and the complex, I (I = K~ [Acid] [DMA]) was reported to be the actual initiating species. Molecular weight distribution
For each A c i d - D M A system, a polymer obtained in bulk (Table 1) under comparable conditions at 40 (concentration of each initiator component in each system being 5 x 10 ~'tool. 1 ~) was purified by re-
1049
1050
PREMAMOY GHOSH and GAURI SANKAR MUKHERJEE 11.2 -
11.2 -
(I) /14n =0.161 x 106
9.6
£O x 3= c o
Mw = 0 . 3 6 5 x 1 0 6
Z" 0
_ ~- ~
M n = 0.117 x 106
9.6
Mw = 0.478 x 106 8D
(II)
(fw l /~l } =2.97
8.0
Mw/Mn) = 3.12
3=
,..-
6.4
6.4
.9 o
o
4.8
4.8
3.2
3.2
1.6
1.6
I 0
I
]
I
I
0.8
1.6
2.4
3.2
~'----d---~+ 4.0
J
4.8
5.6
I
6.4
0
0.6
MoLecuLar weight (Mx 10-6)
~
4.8
~
3.2
~
1.6 -
._~
MoLecuLar
3.2
weight
4.0 (MxlO
4.8
5.6
-s)
Mn=0.069 X 106
8.0
x: 3= 6.4 E;
2.4
(HI)
9.6
O
1.6
k
M w : 0 . 2 4 3 x 106
\
I
0
]
I
0.4
0.8
~
f
~
i
1.6
2.4
MoLecuLar
I
I
3.2
=-J
4.0
weightlMxl0-el
Fig. 1. G P C d a t a and m o l e c u l a r weight d i s t r i b u t i o n curves for p o l y m e r s ( P M M ~ t ) initiated by the A c i d - D M A c o m b i n a t i o n s as p h o t o i n i t i a t o r s at 4 0 : (i) M C A A - D M A system (II) D C A A - D M A system a n d (III) T C A A - D M A system.
peated precipitation [5] and then examined for polydispersity or heterogeneity index (M,~/M°) using gel permeation chromatography (GPC). The GPC data were obtained at 25: on equipment from Waters Associate using a combination of styragel columns (105, 104 and 103A); the mobile phase was tetrahydrofuran (THF) with a flow rate of 1.5 ml/min. The heterogeneity index for the polymer was within the range of 2.9 and 3.6. The GPC data and the distribution curves for the polymers, initiated by the A c i d - D M A combination as photoinitiator, are shown in Fig. I and Table 1. The observed M~/M, values, higher than expected for ideal systems, apparently arise as a consequence of notable deviation of
the kinetics of the present photopolymerization from ideal behaviour, as detailed in the following sections.
Initiator exponent, k ~/k,-l,alue and apparent activation energy Values of Rp in m o l . l - t-see ~ were determined [2] from the slopes of the initial linear parts of plots of conversion vs time (not shown). In each case, the slope of the plot of the log Rp vs log [Acid] [DMA] gives the value of the initiator exponent. All the initiator systems ( T C A A - D M A , D C A A - D M A and M C A A - D M A ) produced non-ideal kinetics, in each case the initiator exponent being <0.5.
Table I. Photopolymerization of MMA in bulk at 40 using TCAA, DCAA and MCAA, each in combination with DMA as photoinitiator Acid I. TCAA 2. DCAA 3. MCAA
IP (min)
Rp × 105 (mol.I i. sec i )
Heterogeneity index, ~ / ~ o
7 14 20
19.79 12.16 7.53
3.52 3.12 2.97
[ A c i d ] = 5 × 10 -'mobl i { D M A ] = 5 x 10 :mol-I '
Kinetics of the photopolymerization of MMA
1051
Table 2. Photopolymerization of M M A in bulk at 40 using Acid D M A combination as photoinitiator: s u m m a r y of kinetic and other parameters [I, 2] Parameters I. 2.
3. 4. 5. 6. 7 8. 9. 10.
Inhibition period, minutes (for concentration of initiator components ranging from 10 3 10 -'mol. I ~). Initiator exponent k~-r,'k~x 10-'l.mol ~sec ~ E~ (k J/tool) C~K~ :,~ 10~ (1" mol ~) C M x 105 Kinetic parameter for chain initiation, ([kdK~) x 106 (1.mol ~.sec t) Primary radical termination parameter. (k~./k,kp) x 10 ~ (mol ~'l-sec t) Direct initiator termination parameter, (k(K~/k r) (I.mol ~) End groups
The values of the kinetic parameter k~/k, for different systems, calculated from the initial slopes of plots [2] of 1/Pn vs Rp,/[M]2, were close to each other and showed good agreement with the reported value for the free radical polymerization of MMA. The values of k~/k, and initiator exponent also determined in the presence of solvents were similar to those for the bulk system; therefore, the solvents seem not to affect the propagation and termination reactions. The apparent activation energy (E,) for the bulk photopolymerization, (E~ = Ep-EU2) determined in the usual manner [6] for the three photopolymerization systems, are close to 20 kJ/mol, in good agreement with those reported for the radical photopolymerization of MMA [7, 8].
Initiator transfer parameter The Mayo equation expressed in the following form was used to determine the initiator transfer parameter CIK~ at 40 in the bulk polymerization of MMA, where C~ is the chain transfer constant of the initiating complex, I, and K~ is the equilibrium constant for complexation between the initiator components [2] in each case.
MCAA-DMA
DCAA-DMA
TCAA DMA
100-125
50 80
10 40
0,25 1.27 18.5 4.76 1.50 4.00
0.28 1.29 20.8 9.00 0.80 20.0
0.23 1.25 20.9 475 1.40 400
8.47
5,2a
4.20
0.74
3.5z
40.3
Carboxyl, DMA-residue
Carboxyl, DMA-residue
Carboxyl, DMA-residue
The values of the monomer transfer constant (c~) for the three systems obtained from the intercepts of the plots were close and of the order of 10 -5 (Table 2).
Monomer order and role of solvents Photopolymerization of MMA was also carried out at 40 '~in diluted systems using various solvents at different concentrations and at fixed concentrations of the initiator components for each Acid-DMA combination. The monomer exponent in each case was determined from the slope of the linear plot of log Rp vs log [M]; results are summarized in Table 3. A monomer exponent of 1.0 is indicative of the inert nature of the solvent or diluent (normal behaviour). Departure of the monomer exponent from unity indicates non-ideal behaviour; a value less than unity implies that the diluent acts as a rate enhancing additive. T C A A - D M A system produced data indicating normal kinetics with respect to monomer exponent in the presence of the selected solvents [2]. The other two initiator systems showed different extents of kinetic non-ideality in this respect (monomer order < 1) depending upon the nature of the solvent.
(1 "Pn - 1.85 k~Rr,/k~[M]:) = CM + CIK~'[Acid][DMA]/[M]. (2) The value of C~K~ for each system was calculated from the slope of the linear plot of the left hand side of equation (2) against ([Acid] [DMA])/[M] (plots not shown). The C~K~values for the three systems, shown in Table 2, are in the following order: (CIKL,)3CAA DMA > (CIKc)DCAA OMA > (CIKc)MCAA DMA.
DISCUSSION
End-group analyses of polymers produced by using Acid DMA initiator systems indicated that initiation took place by radicals derived from both the acid and DMA parts of the Acid-DMA complex and generated by photodecomposition of the complex in each case [1, 2].
Table 3. Photopolymerization of M M A in diluted systems at 40 using A c i d - D M A combinations as photoinitiators: m o n o m e r exponent and k, value (1.tool ~'sec J) M o n o m e r exponent, k: x 10~' Solvent
TCAA-DMA
1. 2. 3. 4. 5. 6.
1.0 1.0
Benzene Cyclohexanone AA Methyl ethyl ketone Chloroform Carbon tetrachloride
I.I 1.0
0 0
0 0
DCAA DMA 1.0 0.80
0.65 0.46
k, - Solvent modified initiation kinetic parameter [see equation {7)].
0 2.00
4.85 9.96
MCAA DMA 1.0
0
0.58 0.55 0.48 0.35
0.5 1.4 1.9 3.5
1052
PREMAMOYGHOSH and GAURI SANKARMUKHERJEE
Kinetic data, inhibitory effects of air (02) and benzoquinone, and the results of end-group analyses indicate a radical mechanism for the photopolymerizations. The reaction steps covering complexation between initiator components and photodecomposition of the complex into radicals may be represented thus [1]:
The value of the degradative initiator transfer parameter, k(Kc/kp for the three systems, obtained from the slope of the corresponding plots (not shown) based on equation (6) and given in Table 2, are in the reverse order, i.e. (TCAA DMA) > ( D C A A - D M A ) > (MCAA-DMA).
X
O
CH 3
I
If
I
I
I
I
K
X
O-
CH 3
I
I
I
I
I
I
X - - C - - C + N - - P h . ~'X---C--C.---+N--Ph X OH CH 3 (Acid) (DMA)
X OH CH 3 (Acid-DMA) complex, I O
I h,. , X - - - C - -
I
X
(3)
CH2 + N - - P h + HX
I
(4)
I
OH CH 3
where X = CI or H. The non-ideality (initiator exponent <0.5) in each of the A c i d - D M A systems is attributed to significant initiator dependent termination of the following kinds in addition to the usual bimolecular mode of termination: (a) (b)
primary radical termination (characterized by the rate constant, kprt) and termination by the initiating complex via degradative chain transfer (characterized by the rate constant, k~) without reinitiation.
Equation (5) may be derived for the present system [2] for analysis of the effect of primary radical termination [10, 11]: log
R 2p [Acid] [DMA] [M] 2
The highest value for the parameter k~Kc/k p was given by the ( T C A A - D M A ) system. Higher polydispersity (Mw/Mn) for polymers from an initiator system corresponding to higher chlorine substitution of acetic acid presumably arises as a consequence of higher incidences of initiator transfer and direct initiator termination by the degradative process (as reflected from the values of k(Kc/kp). The kinetic features of the three systems may be appreciated on a comparative basis from data in Table 2. The value of the chain initiation parameter ( f k dK¢) obtained for each system from the intercepts of the plots based on equations (5) and (6) follows the order: (TCAA-DMA) > (DCAA-DMA) > (MCAA-DMA). This fits with the observed relative order of rates of polymerization for the three systems (Table 1).
Analysis of the role of solvents In order to understand more clearly whether the selected solvents, some showing prominent rate enhancing effects, have any significant role in influencing the process of initiation, the radical generation steps in the presence of a solvent S are described [2] by the following equations:
log(fk d Kc) k2
I kprt Rp
x "P - 0.8684 k~ kikp[M]:"
(5)
The initiator dependent termination process through degradative initiator transfer may be analysed with the help of an approximate equation [9] of the following form [equation (6)] for the present photopolymerizations [2]: 2k~ Rg =2flCdKc k~ [Acid] [DMA] [M] 2
k~K~ Rp kp [M]'
> (TCAA-DMA).
kds
, pair of radical , pair of radical.
Based on this scheme of radical generation and considering significant termination by degradative initiator transfer along with the usual bimolecular termination, equation (7) may be derived [2] to analyse the role of solvents in influencing initiation in the polymerizations.
(6)
In equations (5) and (6), [M] stands for monomer concentration and all other terms have their usual significances. For analysis of kinetic data with equation (5), the l.h.s, was plotted against Rp/[M]2; for equation (6), the left hand side was plotted against Rp/[M]. In each case, the plot had negative slope, giving evidence for the respective initiator dependent termination process. The value of the primary radical termination parameter kp~t/kikp, obtained from the slope of the appropriate plots [2] (not shown) based on equation (5) for the three polymerization systems and given in Table 2, are in the order: (MCAA-DMA) > (DCAA-DMA)
I+ S
kd
2*'k~
[Acid] [DMA] [M] 2 +
k£K¢ R, ko [M]
kl + k2[S] (7)
where k l = 2 f k d K ~ and k:=2J:kd~K ~ (solventmodified initiation kinetic parameter) A plot of the l.h.s, of equation (7) vs [S], the solvent concentration, should produce a straight line with zero slope (k 2 = 0) for inert solvents. However, a straight line with a positive slope will be obtained for a solvent showing rate enhancement. Inert behaviour giving k:-~ 0 was observed with benzene in each of the three A c i d - D M A systems. For T C A A - D M A system, other solvents also produced inert behaviour for all practical purposes. Solvents other than benzene (Table 3) exhibited rate enhancement in the
Kinetics of the photopolymerization of MMA D C A A - D M A and M C A A - D M A systems to different extents, producing finite and positive k2 values (0.5 x 10 6 to 3.5 x 10-61.mol-~.sec-~ for M C A A D M A a n d 2 . 0 x 1 0 6 t o 1 0 x 10 61.tool ~.sec -l for D C A A - D M A in the order of a higher k: value for a solvent giving a lower fractional m o n o m e r exponent (Table 3). CONCLUSION Comparison of the kinetics of polymerization of M M A using the chloro-acetic acids separately (each in combination with D M A ) as photoinitiators, by means of data given in Tables 1-3, reveals that in each case polymerization proceeds by a radical mechanism via equilibrium complexation between the initiator components. In terms of inhibition periods and rates of polymerization, Rp (Table 1) and kinetic parameter for chain initiation, f k d K ~ (Table 2), the effectiveness of the three initiator systems are in the order: TCAA-DMA > DCAA-DMA > MCAA-DMA. Each system is characterized by kinetic non-ideality with respect to initiator (initiator exponent <0.5). The kinetic non-ideality may be understood and interpreted on the basis of both primary radical termination and degradative initiator transfer. The kinetic non-ideality is in the order TCAA-DMA > DCAA DMA > MCAA-DMA and the reason for kinetic non-ideality appears to be degradative initiator transfer (direct initiator termination) rather than primary radical termination, in view of the m o n o m e r exponents being < 1.0. Low monomer exponents ( < 1.0) for most solvents in the
1053
D C A A - D M A and M C A A - D M A systems are explained on the basis of solvent participation in chain initiation and consequent enhancement of Rp. As the initiator transfer process including the degradative effect causing major kinetic non-ideality becomes more pronounced with increasing chlorine substitution in the chloro acid, the product polymers tend to become more polydisperse in that order and this is clearly indicated by the results of G P C analysis (&lw/M n values, Table 1 and Fig. 1). Acknowledgement--Thanks are due to Alchemie Research Centre, Bombay (Dr D. N. Bhattacharyya) for co-operation and help in GPC analyses of some polymer samples.
REFERENCES
1. P. Ghosh and G. S. Mukherjee. Eur. Polym. J. 22, 103 (1986). 2. P. Ghosh and G. S. Mukherjee. J. Po(vm. Mater. 4, 77 (1987). 3. P. Ghosh, P. S. Mitra and A. N. Banerjee. J. Polym. Sci.: Polym. Chem. Edn 11, 2021 (1973). 4. P. Ghosh and A. N. Banerjee. J. Polym. Sei.; Polvm. Chem. Edn 12, 375 (1974). 5. T. G. Fox, J. B. Kinsinger, H. F. Mason and E. M. Schuele. Polymer 3, 71 (1962). 6. P. Ghosh and S. Chakraborty. J. Polym. Sci.: Polym. Chem. Edn 13, 1531 (1975). 7. P. Ghosh and H. Banerjee. J. Polym. Sci.; Polym. Chem. Edn 16, 633 (1978). 8. P. Ghosh and R. Ghosh. Eur. Polym. J. 17, 545 (1981). 9. P. Ghosh and N. Mukherji. Eur. Polym. J. 17, 541 (1981). 10. P. C. Deb. Eur. Polym. J. 11, 31 (1975). 1I. P. C. Deb and G. Meyerhoff. Eur. PoO~m. J. 10, 709 (1974).