Polymerization of Methyl Methacrylate and Other Vinyl Monomers

5 Polymerization of Methyl Methacrylate and Other Vinyl Monomers Activated by Low Concentrations of SO2 Downloaded by UNIV LAVAL on April 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch005

PREMAMOY GHOSH and FRED W. BILLMEYER, JR. Rensselaer Polytechnic Institute, Troy, Ν. Y. 12181

Low concentrations

of SO and TBHP

the

of MMA

DPPH

and hydroquinone

zation.

End-group

sulfonate

and hydroxyl

copolymerization

initiator

The over-all

radical in nature. enhance

initiation

appears other

mechanism

monomers

monomer

is

of

in the polymers, and

system

polymerization

and

MMA-acrylic

and AIBN

the rate of polymerization

to be predominant

polymeri

are in

appears

Inert solvents (benzene,

not of other vinyl monomers

initiate

monomers.

the incorporation

results (MMA-isoprene

good agreement.

An

vinyl

this MMA

indicates

end groups

2

and xylene)

and other

do not inhibit

analysis

acid) with this SO -TBHP primarily

were used to

2

polymerization

to be toluene,

of MMA but

(AN, Sty, VA, EMA, MA, etc.). involving

monomer

and

solvent

in the case of MMA, while

an initiation

reaction

involving

only

with the

predominant.

T t has been observed recently (14, 15) that catalytic concentrations of sulfur dioxide can easily initiate the polymerization of methyl meth acrylate, ethyl methacrylate, η-butyl methacrylate, and styrene at or near room temperature but fails to initiate polymerization i n other monomers such as acrylonitrile, acrylamide, methyl acrylate, ethyl acrylate, vinyl acetate, and vinyl pyridine under similar conditions. However, i n the presence of catalytic concentrations of sulfur dioxide and a hydroperoxide, such as ter£-butyl hydroperoxide, all the above monomers polymerize readily. The results of further investigations on vinyl polymerization i n the presence of low concentrations of sulfur dioxide are reported here. A

75 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

76

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES

Experimental

Downloaded by UNIV LAVAL on April 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch005

Methyl methacrylate ( M M A ) , ethyl methacrylate ( E M A ) , n-butyl methacrylate ( n - B M A ) , styrene (Sty), acrylonitrile ( A N ) , vinyl acetate ( V A ) , methyl acrylate ( M A ) , isoprene ( I P ) , and isobutyl vinyl ether ( I B V E ) were all dried over anhydrous barium oxide and distilled at or below 2 5 ° C . (except n - B M A , 3 5 ° - 4 0 ° C . ) under low nitrogen pressure. Acrylic acid ( A A ) was dried over anhydrous sodium sulfate and distilled under vacuum before use. Baker anhydrous sulfur dioxide of 99.98% purity was used without further purification. Sulfur dioxide was passed through the respective monomers, kept at 0 - 5 ° C , and the dissolved S 0 2 content was determined iodometrically in each case, running a control experiment side by side. terf-Butyl hydroperoxide ( T B H P ) , supplied by the Monomer-Polymer Laboratories of the Bordeh Chemical Co., was used as obtained without further purification. A l l solvents used were reagent grade and were distilled once before use i n polymerization experiments. The polymerization rate at a specified temperature was followed gravimetrically, using methanol or petroleum ether as the nonsolvent; in some cases (where mentioned) a dilatometric technique was also used. A l l polymerization experiments were done i n clean borosilicate glass tubes, sealed in the usual manner under a vacuum of 10" 2 -10" 3 mm. H g , following two freezing and thawing cycles. Intrinsic viscosities were measured i n benzene at 30 ° C . using a Cannon-Ubbelohde viscometer with flow time greater than 200 sec. Re sults are expressed in deciliters/gram and for poly (methyl methacrylate) ( P M M A ) were converted to molecular weights by the following equa tions {4,12): [η] = 7.24 X Î O - W / ™ or [ ] = 5.2 Χ 10·>Μ Μ .°· 7 0 η

G e l permeation chromatography ( G P C ) was carried out with a Waters model 100 gel permeation chromatograph using tetrahydrofuran as the solvent at room temperature. A series of columns of gel porosity 10 6 , 10 5 , 10 4 , and 10 3 A . (Waters designations) was used, and the system was calibrated with P M M A samples of known molecular weight, each of narrow distribution, following the usual procedures (21). A plot of log (molecular weight) against effluent count for the peak of the curve for each known molecular weight standard was used for calibration. Experi mental points for polymers of molecular weight higher than about 6 Χ 10 5 did not fall on the straight-line calibration curve in the lower molecular weight region. Experimental calibration beyond 9 Χ 10 δ molecular weight could not be done because of nonavailability of standard polymer fractions. A n approximate extrapolated line in the higher molecular weight region was used as the calibration curve for distribution analysis. The column system had a plate count of 1240 plates/ft.

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

5.

GOSH AND BILLMEYER

Methyl

Methacrylate

77


Results

Preliminary studies (15) of the kinetics of M M A polymerization initiated by T B P H and S 0 2 showed that the rate of polymerization ( Rp ) had a square root dependence on initiator concentration, and the mono mer exponent was 1.5 (the monomer variation having been done by adding varying amounts of methanol)—i.e., Rp — Κ [ T B H P ] 0 5 [ S O 2 ] 0 n [ Μ ] 1 · 5 . O n the other hand, if the monomer variation were done by adding benzene, an interesting result occurred: considerably higher rates of polymerization were observed on adding small amounts of benzene, and the rate passed through a maximum with increasing benzene concentra tion. Similar results were obtained in the presence of toluene, xylene, chlorobenzene, chloroform, tetrahydrofuran, acetone, methyl ethyl ketone, and cyclohexanone. The rate-enhancing effect was more pronounced in the presence of tetrahydrofuran and the ketones. Effect of Free Radical Inhibitors. Polymerization of M M A induced by the T B H P - S 0 2 system is not inhibited in the presence of hydroquinone or diphenylpicrylhydrazyl ( D P P H ) . Using 0.0075M T B H P and 0.004M S 0 2 , bulk polymerization of M M A was carried out at 3 0 ° C . (a) i n the absence and (b) in the presence of 0.0006M D P P H , and the results based on dilatometric studies are given in Figure 1. Polymerization is somewhat retarded in Case b up to about 15 min., and at this point the violet color of the medium is completely destroyed. Polymerization proceeds thereafter at about the same rate as in Case a. The molecular weight of Polymer b is lower than that of Polymer a. O n purification by repeated precipitation, Polymer b appears very light brown/yellow i n color and exhibits appreciable absorption in the near ultraviolet-visible region (Figure 2) indicating that fragments of D P P H are being incorpo rated in the polymer during polymerization. The polymerization rate in the presence of hydroquinone is not quite reproducible; no inhibition period is observed, but some retardation is apparent at high hydroquinone concentration. The polymers obtained show absorption in the ultraviolet region (Figure 2), indicating that they are incorporating hydroquinone. End Group Analysis. Using 8 r , S-labeled sulfur dioxide as the initiator Ghosh and O'Driscoll (15) have shown that under conditions of less than 2 X 10" 2 M S 0 2 , little or no sulfur is detected in the P M M A prepared. Quantitative analysis of end groups by dye techniques is rendered uncer tain and obscure by the very high molecular weight ( [r;] — 5-10) of the S0 2 -initiated polymers. W i t h the hydroperoxide-sulfur dioxide initiator system, however, the molecular weight of the polymers obtained is fairly low in comparison ( [77] — 0.1-1.5), and the incorporation of sulfonate and hydroxyl end groups in the polymer is possible if the process is of a free radical type (24). By using the dye partition technique to determine


ADDITION


78

AND

CONDENSATION

POLYMERIZATION

PROCESSES

sulfonate (13) and hydroxyl (16) end groups, it is found that P M M A , initiated by the T B H P - S 0 2 system ( concentration of each initiator component being in the region of 10"3 to 1 0 " 2 M ) , has on the average about 1—1.5 end groups (sulfonate and hydroxyl combined) per chain, with the sulfonate end group content varying between 0.5-0.9 per chain. Interestingly, no hydroxyl end group is found i n polymers prepared i n the presence of D P P H or hydroquinone, and only about 0.8-1.0 sulfonate end group per chain is observed i n these cases. Polymers prepared in the presence of high concentrations of S 0 2 ( > 0 . 8 - l M ) in the T B H P - S 0 2 initiator system give a relatively intense response to the dye test for sulfoxy end groups; the rate of polymerization under this condition is relatively slow, and increased incorporation of sulfur dioxide i n the polymer probably takes place by copolymerization of free sulfur dioxide or a sulfur dioxide-monomer complex.

TIME, MINUTES Figure

1.

Polymerization

of MM A at 30°C. initiator system

using

TBHP-S02

Molecular Weight Distribution. The molecular weight distribution of a few P M M A samples initiated i n bulk by S 0 2 , T B H P - S 0 2 , or T B P H at 30 ° C . has been examined by G P C . The description of the samples studied is given in Table I. A l l the polymers were obtained at low conversions ( < 1 5 % ) at 30 ° C . The molecular weight distribution for each sample was computed, and the distribution ratio Mw/Mn for the four samples as obtained from G P C data is shown in Table I. For Sample 3, having intrinsic viscosity 1.07, the G P C weight average molecular weight Mw and number average molecular weight Mn values are i n fairly close


5.

GOSH AND BILLMEYER

Methyl

79

Methacrylate


1.6 h

WAVELENGTH, nm. Figure

2.

Absorption spectra samples

of

PMMA

PMMA prepared with (1) no added reagent, (2) hydroquinone and (3) DPPH. Initiator system: TBHP-SO*. (The unit of wavelength is the nanometer, nm., formerly called the millimicron, m^). Concentration of polymer solution: 2.5% in chloroform

agreement with the respective values calculated from the intrinsic viscosity. For the other three polymers, having very high molecular weights, the G P C molecular weight Mw is much smaller than that calculated from the intrinsic viscosity. This discrepancy with the very high molecular weight polymer suggests that the resolution in the higher molecular weight region is poor, and this is substantiated by the shape of the chromatograms for these samples, showing a sharp rise in the detector response in the higher molecular weight (lower effluent count) region. The uncertainty in calibration in this region (owing to extrapolation) is also a factor to be considered. Thermogravimetric Analysis. P M M A prepared by initiation with low concentrations of sulfur dioxide at or near room temperature has been found to incorporate from negligible amounts to trace quantities of SOo at chain ends and/or at any other part of the chain presumably by copolymerization (15). Thermogravimetric analysis of a few P M M A samples prepared in the presence of S 0 2 under varied conditions has


80

ADDITION AND CONDENSATION POLYMERIZATION

PROCESSES

Table I. PMMA Samples for GPC Analysis

Sample

Initiator, M

1 2 3

S 0 2 (3.75) S 0 2 (0.0037) TBHP (0.007) S 0 2 ( 0.004) TBHP (0.03)


4

M

from

from GPC

[η]

Mw Mn

6.7 7.4 1.07

3.96 Χ 10 3.98 Χ 106 5.06 X 10*

5.26 6.03 4.73

Χ

106 Χ 106 Χ 105

1.67 1.85 1.75

5.08

2.41 X 10«

3.67

Χ 106

1.55

6

been done to see i f the incorporation of S 0 2 , in trace quantities, affects the thermal properties of the polymers. Five P M M A samples were studied: (1) d u Pont Lucite 140, (2) initiated by S 0 2 ( 4.25M) i n bulk at 3 0 ° C , (3) initiated b y S 0 2 ( 0.0375M) in bulk at 3 0 ° C , (4) initiated by 0.05M T B H P at 3 0 ° C , (5) initiated by T B H P (0.01M) and S 0 2 (0.004M) at 3 0 ° C . Polymers 2-5 were isolated by precipitation i n petro leum ether ( 4 0 ° - 6 0 ° C . ) and then dried in air and finally in vacuum at 40 ° C . The numbered curves i n Figure 3 represent the thermograms of the respective samples. The thermograms of Samples 2, 3, and 4 appear more or less comparable, showing a very slow decomposition (given b y loss i n weight) beginning at about 1 3 0 ° - 1 5 0 ° C . and a fast decomposition beginning at about 300 ° C . for Samples 2 and 3 and at about 2 9 0 ° C . for Sample 4. Sample 5 shows much lower thermal stability, decomposing in three stages. It leaves a very fine brown residue beyond about 4 0 0 ° C ,

T,°C. Figure 3. Dynamic thermogravimetric analysis of PMMA samples. Heating rate, 5°C./min., with a steady flow of nitrogen, 80 ml./min.


5.

GOSH AND BILLMEYER

Methyl

81

Methacrylate

M MA (M,) Isoprene ( M2 ) 0.8 h

οο o.6

0.4

2

° ΙΤΒΗΡΌ.0044/ *

•S 0.2 h Downloaded by UNIV LAVAL on April 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch005

|S0 : 0.003Λ/

o

f

1

0.2

AIBN'· 0.0084/

1

1

0.4

1

0.6

Mole fraction

0.8

t l]feed M

MMA (M,) Acrylic Acid (M ) 2

0.8 h 0.6 rso 0.4

#

•

: 2

o.oi/i/

ITBHP: 0.0064/ AIBN: 0.0084/

-= 0.2 1

0.2

1

0.4

1

1

0.6

0.8

Mole fraction Γ Μ ι ] , feed , J

Λ

Figure 4. Copolymer composition curves Top: MMA—isoprene. Bottom: MMA-acrylic acid. Temperature: (A) 30° C. for TBHP-SO2 system and 65°C. for AIBN system; (Β) 30°C. for both the ini tiator systems

while the other samples leave no residue. Sample 1 decomposes quickly, beginning at about the same region as Samples 2 and 3. Copolymerization. Copolymerization of M M A (Mi) with both iso prene and acrylic acid (M 2 ) was carried out using both the TBHP-SO2 system and azobisisobutyronitrile (AIBN) as the initiator, and the com-



82

ADDITION AND CONDENSATION

POLYMERIZATION

PROCESSES

positions of the respective copolymers obtained at low conversions (^5% ) with the two initiator systems have been compared (Figure 4). Isoprene and acrylic acid were chosen as the second monomers because of the wide difference in their physical and chemical properties. The reactivity ratios for these two copolymer systems have not been reported i n the literature. In each binary system investigated herein, the results for the two initiator systems agree closely. Compositions of the MMA—isoprene copolymers were determined by estimating unsaturation in the copolymer samples, employing a modified bromination method (17,18) to determine polyisoprene unsaturation, while those for the M M A - A A copolymers were determined by estimating the carboxylic acid content of the purified copolymers i n acetone solution b y titration with methanolic sodium hydroxide. The reactivity ratios for the copolymers were derived graphi cally by the method of intersection. For the M M A - i s o p r e n e system, fi = 0.60, and r 2 = 0.47; for the M M A - A A system, η = 1.59, and r 2 = 0.21. This fi value of 1.59 is i n good agreement with fi = 1.28 for the same system reported by Palit and Ghosh (23) where the carboxyl content in the copolymers, prepared under limiting condition of very low M 2 / M i ratio i n the feed monomer mixture, was determined b y a sensitive dye interaction method. Discussion

Acrylonitrile, methyl methacrylate, vinyl acetate, and styrene were polymerized at 30 ° C . i n bulk using the same concentration of S 0 2 and T B H P as the initiator components. The yield curves are given in Figure 5. The initial (steady) rates in moles/liter/sec. are 6.2 X 10"3, 0.83 Χ 10"3, 1.9 Χ 10"3, and 0.019 χ 10'3, respectively for A N , M M A , V A , and Sty. These rates are thus in the proportion 1 : 0.13 : 0.31 : 0.0031. From values of kp/kt1/2 at 30 ° C . for radical polymerization, taken from the literature (7) the rates would be expected to be in the proportion 1:0.11:0.26:0.016. Thus, they appear to be i n the same order as expected for radical polymerization. Isobutyl vinyl ether ( I B V E ) is known to polymerize by cationic mechanisms (5, 27). It has been found, however, that S0 2 , dissolved i n I B V E , turns the system to a brown viscous liquid on standing overnight in air; when the solution is allowed to stand i n vacuum, there is little change i n 24 hours, but gain i n viscosity and gradual darkening of the medium occur in about 3-4 days. The rate of polymerization is enhanced by the presence of T B H P , and using high concentration of T B H P (>0.5M), the polymer obtained is found insoluble, presumably owing to crosslinking. In all cases the final yield of polymer is low (•—2-10%



5.

GOSH AND BILLMEYER

20

Methyl

83

Methacrylate

40 60 80 TIME, MINUTES

100

Figure 5. Polymerization of AN, MMA, VA, and Sty at 30°C. using fixed concentrations of SO, and TBHP

in 3-5 days ). The intrinsic viscosity of the soluble polymers obtained is in the range of 0.04-0.1 dl./gram, as determined in benzene solution at 30°C. Some interesting solvent effects observed in the T B H P - S 0 2 - i n i t i a t e d polymerization of M M A have been mentioned before. It has been further observed that when polymerization of M M A and A N is carried out i n the presence of alcohols, the rate of polymerization is in the order: methanol > ethanol > isopropyl alcohol > tert-buty\ alcohol, cyclohexanol. This suggests that the over-all polymerization mechanism (or the initiation mechanism itself) depends on the polar contribution of the alcohol. A similar observation was made by Imoto et al. with a benzoic anhydridedimethylaniline N-oxide system as the initiator (20). Although the polymerization of A N in bulk and also in the presence of 5—15% alcohol was heterogeneous, that of M M A in the presence of the alcohol remained homogeneous. Radical occlusion undoubtedly took place in the A N polymerizations, but this complication has not been considered i n detail. Mechanism. The ability of SOL» to participate in complex formation with unsaturated hydrocarbons and a host of other compounds such as amines, ethers, phenols, and aromatic hydrocarbons is known (2, 6, 11, 22). SO2 has been found to initiate polymerization of some monomers


84

ADDITION

AND CONDENSATION

POLYMERIZATION

PROCESSES


such as the methacrylates and styrene at room temperature but not of some other monomers such as A N , acrylamide, the acrylates, and vinyl acetate. This indicates that the S 0 2 complexes formed with the latter monomers are relatively stable compared with those formed with the former monomers. L o w concentrations of T B H P easily initiate the poly merization of all the above monomers in the presence of S 0 2 at room temperature. It appears that the stability of the S 0 2 complexes with the latter monomers does not arise from cyclic sulfone formation, and the use of η-butyl sulfone with T B H P does not cause these monomers to polymer ize at room temperature. When the hydroperoxide, T B H P , is used, polymerization is much faster than when a peroxide such as tert-butyl peroxide or benzoyl per oxide is used along with S 0 2 as the initiator. Thus, it may be assumed that T B H P reacts more rapidly with S 0 2 than do other peroxides to give free radicals or also may form complexes with S 0 2 and probably with the monomers as well (25) to give less stable species, thus bringing down the energy barrier for the initiation reaction. S 0 2 in methanol solution shows a strong absorption at 275 nm. It is greatly and rapidly reduced i n the presence of an equivalent amount of T B H P , which does not show any significant absorption at that wavelength. The non-inhibitory effect of D P P H and hydroquinone on polymerization initiated by the T B H P - S 0 2 system and the ability to initiate polymerization of isobutyl vinyl ether indicates that the polymerization concerned is not of typical free radical type; on the other hand, copolymerization data and end group studies suggest that the T B H P - S 0 2 - i n i t i a t e d polymerization is, by and large, radical i n nature. The importance of environment on the T B H P - S 0 2 initiated polymerization was clearly shown when the polymerization was carried out i n the presence of different solvents. The following reactions may possibly take place before propagation of the chain reaction sets i n : R O O H + S 0 2 -> [ R O O H . . . SO a ] k2

(1)

R O O H + S -> [ROOH . . . S]

(2)

R O O H + M - » [ROOH . . . M ]

(3)

k4

S02

+ M ->

[S02 . . . M]

*5

SO, + S - » [ S 0 2 . . . S]


(4) (5)

5.

GOSH AND BILLMEYER

Methyl

Methacrylate

85

[ R O O H . . . S 0 2 ] + M -> RO- + HSO ; i MR02- + HS02MRS0 3 - + H O M RO" + H S 0 3 M

V

(6)

+

ROH + S03M-


k7

[ R O O H . . . S] + S 0 2 -> R O H + H S 0 3 - + S-

(7)

[ROOH . . . M ] + S 0 2 - » ROH + S03M-

(8)

ROOH + [S02 . . . M ] - » ROH + S03M-

(9)

R O O H + [ S 0 2 . . . S] -> R O H 4- H S 0 3 - + S-

(10)

R O O H stands for T B H P , and the species in the brackets represent respective complexes. In Reactions 7 and 10 abstraction of a hydrogen atom from the solvent molecule, S, is considered. Reactions between two or more complexes are neglected for simplicity; radical generation involv ing dissociation of the binary complexes as such is considered insignificant and therefore neglected. The rate of initiation, R { , can then be expressed as =

[ROOH] [ S 0 2 ] [ M ] + fc2fc7[ROOH] [ S 0 2 ] [S] +

fc3fc8[ROOH] [ S 0 2 ] [ M ] + fc4fc9[ROOH] [ S 0 2 ] [ M ] + fc5fc10[ROOH] [ S 0 2 ] [S] Thus, R( may be simply expressed as: R { = K a [ R O O H ] [ S 0 2 ] [ M ] + Kb[ROOH]

[ S 0 2 ] [S]

(11)

If termination of polymerization is bimolecular in nature, then

where Rp = rate of polymerization, kp and kt are rate constants for propa gation and termination, respectively. Hence, 2k R p 2

"kj-

1

Xi^F=KatR00H] ΟΓ

[ s

°

2 ]

[ M ]

'V[ROOH][S02][M]3

+

Λ

K

«

^

R

Τ Λ 6

0

0

H

]

t °2] s

[Μ]


PI V

'

86

ADDITION A N D C O N D E N S A T I O N


wo-

POLYMERIZATION

PROCESSES

y

I

I

»

»

0.4

•

0.8

»

•

1.2

I

I

—L.

1.6

_J

1

2.0

[S]/[M] Figure

6.

Polymerization of MMA in benzene solution at 30°C. (A), Left-hand expression of Equation 12

(B), i/Pn, plotted against [S]/[M]

Table II. Polymerization of MMA at 3 0 ° C . in Benzene Solution" [S]/[M] 0 0.135 0.324 0.812 1.645 β

=

[M],M 9.36 8.44 7.49 5.62 3.98

R X 10* M/sec/ 1.761 2.921 2.812 1.897 1.395

[TBHP] = 0.0075M; [S0 2 ] = 0.004M; C = ktlk? = 3.69 X 102.

[η] 0.99 0.53 0.43 0.38 0.30 [TBHP] [S0 2 ] =

2A" R 2 C[M]^

0.00092 0.00339 0.00462 0.00500 0.00757 3 X 10"~>; A"

According to the initiation mechanism given above, a plot of the left hand expression against [ S ] / [ M ] in solution polymerization would give a straight line i f termination is bimolecular i n nature. Such a plot for the polymerization of M M A in benzene at 3 0 ° C . and at constant concen trations of T B H P and S 0 2 is given in Figure 6, Curve A , and the results are given in Table II. The values of kp and kt for free radical polymeriza tion of M M A at 3 0 ° C . are taken from the literature ( 7 ) . A reasonably good linear plot is obtained over a wide range of [ S ] / [ M ] , excepting points at near-bulk or bulk polymerization. In the solution polymerization of M M A at high temperature initiated by cumene hydroperoxide, Tobolsky and Matlak (25) also observed effects of this kind in the presence of solvents such as benzyl alcohol or dimethylaniline, but no solvent effect


5.

GOSH AND BILLMEYER

Methyl

Methacrylate

87


was noticed with benzene. Burnett and co-workers (1, 10) also reported an enhanced rate of M M A polymerization in the presence of some halogenated benzenes, but no solvent effect was reported for benzene. The difference between the intercept obtained by extrapolating the straight line portion of Figure 6, Curve A , to zero [ S ] / [ M ] and the experimental value of the ordinate at zero solvent concentration is sig nificant. A plot of reciprocal degree of polymerization ( 1 / F n ) against [ S ] / [ M ] has similar characteristics (Figure 6, Curve B ) ; the molecular weight of the polymer at zero [ S ] / [ M ] is higher than that obtained from the intercept given by extrapolating the straight line at high [ S ] / [ M ] region. The slope of this line is 0.4 Χ 10~3, and this is apparently much higher than the expected chain transfer constant for benzene at 30 ° C . The slope of the straight line portion of Curve A (Figure 6) gives the value 2.5 Χ 10"3 for Kb i n Reaction 12 for the present system. The value of Ka as given by the intercept on extrapolating the linear portion of Curve A to zero [ S ] / [ M ] is 3.4 Χ 10"3, while the experimental value of the ordinate at zero [ S ] / [ M ] is 0.93 Χ 10Λ Similar observations were made with T B H P - S 0 2 - i n i t i a t e d polymeri zation of M M A at 5 0 ° C . using benzene, toluene, and xylene as solvents and at 6 0 ° C . using chloroform as the solvent (Figure 7). Among the three aromatic hydrocarbons used, the rate enhancement is of the order: xylene > toluene > benzene. Polymerization of Some Other Vinyl Monomers. The polymeriza tion of acrylonitrile ( A N ) with T B H P - S 0 2 initiator system occurs at a fairly high rate with little inhibition period. A log-log plot of rate of polymerization Rp i n bulk at 30 ° C . vs. the product of the concentrations of T B H P and S 0 2 gives a straight line having a slope of 0.75. The rate varies directly with the 1.8 power of monomer concentration (Figure 8), the monomer variation being done with benzene as the solvent. Polymeri zation proceeds at an enhanced rate in the presence of low concentrations of tetrahydrofuran, acetone, and cyclohexanone but is appreciably slower in the presence of higher alcohols and dimethyl formamide. Hydroqui none (0.01M) produces a small inhibition period (.—2-3 min. using 0.0025M S 0 2 and 0.01M T B H P ) , and the polymerization proceeds at a very retarded rate thereafter. Under the same conditions, 0.0012M D P P H induces an inhibition period of 3 hours, and about 8-10% conversion to polymer takes place in about a week thereafter. The purified polymer is faint yellow and shows significant absorption in the visible/near-ultraviolet region, indicating the incorporation of fragments of D P P H i n the polymer. The over-all activation energy of A N polymerization by the T B H P - S O o initiator system is 8.74 kcal./mole.


88

ADDITION


MMA in: Φ • A Φ

0.2

A N D CONDENSATION

POLYMERIZATION

PROCESSES

Xylene Toluene Benzene Chloroform

0.4

0.6 0.8 [S]/[M]~

1.2

1.0

MMA in: · Toluene c Chloroform 3 to

?

2 Slope: · 2.4XI0"

3

Φ

1,

0.2

1

0.4

0.6

0.8

1

1.0

2.1 χ ΙΟ

3

ι

1.2

I

[S] /[M] Figure 7.

Polymerization of MMA at 50°C. in benzene, toluene, and xylene solution and at 60°C. in chloroform solution Top: left-hand expression of Equation 12 plotted against [S]/[M] Bottom: I/P„ vs. [S]/[M] for polymerization in toluene

Polymerization of some other vinyl monomers such as E M A , n - B M A , V A , Sty, and M A was studied in the presence of certain solvents using the T B H P - S 0 2 initiator system. Tetrahydrofuran, acetone, and cyclo-


5.

GOSH AND BILLMEYER

Methyl

89

Methdcrtjlate

hexanone enhanced the rate of polymerization of all these monomers, but benzene, toluene, or xylene d i d not. The rate dependence on mono mer concentration, using benzene as the diluent for the various monomers, is shown in Figure 8. The slopes of the logarithmic straight-line plots of Rp vs. [ M ] for these monomers are between 1.5 and 1.8. Only M M A is different, showing a maximum i n the plot. Log [M] I

ι

1

0.2

0.4

1

1

1

1

0.6^

1

1

0.8 1.8


0.9

1.6

0.7 ο. ~~A

QC

/

o» ·/ 0.5 - ©

/

'J

/

/

-J /

/

- -h

• nBMA, 30°C α EMA, 30°C • MMA, 50°C ο VA, 50°C A AN, 30°C φ MA, 30°C © Sty 30°C • MMA 30°C

/

0.3

0.1 1

0.6

Figure 8. Log Initiator, TBHP-SO2

1

0.8

1

1

1.0 Log [M]

1

1

1.2

1

- 1.4 _ 1.2 . _

1

R D vs. /og[M] for various monomers system. Slope: AN =

1.8, MA =

1.5, VA

= 1.8, Sty = 1.8, n-BMA = 1.6, EMA = 1.6

The plot of the left-hand expression of Equation 12 against [S] / [ Μ ] , where S stands for benzene, for some of these monomers is shown in Figure 9. For methyl acrylate the rate measurements were done dilatometrically at high [ S ] / [ M ] ratio to avoid difficulties owing to gelation and heat buildup. The slope of the plots in Figure 9 is negative (or negligible compared with the value of the intercept as for M A ), indicating that benzene does not participate in the initiation step in the polymeriza tion of these monomers. The monomer exponent of 1.5-1.8 for these monomers (except M M A ) indicates that the initiator-monomer initiation reaction is dominant, and the initiator-solvent initiation reaction is neg ligible. For M M A in benzene solution, in the high solvent concentration region the log-log plot of Rp vs. [ M ] shows an order in [ M ] close to the value of 1 (Figure 8) indicating, according to the initiation mechanism detailed above, the predominance of an initiator-solvent initiation reac-


90

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N

PROCESSES

tion (25). Plots of ( 1 / P n ) vs. [ S ] / [ M ] for some of the monomers polymerized i n benzene solution are given i n Figure 10; i n each case the slope is higher than the expected chain transfer constant for benzene in the respective systems ( 8 ) . Thus, i n the present system of polymerization initiated by TBHP-SO2, M M A behaves quite differently from other monomers such as Sty, E M A , n - B M A , A N , V A , etc. The precise reason for this difference is not underSty AN n-BMA MA xl0 xl0 xl0 xl0 I6 8 4 5

4

3

r

2

r

9


J.

φ

Ο-"™

3

n-BMA

CVJ Oίε. Ο

m

0.2

0.4

0.6

0'.4 2

ι

1.2 ι 6

0.8 ι 4

0.8 η-ΒΜΑ ι 1.6 ΜΑ ι 8

[S]/M Figure 9. Left-hand expression of Equation 12 vs. [ S ] / [ M ] for the polymenzation of MA, n-BMA, AN, and Sty in benzene solution Slope: MA — 0.21 X 10~s, n-BMA = 0.5 X 10rS, AN = -2 X 10~^ and Sty =

-4X

IO-

5

EMA AN -0.7 EMA

.AN

0.3 -

Sty VA 1.2- 3

^^Sty 0.2

0.8- 2

r"VA

-0.3

Slope: • 4.9 xl0~ 0 0.71x10

ro Ο

~

4

0.1

4

-0.1

-A

7.2 χ 10* • Il.4xl0*

A

0.4- 1

4

1

0.5

1

1.0 [S]/[MJ

Figure 10. 7/P n vs. [S] / [ M ] for the polymenzation of AN, EMA, VA, and Sty at 30°C. in benzene solution initiated by TBHP-S02 system



5.

GOSH AND BILLMEYER

Figure

11.

Methyl

Methacrylate

91

Pn vs. [M]2/RO for the polymerization of MMA

(1) In benzene solution at 30°C. (2) In toluene solution at 50°C. (3) In chloroform solution at 60°C. at varying monomer concentration (4) In benzene solution at 30° C. and varying initiator concentration • , [M] =8.2M O , [M] = 4.68M Initiator system: TBHP-SO>

stood from our experiments; the enhanced rate of polymerization i n the presence of some ketones and tetrahydrofuran for all the monomers can, on the other hand, be explained in terms of the observation of Uebereiter and Rabel (26)—i.e., it arises from activation of the hydroperoxide by hydrogen bond formation between the peroxy hydrogen of the hydroperoxide and the ketonic or etherial oxygen. W h i l e considering the rate-enhancing effect of bromobenzene i n M M A polymerization initiated by A I B N , Henrici-Olivé and Olive (19) noted that the effect can be explained as the consequence of electron donor-acceptor complex formation between polymer radicals and monomer or solvent molecules. Based on this view, these authors have shown that i n polymerization i n active solvents (which enhance the rate), the degree of polymerization Pn appears as a linear function of M2/Rp with



92

ADDITION A N D C O N D E N S A T I O N P O L Y M E R I Z A T I O N PROCESSES

zero intercept (19), assuming the absence of any side reactions. F o r M M A polymerization i n benzene, toluene, and chloroform (which show the rate-accelerating effect), using T B H P - S 0 2 as the initiator, fairly good linear plots of Pn vs. M2/Rp, passing through the origin, have been obtained (Figure 11). Thus, it appears that our experimental results can also be explained in the light of the hypothesis of Henrici-Olivé and Olive. O n reviewing and reinvestigating the polymerization of M M A i n the presence of halogenated benzenes and other aromatic solvents, Bamford and Brumby (3) have shown that the dependence of the rate of polymerization on the nature of the solvent arises partly from the viscosity dependence of the termination coefficient and partly from the dependence of the propagation rate constant on the nature of the solvent, and that the rate of initiation is not influenced by the nature of the solvent. Their results are compatible with the hypothesis of Henrici-Olivé and Olive, but they contradict the suggested mechanism of Burnett et al. ( I , 10) which considers solvent participation in the initiation step. The above authors (1, 3, 10, 19) d i d not observe any rate-enhancing effect in M M A polymerization in the presence of benzene. However, using T B H P — S 0 2 as the initiator system for the polymerization of M M A , we have observed enhanced rates i n the presence of benzene and also i n the presence of toluene, xylene, chlorobenzene, chloroform, etc. Kinetic analysis indicates that these solvents take part i n the initiation process, and the treatment of data relating the degree of polymerization and rate of polymerization shows that the hypothesis of Henrici-Olivé and Olive is also compatible with our results. A cknowled

gment

Financial support of this work by the National Aeronautics and Space Administration under Grant NsG-100 is sincerely acknowledged. The thermogravimetric analyses were obtained through the courtesy of J. C h i u , Plastics Department, Ε . I. d u Pont de Nemours & Co., Experimental Station, Wilmington, D e l . , and for this we express our sincere thanks. W e also appreciate the assistance and cooperation of R. N . Kelley of our laboratories i n obtaining the G P C data. Literature

Cited

(1) Anderson, D. B., Burnett, G. M., Gowan, A. C., J. Polymer Sci. A, 1, 1465 (1963). (2) Andrews, L. J., Keefer, R. M., J. Am. Chem. Soc. 73, 4169 (1951). (3) Bamford, C. H., Brumby, S., Makromol. Chem. 105, 122 (1967). (4) Baxendale, J. H., Bywater, S., Evans, M. G., J. Polymer Sci. 1, 237 (1940).



5.

GOSH AND BILLMEYER

Methyl Methacrylate

93

(5) Blanke, G. J., Carbon, A. M., J. Polymer Sci. A-1 4, 1813 (1966). (6) Booth, D., Dainton, F. S., Ivin, K. J., Trans. Faraday Soc. 55, 1293 (1959). (7) Brandrup, J., Immergut, Ε. H., Eds., "Polymer Handbook," pp. II-(5765), Interscience, New York, 1966. (8) Ibid., pp. II-(77-133). (9) Ibid., pp. IV-(1-45). (10) Burnett, G. M., Dailey, W. S., Pearson, J. M., Trans. Faraday Soc. 61, 1216 (1965). (11) Dainton, F. S., Ivin, K. J., Discussions Faraday Soc. 2, 376 (1947). (12) Fox, T. G., Mason, H. F., Cohn, E. S., unpublished work (cited in Ref. 9). (13) Ghosh, P., Chadha, S. C., Mukhrejee, A. R., Palit, S. R., J. Polymer Sci. A 2, 4433 (1964). (14) Ghosh, P., O'Driscoll, K. F., J. Polymer Sci. B, 4, 519 (1966). (15) Ghosh, P., O'Driscoll, K. F., J. Macromol. Sci. A, 1, 1393 (1967). (16) Ghosh, P., Sengupta, P. K., Pramanik, Α., J. Polymer Sci. A, 3, 1725 (1965). (17) Ghosh, P., Sengupta, P. K., J. Appl. Polymer Sci. 11, 1603 (1967). (18) Gowans, W. J., Clark, F. W., Anal. Chem. 24, 529 (1952). (19) Henrici-Olivé, G., Olivé, S., Makromol. Chem. 96, 221 (1966). (20) Imoto, M., Sato, T., Takemo, K., Makromol. Chem. 95, 117 (1966). (21) Johnson, J. F., Porter, R. S., Cantow, M. J. R., Rev. Macromol. Chem. 1, 393 (1966). (22) Maine, P. A. D., J. Chem. Phys. 26, 1036, 1042, 1049 (1937). (23) Palit, S. R., Ghosh, P., J. Polymer Sci. 58, 1225 (1962). (24) Schulz, R.C.,Banihaschemi, Α., Makromol. Chem. 64, 140 (1963). (25) Tobolsky, Α. V., Matlak, L. R., J. Polymer Sci. 55, 49 (1961). (26) Uebereiter, K., Rabel, W., Makromol. Chem. 68, 12 (1963). (27) Yamaka, H., Takakura, K., Hayashi, K., Okamure, S., J. Polymer Sci. B, 4, 509 (1966). RECEIVED April 1,

1968.


Polymerization of Methyl Methacrylate and Other Vinyl Monomers

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