Catalytic System for Low Temperature Polymerization of Vinyl Chloride Corrado Mazzolini, Luigi Patron, Albert0 Moretti, and Marcello Campanelli Chatillon, Societa A n o n i m a Italiana per le Fibre Tessili Artificiali S.p.A., V i a Conservatorio 7-13, M i l a n , Italy
Organic hydroperoxides react with SO1 in the presence of nucleophilic agents, giving RO. and SOa. radicals that are efficient initiators of vinyl chloride polymerization in bulk a t subzero temperatures. When cumyl or tert-butyl hydroperoxides and SO2 are used together with ethers, ketones, or alcohols, sulfone groups are incorporated in the polymer chain because of the copolymerization of SOZ. When the hydroperoxides and SO2 are used with MeO- or EtO- (from Na or Mg alkoxides), SO2 copolymerization is completely suppressed, provided the ratio (MeO-) or (EtO-)/(SOz) is at least 1 . When the feed rate of hydroperoxide is constant, maximum monomer conversion in continuous bulk polymerization is reached when the ratio (S02)/(hydroperoxide) is 1.5 or higher. Then the dependence of monomer conversion (c) on hydroperoxide concentration (C), and dwell time (Q) is expressed as c/( 1 - c)' = (K,/K,' ') f' (C),1 2Q 1 2, where K , is the rate constant for propagation, K, is the rate constant for combination, and f is the radical efficiency. Polymerization examples are given for vinyl acetate, vinyl formate, acrylonitrile, acrylamide, and 2-hydroxyethyl acrylate at -30"and -60" C.
'
T h e interest in low temperature PVC polymerization predates the previous decade. Huggins (1944) deduced from theoretical considerations and Fordham et al. (1959) experimentally demonstrated that, as the polymerization temperature is decreased, the formation of syndiotactic sequences is favored over the formation of isotactic, because of a lower activation energy for monomer addition in the syndiotactic pattern. These results were confirmed by several authors through N M R and infrared measure1967; Garbuglio et d . , 1964; Garbuglio ments (Bovey et d., and Gallinella, 1965; Krimm et al., 1963; Nakajima et al., 1966; Talamini and Vidotto, 1967). Though a perfect agreement on absolute values is not found among their determinations, all existing data indicate an increase in syndiotacticity a t increasingly lower polymerization temperatures. Chain branching also diminishes a t low polymerization temperatures. Other investigators (Boccato et al., 1967; Nakajima et al., 1966) agree on the trend, although differing in ratios. Both trends contribute to an increase in crystallinity of the polymer, higher glass transition temperature, higher resistance to solvents, and better mechanical properties a t and above the glass transition temperature (Bockmann, 1965; Garbuglio et al., 1964; Reding et al., 1962). Such polymers of high syndiotacticity have been used for the production of new PVC fibers of improved properties, 504
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
'
substantially upgrading those of conventional PVC fibers ( M a n - M a d e Textiles, 1966; Mazzolini, 1968). Other applications of low temperature-polymerized PVC have been considered, although the processing of such polymers is believed to encounter difficulties (Bockmann, 1965; Braun, 1968; Brighton, 1962; Pungs, 1967; Trautvetter, 1967). The objective of our research was to find a suitable catalytic system for the bulk polymerization of vinyl chloride a t temperatures from -20" to -60°C which would fulfill the requirements of commercial operation-Le., sufficiently high activity a t those low temperatures, easy polymerization control, no adverse effect on polymer stability, safe operation, and low cost. All requisites are more or less lacking in the previously known catalytic systems, based mainly on organometallic compounds like boron alkyls (Borsini et al., 1966; Furukawa and Tsuruta, 1958; Talamini and Vidotto, 1961, 1962), cadmium alkyls (Furukawa et al., 1958), aluminum alkyls (Montecatini, 1963), lead alkyls (Borsini et al., 1968; Edison, 1966), and silver alkyls (Collinson and Jones, 1962). Decomposition of Organic Hydroperoxides by SO?
The decomposition of organic hydroperoxides by the action of SO, depends on the reaction medium. Taking as an example cumyl hydroperoxide ( C H P ) , it is quan-
titatively decomposed into phenol and acetone, if the reaction is carried out in anhydrous weakly nucleophilic or nonnucleophilic medium-e.g., CClr, CH ,CN, CH3CHzCl. CH?=CHCl. This type of decomposition, which proceeds through an ionic mechanism without formation of radicals, can also be obtained with perchloric acid, ferric chloride in benzene, and sulfuric acid (Seubold and Vaughan, 1953; Tobolsky and Mesrobian, 1954). Following the decomposition by means of ultraviolet measurements, we have determined constants to be pseudo-first-order with respect to C H P and found them to be 2.8 x 10 ' and 1.9 x 10 ' min ' for the decomposition of CHP in acetonitrile at -20°C. respectively, in the presence of 0.02 mole per liter of SO?and 0.02 mole per liter of H?SOI. No reaction product other than phenol and acetone was detected by ultraviolet and chromatographic analysis. I t is therefore inferred that SO, behaves as a strong acid toward the decomposition of C H P in anhydrous weakly nucleophilic or nonnucleophilic solvents. For a redox reaction to take place, according to the Le%is theory of acids and bases, it is necessary that the reductant (SO,) act as a base toward the oxidant (hydroperoxide) to allow the transfer of electrons from the former to the latter (Gilman, 1948). This condition is fulfilled by the addition t o the reaction medium of a strongly nucleophilic agent X --e.g., OH-^in order to transform the SOz into the conjugate base ~
xso, .
We have followed this clue when investigating the effect of water on the reaction between C H P and SO? in acetonitrile a t -20" C. As the water concentration increases, t h e , absorbance a t 272 mp, characteristic of phenol, diminishes, while a new absorbance maximum. ranging between 23'7 and 255 mw, emerges due t o a mixture of 1-methylstyrene ( 3 5 ) . acetophenone (60',), and cumyl alcohol ( 3 7 5 ) (Figure
inlet tubes for the addition of vinyl chloride, C H P , and, through the same neck. SO? and methoxide solutions. Gaseous vinyl chloride was metered with a rotameter. SO?and Na or Mg methoxide were fed in methanol solution at concentration and metered by motor-driven syringes. C H P was also metered and fed by a motor-driven syringe. When SOL and methoxide are fed separately. they react in the reactor to give the corresponding salt immediately, Na or Mg methyl sulfite. K O difference was found in polymerization behavior when feeding a preformed solution of Na or Mg methyl sulfite. When preformed solution was used, the methoxide was added to a 1 0 5 methanolic solution of SO?.As E a methoxide would form a precipitate if added to the SO? solution in an amount higher than 1 mole per mole of SO?, its concentration in the preformed solution was kept below this limit. The excess of methoxide, when required. was fed separately. I n the presence of alkoxides, the presence of methanol did not affect the rate of reaction. In the absence of alkoxide, the presence of methanol as a nucleophile caused vinyl chloride to polymerize. Therefore, the polymerization reactions were conducted by feeding pure gaseous SO,, instead of a methanol solution, only when no strong base was present (Table I and the first row of Table 11). To start the continuous polymerization, the reactor was initially charged with vinyl chloride and cooled to a desired temperature. Then all feeds were started a t a prefixed rate. Samples were taken at 1- or 2-hour intervals for the determination of percentage conversion and specific viscosity. Consistent results were always reached within five dwell times. All the results presented were determined
10
1).
These products, isolated by chromatography according to Vlodavets and Golbert (1965), and identified by infrared spectroscopy, indicate that the decomposition of C H P proceeds in this case through radical intermediates. Acetophenone, cumyl alcohol, and 1-methylstyrene are, in fact, according t o Tobolsky and Mesrobian (19541, the rearrangement products of the reaction intermediate a-oxycumyl radical. When water is added to the system hydroperoxideSO, in an organic medium, a situation analogous to the emulsion polvmerization by hydroperoxide and SO? is induced (Kutsenok et al., 1959). This demonstrates the possibility of switching the mechanism of reaction between hydroperoxide and SO?in an essentially organic medium from an ionic mechanism to a radical one, thus offering a way for the initiation of vinyl polymerization a t low temperature. Experimental
hIaterials. Vinyl chloride monomer was distilled, cooled to -50"C, and flushed with nitrogen free from oxygen. C H P was 80°C pure, the balance being cumene. Other reagents and solvents were of analytical grade. CAUTION. Addition of S O p to C H P solutions containing more than 10% C H P can cause an explosive reaction. Procedure. Continuous polymerizations were carried out using a 2-liter three-necked glass reactor, fitted with four
20 30
s 40 W c,
5 50 I-
4
5
60
Q
Tr
70 80
90 100
290280 270 260 250
240
230
m)l Figure 1. Ultraviolet spectra showing influence of water on decomposition of CHP in acetonitrile
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
505
Table I. Influence of Nucleophilic Agents on Bulk Polymerization of Vinyl Chloride Batch polymerization conditions. Temperature, -30" C. CHP. 0.15(; (0.m.w.) SO1, 1.6'~;(o.m.w.1. Nucleophilic agent as specified. Addition time of catalyst components into monomer. 1 hour. Total reaction time, 2 hours Nucleophilic Agents
Kone Acetophenone Cycl(ihexanone Acetone Methyl ethyl ether Ethyl ether Methanol Methanol Butanol Dimethy lamine Dimet hylf'ormamide
Yo o.m.w.
0.00 0.60 0.49 0.29 0.36 0.52 0.16 5.00 0.36 0.22 0.36
Conversion,
Oh
0.00 1.9 5.1 1.2 3.7 3.2 6.8 10.5 6.0 0.4 0.51
on samples collected after five dwell times. The average of a t least three determinations is always given. I Batch polymerizations were carried out by precharging the reactor with monomer and then feeding the catalyst components at constant rate for a definite time (Table I ) . The polymerization was allowed to proceed, if necessary, for additional time after the catalyst feed was discontinued. The polymerization slurries were mixed by means of conventional tilted-blade agitators revolving a t 200 to 300 r.p.m. Under the experimental conditions, the polymerization slurries were free-flowing. There was adequate mixing. and the regular discharge was by overflowing. Samples for conversion and analytical determinations were taken by the following procedure. About 160 grams of slurry were collected in a glass flask maintained a t -70°C. weighed, and poured into methanol chilled at -70" C. The polymer was filtered on a sintered glass filter and repeatedly washed with methanol, then dried in a vacuum oven a t 60' C for 8 hours.
Figure 2. Infrared spectra of PVC showing copolymerized SO?
506
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
Conversion was calculated as per cent dry polymer weight on slurry weight. Specific viscosity ( N s p ) was measured in a 0.15 cyclohexanone solution at 25" C with a Cannon-Fenske viscometer, and is expressed in decilkers per gram. The number average molecular weight, PM,, was determined in cyclohexanone a t 37" C with a Hewlett-Packard 502 high speed membrane osmometer. The amount of sulfonate end groups was determined by titration with tetramethylammonium hydroxide of a 0.5('r polymer solution in tetrahydrofuran, after acidification of the sulfonate end groups and elimination of impurity ions by treatment with cationic (Amberlite IR-120) and anionic (Amberlite IRA-410) ion exchange resins. The sulfur content of the polymer was determined according to Bertolacini and Barney (1967). The glass transition temperature, T,, was determined bv differential thermal analysis. Differential scanning calorimetry curves were obtained a t a rate of 16" C per minute, and TL,was determined by the extended line intercept method. Samples for measurement of dehydrochlorination rate were collected by simple filtration of the polymer slurry and evaporation of the residual monomer, this procedure being close to commercial operation of a commercial product. The dwell time-i.e., the average residence time of each species in the reactor-is conventionally calculated as the ratio between reactor volume (V,)) and the feed rate of all liquid streams to the reactor. The concentrations of catalyst components are given as per cent on monomer weight (o.m.w.), based on the monomer charged in the reactor for batch polymerizations and the feed rate of monomer for continuous polymerizations. Results
The hydroperoxide-SO>system reacted in vinyl chloride monomer a t -30OC. Without any nucleophilic agent, the reaction proceeds via the usual ionic path and no poly-
Table II. influence of Nucleophilic Agents on Polymerization Conversion and SO?Copolymerization in 0- Catalytic System at Continuous Bulk Polymerization of Vinyl Chloride by the CHP/SO?/CH 1 -30' C Dwell Time, Min.
YO 0.m.w.
(SO,)/ (CHP)
0.10
1
CHP,
90
Alkoxides .
.
I
NaOCH 120
0.18
2
Mg(0CH 3 ) 2
(CH IO-)/ (SO?)
Conversion,
...
0 7.2 9.8 9.8 4.1 4.7 8.6 10.6 10.6
0.5 1.0 1.5 0.15 0.45 0.85 1.oo 1.30
O h
Sulfonate End Groups, Mole/G
Sulfur, Mole/G
...
... 32.0 x 31.5 x 30.8 x 30.8 x 34.5 x 28.0 x 37.8 x 38.4 x
10 10 10 10 10 10 10 10
A1 130/ A1425
"
'' "
''
'> 'I
"
''
175 x 31.4 x 31.4 x 338 x 281 x 34.4 x 38.2 x 37.4 x
...
10 " 10 ' 10 ' 10 ' 10 '' 10 ' 10 " 10 '
0.28 0.13 0.13 0.54 0.48 0.18 0.13 0.13 0.13
PVC prepared by triethylboron-oxygen catalytic system. Table I l l . Influence of (CH1OSOS )/(CHP) Ratio on Continuous Bulk Polymerization of Vinyl Chloride by the CHP/SOY/Mg(OCH 1) 2 Catalytic System
Polymerization conditions. Temperature, -30'C. Dwell time, 150 min. (CHIO-) / (SO?) = 1.1 CHP,
O h
0.m.w.
0.100
0.145
0.187
(CHiOSO?)/ (CHP)
1.0
1.5 2.5 3.5 1.o 1.5 2.0 2.5 3.5 1.0 1.5 2.5 3.5
Conversion,
6.5 8.5 10.0 9.9 6.9 10.3 10.8 12.2 11.2 8.7 11.9 12.8 12.5
O h
NsP, DI/G
1.16 1.36 1.40 1.38 0.97 1.13 1.16 1.16 1.20 0.80 1.00 1.00 1.OO
merization is detected. With the addition of alcohols, ketones, and ethers, the redox reaction is promoted and substantial quantities of polymer are formed. When weak nucleophilic agents, like ethers and ketones, are used, polymerization yields are low. A set of polymerization results is presented in Table I. Higher conversions were obtained with alcohols. The best yield was obtained by addition of 5% methanol on monomer weight. I n every case analysis showed the presence of a t least one SO, end group per polymer chain. In addition, determination of sulfur content, and the infrared absorption [corresponding to bands at 520, 1130, and 1326 cm -SO?- groups (Bellamy, 1958) 1, showed the presence of substantial amounts of copolymerized SO?(Figure 2 ) . The picture changes if an alkoxide-e.g., NaOCH,is used. Very small quantities of alkoxide are sufficient for the initiation. When the (CH,O-)/(SO?) ratio is a t least 1, the copolymerization of SOr is completely suppressed, since only free SO, can undergo copolymerization. Polymers so prepared do not show any infrared band corresponding to - 4 3 0 2 - groups (Figure 2 ) , and no sulfur is found by elemental analysis other than the amount corresponding to the sulfonate end groups (Table 11). The infrared spectrum shows that the ratio of absorbance (--SO2- groups) to absorbance at 1425 a t 1130 cm cm (CH, bending) reaches the value 0.13 characteristic of PVC free from sulfur-e.g., polymer by the triethylboron-oxygen catalytic system. The polymerization rate, a t constant C H P and SO, concentrations, approaches the maximum when the
Sulfonate End Groups, Mole/G
32.0 x 10 35.2 x 34.2 x 36.8 x 38.3 x 37.1 x
10 ', 10 10 '' lo-" 10 '
40.8 x 10
"
rn"
Sulfonate End Groups, per Polymer Chain
47.000
1.5
36.000
1.38
32.800
1.34
(CH10-)/(S02) ratio is a t least 1, employing either sodium or magnesium methoxide. Analogous results were obtained by substituting tert-butyl hydroperoxide for C H P , or Na and Mg ethoxides for methoxides. At a (CH,O-)/(SO?) ratio = 1, SO, is completely transformed into the salt of methyl sulfurous acid. Salts of alkyl sulfurous acids are, in fact, prepared by reaction of SO?with alcoholic alkoxide solutions. The systematic polymerization study was carried out using a (CH,O---) 1 (SO,) ratio of 1.1 to assure complete neutralization of SOy and avoid its copolymerization. Under this condition. it is of particular interest to note the dependence of the polymerization rate on the (ROSOF) / (CHP) molar ratio. The SO?, or, more exactly, the methyl sulfurous acid salt, shows a positive effect on the conversion up to a (ROSO,)/(CHP) ratio of 1.5 to 2.0 (Table 111). At constant feed of CHP. the increase in polymerization rate. when going from a (ROSO? ) / (CHP) ratio of 1 to a ratio of 1.5 to 2.0, is 1.5- to 1.7-fold, which corresponds to a 2.2- to 2.9-fold increase in initiation rate. The actual (ROSOY)/(CHP) ratio, when methyl sulfite is in excess over C H P , becomes a t steady state in the reactor much higher than in the feed. In fact, as 1 mole of ROSO? is consumed per mole of C H P , the actual ratio becomes [(ROSO, ) - (reacted C H P ) ] / [ ( C H P ) - (reacted C H P ) ] , which is very high when the fraction of reacted C H P approaches unity. This influence on the polymerization rate of the methyl sulfite, in excess of the amount required for stoichiometric reaction with C H P , indicates that the rate-determining step of the radical formation reaction Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970 507
should be the bimolecular reaction between methyl sulfite and the hydroperoxide. On the other hand, when methyl sulfite is in very high excess in the reactor, the increase in polymerization rate must become asymptotic, as the decomposition of the hydroperoxide becomes practically complete. In fact, neither the conversion nor Nsp and the amount of sulfonic end groups vary appreciably, a t constant CHP. when the amount of Mg methyl sulfite is increased beyond a certain limit [a (ROSO? ) / (CHP) ratio in the feed of about 2.01, showing that no higher amount of radicals is available from the methyl sulfitehydroperoxide reaction. The independence of Nsp of the excess of methyl sulfite also rules out any transfer reaction to methvl sulfite itself. The half life of the catalyst should be of the order of minutes. This was demonstrated by carrying out batch polymerizations. If the catalyst) components are added a t once. the conversion to polymer is very low. If the same amount of catalyst is added continuously over 1to 3-hour intervals, conversions comparable to continuous polymerization are obtained. When catalyst addition is discontinued, no further increase in conversion or change in Nsp is detectable. The dependence of polymer N s p on hydroperoxide concentration (Table 111) is a feature of this low temperature polymerization not found in traditional PVC polymerization (Talamini and Peggion, 1967). I t induces the hypothesis that a t low temperature (-30°C or lower) the chain transfer to monomer is low. Again taking into account polymerizations carried out a t a (ROSO, ) / (CHP) ratio over 1.5, where, because of the excess of methyl sulfurous salt the catalyst decomposition can be assumed to be almost complete, the ( m n ) ( C H P ) ' ' product or the (CHP)' '/(end groups mole/gram) ratio is fairly constant, according to the well known rule for polymerizations in the absence of chain transfer. Table IV shows the influence of hydroperoxide concentration and dwell time on conversion and polymer characteristics, and that, for a given catalyst formulation. the
sulfonate end group content decreases as the conversion increases by increasing the dwell time. Accordingly, there is an increase in polymer N s p a t increasing dwell times. The product (conversion) x (sulfonate end groups) within each set of polymerizations (carried out a t increasing dwell times in the absence of transfer agents) is fairly constant. This product is correlated to the amount of primary radicals produced and reacted for a given catalyst concentration. Its constancy is again an indication that the amount of radicals produced does not increase when the dwell time is lengthened. The fraction of catalyst decomposed approaches unity within the variation of the dwell times. PVC polymerizations were also carried out in the presence of transfer agents-namely, mercapto compounds. No adverse effect on polymerization rate was detected, while the expected reduction of polymer Nsp was achieved. PVC polymers obtained by the catalytic system ROSO; / C H P were characterized for sindiotacticity index. glass transition temperature, and dehydrochlorination rate. The sindiotacticity index [determined according to Fordham et al. (1959)l was 2.1 to 2.2 for polymers prepared at -30" C, and 2.4 to 2.5 for polymers prepared a t -5O'Cthat is, in the same range of the polymers obtained by the triethylboron-oxygen catalytic system. T,[determined by differential thermal analysis according to McKinney (196711 was 1000C for polymers obtained a t -3O"C, and 104OC for polymers obtained at -5O"C, as in the case of polymers from triethylboron-oxygen. These values stand against a sindiotacticity index of 1.6 and a T , of 75.C for conventional PVC. The dehydrochlorination rate, at 180°C in a nitrogen stream [according to Geddes (196711, ranged between 0.3 and 0.5 pmole of HCI per gram minute for polymers prepared under selected conditions. These are comparable to the dehydrochlorination rate of polymers prepared by triethylboron-oxygen (0.4 pmole per gram minute) and are lower than conventional PVC as reported by Geddes (1967). The above-described catalytic system was also efficient on other vinyl monomers, over wide temperature ranges.
Table IV. Continuous Bulk Polymerization of Vinyl Chloride by the CHP/SO?/CH,30-
Catalytic System
Polymerization a t various levels of hydroperoxide and different dwell times. Polymerization conditions. Temperature, -30" C. (CHqO-) / 602) = 1.1 CHP,
Dwell Time, Min.
Yo 0.m.w.
120 150 120 150 180 120 150 180 210 120 150 180 210 270 600 600 120 360 480 600
0.100 0.100 0.130 0.130 0.130 0.145 0.145 0.145 0.145 0.187 0.187 0.187 0.187 0.187 0.050 0.100 0.050 0.080 0.080 0.120
508
(CH?OSO;)/ (CHP)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2 2 2 1.5 1.5 1.5
Alkoxide Used
Mg(0CH0 2 Mg(OCH?), Mg(OCHIl2
Mg(OCH,), Mg(OCH,)? Mg(0CH i ) ? Mg(OCH,), Mg(OCH3)J Mg (OCH?I > Mg(OCH,l? Mg(OCH,Iz Mg(0CH312 Mg(0CHi)i Mg(OCH812 Mg(OCH3): Mg(OCH $ 1 2 NaOCH t 6aOCHI NaOCHI NaOCH ,
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 4, 1970
2-Mercaptoethanol, PPM
Conversion, Oh
DI/G
... ... ... ...
9.0 10.0 7.9 11.0 12.3 9.5 12.2 12.8 14.5 11.2 12.8 13.8 15.5 18.5 17.0 23.2 6.3 14.1 18.4 22.6
1.24 1.40 1.17 1.16 1.25 1.10 1.16 1.21 1.30 1.OO 1.00 1.03 1.17 1.24 1.30 1.13 1.70 0.90 1.18 0.90
...
...
... ... ... ... ...
125 200
... 165 240 300
Sulfonote End Groups, MolelG
30 x 10 32 x 10
43.0 x 38.3 x 38.2 x 28.2 x 46.5 x 40.8 x 36.0 x 35.2 x 33.0 x 9.9 x 15.1 x 25.0 x 22.0 x 14.1 x 16.5 x
" "
10 10 " 10 ' 10 10 " 10 " 10 " 10 " 10 '' 10 " 10 " 10 10 " 10 '' 10 "
where Table V. Polymerization of Vinyl Monomers by the CHP/SO?/Mg( OCH 1) 2 Catalytic System
F
Catalvst formulation. CHP. 0.25'; 0.m.w. SO?, 0.2'r o.m.u. M g ( 0 C H ) . 0.14' 0.m.w. Batch polymerization. Catalyst addition time, 5 hours. Total reaction time, 5 hour5 Monomer
Vinyl acetate Tiny1 formate Acrylonitrile Strvene Acrylarnide (30' in methanol1 ZHydroxvethyl acrylate tw-Butvlaminoethyl methacrylate
Temp, 'C
Conversion, %
-30 -60 -30 -30 50 -30 20 -30 20
22 6 19 23 15.5 21 56.5 27.5 46.5
= feed rate of all liquid streams to reactor, volume per unit time F = output rate of the liquid fraction a t overflow from reactor. same units (C) = hydroperoxide concentration in liquid feed (C) = hydroperoxide concentration in reactor (or in reactor overflow) (S) = concentration of compound C H , 0 S 0 2 in reactor (or in reactor overflow) V = volume occupied by liquid phase in reactor
At sufficient dwell time and (ROSO;)/(CHP) molar ratios, F(C) is negligible if compared t o F,(C), and K,(C)(S) V . As pointed out previously, the catalyst decomposition approaches its completion. As an expression for ( C ) it can therefore be assumed that
(C) = F,(C),/Kd(S)V
(6)
The balance for the monomer is Examples are given in Table V for vinyl acetate, vinyl formate. acrylonitrile, styrene, acrylamide, 2-hydroxyethyl acrylate. and tert- butylaminoethyl methacrylate. Discussion
A kinetic investigation was conducted to determine the correlation among catalyst formulation, dwell time. and polymerization rate in continuous bulk polymerization of vinyl chloride. Continuous polymerization was the object of this investigation not only for a prominent commercial interest. but also because, being a steady-state operation, it allows a much simpler mathematical treatment for the understanding of the polymerization kinetics. The following reactions and equations are considered representative of the entire polymerization process. PRODUCTION OF RADICALS
ROOH
+ CHtOSOi
4
CH;OH + RO.
+
-SO,*
and
d ( R 4 d t = 2Kd (ROOH)(CH.jOSOF)
(1)
where K,! = velocity constant for the reaction of radical production. INITIATION O F POLYMERIZATION
(2)
PROPAGATION
+M
--i
(3)
where K,, = velocity constant for the propagation reaction.
TERMINATION 4
and
(4)
where K . = velocity constant for the combination reaction. Under stationary conditions (input = output + reacted amount). the balance for the catalyst will be (cf. Equation
1)
F,,(C).,= F ( C ) + Kd(C)(S)V
c =
K,(M.)V/F,
(9)
The balance for ( M a ) is
2fKd(C)(S)V = F ( M . )+ 2K,(M.)?V
(10)
where f is the efficiency of initiating radicals-i.e., the fraction of radicals giving place to polymer chain initiation. The term 2 f K d ( C )(S)V, according to Equation 6, can be assumed as equal to 2f(C),F,. 2 K , ( M . ) 2 V is equal to twice the number of macromolecules formed per unit time. Both can be experimentally estimated. Under the experimental conditions of the set of results considered. F ( M . ) appears t o be negligible if compared to 2 f K d ( C )(S) V and 2 K , ( M .) * V. Equation 10 then becomes
2 K , ( M . ) ?= 2fKd(C)(S)
(11) (12)
The conversion, combining Equations 9 and 12, can be expressed as
c =
(K,/K,' ') f'
'(QO1
' V ' 'Fq-'
(13)
(51
(14)
thus, V,/ F , being the conventional dwell time, Q , Equation 13 can be finally written as
C / ( 1 - C ) ' ' = ( K p l K ~f"(C)d'Q" *)
P
- d ( M * ) , ! d=t 2 K , ( M * ) '
(8)
v = V,(1 - e )
and
M * + Me
(F, - F)/Fo
c =
or, from Equation 7,
For conversions not exceeding about 20%, V can be assumed as
M.
- d ( M ) i d t = K , ( M * )( M )
( M ) ,(monomer concentration in feed) and ( M ) (monomer concentration in liquid phase of overflow) being equal for a bulk polymerization, the monomer conversion can be expressed as
2 K r ( M * I= 2 ZfF,(C),/V
where K,, = velocity constant for monomer addition to primary radicals.
M.
(7)
or, substituting the value of ( C ) of Equation 6 ,
R*+M-M* where M is vinyl chloride monomer, and d ( M * ) / d t = K , ( R * )( M )
F , ( M ) , = F ( M ) + K , ( M * )( M )V
(15)
In other words, the conversion is proportional t o the square root of the hydroperoxide concentration and the dwell time. The good agreement of the experimental results with the relationship of Equation 15, as shown in Figure 3, confirms the validity of the assumption introduced-i.e., that catalyst decomposition is almost complete in the reactor, a t least within the range of polymerization condiInd. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 4, 1970
509
Nucleophilic agent oMg(OCH,),
Chain transfer agent
-
HO-CHzCHiSH HO-CHiCHiSH
-
Nucleophilic agent o Mg(OCH,),
Chain transfer agent -
e
41
HO-CH,-CH,-SH
A
NaOCH,
A
@
/
A
loo.lo6
50.10-6 (C),
1ooo.c
mole/g
Figure 3. Conversion as a function of catalyst concentration and dwell time
Figure 4. Sulfonate end groups content as a function of catalyst concentration and monomer conversion
tions taken into account. All the values plotted were in fact calculated from experimental results taken from Table I V a t a (CH,OSO;)/(CHP) ratio of 1.5, as assumed for Equation 6 to be valid, and they include polymerizations carried out with either sodium or magnesium methoxide, together with results obtained in the presence of mercapto compounds. The latter results demonstrate that the mercapto compound does not interfere with the catalytic system, and that the radicals formed in the transfer process display approximately the same activity as the original chain radicals. The slight deflection from a linear relationship between c / ( l - e ) ' ' and [(C),Q]' ' in Figure 3 could be suppressed by using a power of 0.59 for (C),Q. This deflection can be attributed to the accelerating effect of the solid polymer phase on the polymerization rate. The efficiency, f - , of -SOs. radicals in initiating polymerization can be calculated, again on the assumption that the catalyst decomposition is nearly complete during the polymerization. If so, the absolute amount of sulfonate end groups originated by initiation by the -SO,.radical in a unit. time is equal to fs(C),F,. As in the same unit time the amount of polymerized monomer is F,, - F , the amount of sulfonate end groups (SEG) relative to the formed polymer is
polymer chain found in the absence of chain transfer agents, which is on the average higher than 1 (Table 111), it is inferred that the organic radical has an efficiency lower than the SOi. radical. The average efficiency for both types of radicals is estimated, on the basis of these results, as 0.33.
SEG = f,F,(C),/ ( F , - F )
Conclusions
On the grounds of the experimental results, the following mechanism may be proposed for the radical decomposition of the hydroperoxide by SO? and nucleophilic agent.
SO,+ C H 3 0 - d CHqOSOY ROOH
+ CH7OSO-T
-
ROOSOT RO.
+ CH3OH + -SO;. + CHIOH
Or,
(16)
or, the monomer density being ca. 1,
SEG (mole/g) = f5(C),/1000.c
(17)
From Figure 4 a value of 0.46 for is calculated. The efficiency of the organic radicals RO. originated by the hydroperoxide component of the catalytic system cannot be calculated, as it was impossible to identify RO-end groups by common analytical methods. A determination of such end groups by the aid of Clr-traced hydroperoxide is planned. However, based on the amount of SEG per f9
5 10
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 4, 1970
CH
@ - L o . +so CH i
The oxycumyl radical may further decompose into 1-methylstyrene, acetophenone, and cumyl alcohol, or the radical itself, its fragments (CO,., O H . ) , or radicals derived from chain transfer reactions may initiate polymerization. The S O 3 . radical is easily identified as an end group in the polymer chain. The rate-determining step for the whole catalytic reaction appears to be the formation of the complex ( I ) , as indicated by the fact that an asymptotic limit for the polymerization rate is reached, only when the ( C H 3 0 S 0 2) / ( C H P ) ratio is in considerable excess over the stoichiometric ratio of 1. literature Cited
Bellamy, L. J. “Infrared Spectra of Complex Molecules,” 2nd ed., pp. 360-3, Wiley, New York, 1958. Bertolacini, R . J., Barney, J. E., Anal. Chem. 29, 281 (1957). Boccato, G., Rigo, A., Talamini, G., Zilio-Grandi, F., Makromol. Chem. 108, 218 (1967). Bockmann, 0. C., Brit. Plast. 38, 364 (1965). Borsini. G., Nicora, C., Segalini, A. (to MontecatiniEdison), U. S. Patent 3,409,601 (Nov. 5, 1968). Borsini, G., Ragazzini, M., Peron, U. (to MontecatiniEdison), U. S. Patent 3,255,168 (June 7, 1966). Bovey, F. A., Hood, F. P., Anderson, E. W., Kornergay, R. L., J . Phys. Chem 71, 312 (1967). Braun, D., Chem. Ztg. 92, 101 (1968). Brighton. C. A,, “Advances in PVC Compounding and Processing,” p. 11, M. Kaufman, Ed., McLaren, London. 1962. Collison, G. R., Jones, T. T. (to Bakelite Ltd., London), U. S. Patent 3,035,032 (May 15, 1962). Edison S.p.A., Brit. Patent 1,043,026 (Sept. 21, 1966). Fordham, J. W. L., Burleigh, P. H., Sturm, C. L., J . Pol-vm.Sei. 41, 73 (1959). Furukawa, J., Tsuruta, T., J . Polym. Sei. 28, 227 (1958). Furukawa, J., Tsuruta. T., Fueno, T., J . Polym. Sci. 28, 234 (1958).
Garbuglio, C.. Gallinella, E., Chim. Ind. (Milan) 47, 1208 (1965). Garbuglio, C.. Rodella, A., Borsini, G., Gallinella, E.. Chirn.Ind. (Milan) 46, 166 (1964). Geddes, W. C. Eur. Pol.vm. J . 3, 267 (196‘7). Gilman, H., “Organic Chemistry, Advanced Treatise,” 2nd ed., Vol. 11, pp. 1858-9. Wiley, New York, 1948. Huggins, M. L., J . Amer. Chem. Soc. 66, 1991 (1944). Krimm, S., Folt, V. L., Shipman, J. J.. Berens, A. R., J . Po/ym. Sci. A-1, 2621 (1963). Kutsenok, B. E., Kulakova, M. N.. Tinyakova, E. I., Dolgoplosk, B. A., Dokl. Akad. Nauk S S S R 125, 10736 (1959). Man-Made Text. 43, 26 (1966). McKinney. P., Foltz, C., J . Appl. Polym. Sci. 1 1 , 1189, (1967). Mazzolini, C., Tex. Chim. 24, 673 (1968). Montecatini S.p.A., Brit. Patent 944,741 (Dec. 18, 1963). Nakajima, A., Hamada, H., Hayashi, S., Mahromol. Chem. 95, 40 (1966). Pungs, W., Kunststoj‘fe 57, 317 (1967). Reding, F. P., Walter, E. R., Welch, F. J., J . Polym. Sei. 56, 225 (1962). Seubold, F. H., Vaughan, W. E., J . Amer. Chem. Soc. 75, 3790 (1953). Talamini, G., Peggion, E., “Vinyl Polymerization,” Vol. 1, Part 1, p. 332, G. E. Ham, Ed., Marcel Dekker, New York, 1967. Talamini, G., Vidotto, G., Makromol. Chem. 50, 129 (1961). Talamini, G., Vidotto, G., Makromol. Chem. 53, 21 (1962). Talamini, G., Vidotto, G., Makromol. Chem. 100, 48 (1967). Tobolsky, A. V., Mesrobian, R . B., “Organic Peroxides.” pp. 99-100, 117-20, Interscience, New York, 1954. Trautvetter, W., Makromol. Chem. 101, 214 (1967). Vlodavets, M. L., Golbert, K. A., Whirvinskaja, E. J., Perovskaja, N. V., Ternovskaja, I. P., Neftekhim. 5 , 613 (1965). RECEIVED for review November 24, 1969 ACCEPTED June 1. 1970
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No.4, 1970
511
