Kinetics of ethylene polymerization over supported TiCl4SiO2 Ziegler catalyst modified with binary metal chlorides

Kinetics of ethylene polymerization over supported TiCIJSiO 2 Ziegler catalyst modified with binary metal chlorides A. Munoz-Escalona,

C. Martinez

a n d J. H i d a l g o

Laboratorio de Polimeros, Centro de Ouimica, Instituto Venezolano de Investigaciones Cient#icas IWC, Apartado 1827, Caracas, Venezuela (Received 8 April 1980; revised 16 June 1980) The polymerization of ethylene over metal chloride-exchanged silica-gel has been studied. Basic, amphoteric and electropositive metals on the surface of the silica-gel were found to promote the polymerization reaction. I.r., e.s.r, and kinetic studies suggest that the active sites were highly dispersed in a solid solution of TiCI 3 and MCI 2 formed from the supporting TiCI 4. The influence of polymerization time, temperature and pressure on the polymerization rate were also investigated.

INTRODUCTION The addition of A1CI3 as solid solution in the crystal lattice of the titanium trichioride, increases its activity significantly1"2. The increase in the activity is. related to the reduction of crystallite sizes by increasing the stacking distortion owing to the isomorphous substitution of titanium atoms for aluminium cations. The synthesis of solid solutions containing TiCi 3 as a component has been described in the literature; a broad range of preparative methods has been reported, e.g. reduction of TiCI3.0.33AICi3 crystals by dry milling with A1CI33 or with organomagnesium halides 4. Similarly, TiCI4 has been reduced under a variety of conditions including the use of Grignard reagents, either in the presence or absence of a support 5'6, by use of the binary chlorides of highly electropositive metals, e.g. MgCI 2 and MnCI27 and also by co-crystallization with transition metal carbonyis in low valency states 8'9. Reduction of TiCI 4 to TiCI a can be achieved by the latter procedure, resulting in a stoichiometrically well-defined solid solution with the general formula TiCI3.MCIx, which shows high activity for ethylene polymerization. We have synthesized high active Ziegler Natta catalysts by supporting on SiO: metal halides which have been found to be excellent carriers for Ti chlorides, i.e. MgCl 2. Using this procedure we expected to find higher catalytic activity by dispersing the active centres on the carrier surface ~°. Recently, we expanded the number of metal chlorides to be supported by selecting those metals showing basic or amphoteric properties and also highly electropositive ionic character, such as: BaCI 2, CaCI 2, SrCI2, ZnCI2, etc. 11. It was inferred that the promoter effect of those metals may be accounted for by the increase in the surface acidity of the catalyst and the chemisorption of the olefin onto the catalyst surface, and also by the reduction of the TiCI~ to the active TiCl a under mild conditions and lastly by increasing the stability of the active sites a2. The object here was to investigate the kinetics of the ethylene polymerization over TiCl4/SiO 2 supported Ziegler catalyst modified with metal chlorides. The results 0032-3861/81/0811184)6502.00 ©1981 IPC Business Press

1118

POLYMER, 1981, Vo122, August

found for BaCl 2 were selected as they showed good promoter effects. The study was directed towards the determination of kinetic parameters which govern the formation of the polymer over the catalyst. The values obtained are compared with those found in the literature for TiCl a. Although, in the case of heterogeneous ZieglerNatta catalyst, kinetic studies are of limited value in an attempt to establish polymerization mechanisms, they are of inestimable help to elucidate certain questions concerning the mechanism of polymerization. Finally, attempts were made to characterize the active sites on the catalyst. EXPERIMENTAL

Materials Polymerization grade ethylene supplied by Matheson (U.S.A.) having a purity of 99.5~o was purified by passing through a series of columns containing BASF (W. Germany) activated R3-11 catalyst, KOH, Linde molecular sieve 4 ~, P 2 0 5 and aluminium triethyl in n-heptane (l:l). The nitrogen was also treated in the same way. Aluminium diethyl chloride obtained from Ethyl Corporation (U.S.A.) and TiCl 4 from Merck (Darmstadt, W. Germany) were used as received. The n-heptane used as solvent for the reaction was firstly dried with sodium wire and then distilled over AlLiH4 in N 2 stream. After this treatment the purity was greater than 99.9~o. Grace Davison (U.S.A.)commercial silica-gel grade 951 having a specific surface of about 429 mZg-t and containing 99.5 weight ~o of SiO 2 and with the main impurity consisting of A1203 (0.15 wt~o) was chosen as the support. The BaC12 (99 Wt~o) from Merck (Darmstadt, W. Germany) was employed as the representative metal halide to modify the catalyst system t ~and was used without purification before being supported on the silica-gel. Catalyst preparation and characterization The supports containing CaC12, BaCI2, SrC12, ZnC12, were prepared by previous treatment of the silica-gel at different temperatures under N2 flow for 2 h followed by

Kinetics of ethylene polymerization." A. Munoz-Escalona et al. 00

This injection was taken as the zero time of the polymerization. The ethylene was conducted into the reaction medium, via a pressure sensitive regulator and bubbled into the solvent through a dip tube gas inlet. The polymerization was followed at constant ethylene pressure by measuring the amount of ethylene consumed during the reaction and the difference in weight recorded as time elapsed. At the end of the polymerization the reactor was depressurized, the catalyst destroyed by adding a 10~o mixture of HC1 in methanol and the polymer washed and oven dried. The reproducibility of the kinetic data was determined being within _ 10~o.

o

c

0.1C

B

~u 0 2 o

A

cO

Polymer characterization The intrinsic viscosity of the polyethylene samples was measured at 135+_0.05°C in decalin using standard procedures with Ubbelohde viscosimeters. The errors of the values of ]~/[were within _+ 10~o. The viscosity average molecular weights were calculated from the intrinsic viscosities using the relationship14:

0.30

040 0"50 0'60

Ir/[ =6.2 × lO-4.191,.°V°(dl/g)

(1)

0"70

[

4000

I

I

I

~

I

3 500

I

I

i

I

I

3000

(cm-I) Figure I

Infra-red spectra of SiO 2 after different treatments: (A) as received; (B) after heating at 120°C during 2 h in N2; (C) after heating at 150°C, 2 h; (D) after mixing with aqueous solution of BaCI 2

impregnation with a 25 Wt~o aqueous solution e,g. BaCI 2 at 6 0 C for 4 h at stirring speed of 80 rotations m i n - 1 After the impregnation was complete the excess solution was removed by filtering and dried at 150°C for 2 h under N 2 flow. The Ti metal was supported by reaction of 15 cm 3 ofTiCl 4 with 1 g of carrier (SiO2/BaClz) at 5 0 C for 24 h at stirring speed of 80 rotations rain- 1 under N 2. The excess TiC14 was removed by vacuum distillation at 60 C. The chemical composition of the catalyst was determined by a colorimetric method using a PerkinElmer Model 124 spectrophotometer for Ti contents in the form of its peroxide and by atomic absorption techniques using a Varian Techtron Model 63 for the other metals.

Polymerization procedure The kinetics of polymerization of ethylene were studied in a system previously described in the literature 13. Careful precautions were taken to ensure anaerobic and anydrous conditions for all polymerization experiments. The reaction was carried out in a batch using a one-litre propeller stirred glass autoclave (Bfichi, Zurich, Schweiz) which allowed direct observation of the stirring efficiency during polymerization. A stirrer speed of 1200 rotations rain-1 was used. The temperature in the reactor was controlled by water, pumped from a thermostat bath through the jacket. The solvent (n-heptane) was saturated with ethylene at the selected reaction temperature and pressure before addition of the mixture of the Ti supported catalyst and the co-catalyst metal alkyl (AIEtzC1). Both catalyst components were previously aged for 3 rain before injection into the reactor by overpressure of N 2.

l.r. and e.s.r, spectroscopy l.r. and e.s.r, measurements were carried out in an attempt to determine the nature of the active centres. All measurements were recorded at room temperature. I.r. spectra were recorded on a 557 double-beam PerkinElmer spectrophotometer using the nujol technique for sample preparation. E.s.r. spectra were recorded on a Varian V-4502 spectrometer, using the signal of TiCI 3 crystals grade AA from Stauffer Chemicals (U.S.A.) for calibration. The signal appeared at 3255.95 Gauss giving g = 1.98. RESULTS AND DISCUSSION

Nature of the actit, e centres The synthesis of supported Ziegler catalysts is based mainly on the interaction of an active transition metal compound with the surface groups of suitable carriers, e.g. TiCI 4 with the surface hydroxyl groups of the silica 16-18 The catalytic activity of the supported catalysts can, therefore, be changed by using different inorganic or organic substances as carriers, e.g. Mg(OH)CI, ethylenevinyl alcohol copolymers 19, or by changing the properties of the carrier surface introducing new functional groups. Thus, when Davison 951 silica-gel was mixed with the aqueous solution of BaC12, exchange of the Ba: + ions takes place with the weakly acidic silanol groups of the silica, as inferred from the spectra of Figure 1, recorded in the region 380(Y 3000 cm- 1. Spectrum (A) corresponds to the silica as received. The band at 3750 cm- 1 is related to the isolated silanol groups, while the broad band appeared between 3600 and 3400 c m - 1, due to the weakly resolved frequencies in this region, is attributed to the weakly and strongly hydrogen-bonded hydroxyl groups 21. Spectrum (B) was recorded after heating at 120C during 2 h under N 2 atmosphere, resulting in a better resolved band of the isolated silanol groups. By increasing the heating temperature to 150cC a decrease of the peak at 3750 c m - 1 resulted, due to that, the number of the isolated hydroxyl groups declines. Spectrum (D) corresponds to the silica after it has been mixed with a 25'~ aqueous solution of BaC12 at 60°C for 4 h followed

POLYMER, 1981, Vo122, August

1119

Kinetics of ethylene polymerization: A. Muhoz-Escalona et al.

B

___j

C

D

Figure 2

Variation of the form of e.s.r, signals with the ratio ITil/IBal supported on the surface of SiO2: (A) Stauffer TiCI3, reference; (B) 4 : 1 ; ( C ) 3:1; (D) 1:1.5

as

by the same treatment as (C). The absorption peaks disappearance can be explained by the cationic exchange of the silanol groups with the Ba / + ions as described in the literature 20,2a. Ba z÷ + 2(_= S i O H ) ~ ( = SiO)-zBa 2+ + 2 H +

(2)

This assumption is favoured, because the impregnation proceeded the value of the pH of the solution changes from 6.5 to 3.7. An ion exchange of ~100~o can be attained under these conditions. Furthermore, it is noteworthy that the silica acquires a stronger ion character thus increasing considerably its surface acidity 12. E.s.r. spectroscopy was used in addition to study the possible effects of the supported Ba metal on the SiO 2 after treatment with TiCI4. This technique is useful in detecting the formation of paramagnetic species after the reaction of TiCI 4 with the SiO2 and with the SiO 2 modified with BaCI 2. When SiO 2 was employed as carrier, only a broad signal, depending on the pretreatment temperature of the silica ~1, was observed at room temperature after the addition of TiCI 4. However, by using the modified SiO 2 containing Ba metal a characteristic paramagnetic spectrum similar to those obtained with Stauffer grade AA TiCI 3 was clearly found, after the reaction with TiCI 4 at room temperature and before any alkyl compound was added (Figure 2). This result indicates that the supported Ba reduces the Ti(IV) ions to the active Ti(III) probably giving rise to solid solutions of both halide metals highly dispersed on the silica surface, as described by the following equation: ( -= SiO)~ Ba 2 + + TiC14 ~ ( = SiO)2(TiCI 3 x BaClz) (3) This reaction is accompanied by a colour change from

1120

POLYMER, 1981, Vol 22, August

pale yellow to brown. Similar results were found using CaCI 2, SrCI 2 and ZnCI2 T M When the supported systems loaded with the metal chlorides were combined with Et2AICI a highly active supported Ziegler catalyst for ethylene polymerization was obtained. This catalyst might be considered to be similar to those reported by Short and Shoka123, Hewett 24 and Greco et al. 8'9, involving either the direct use of a binary metal halide as carrier or the synthesis of a solid solution of the TiCI 3 with binary chloride by reducing selectively TiCI 4 to TiCI 3 by means of low valence transition metal carbonyls. Here, however, it was intended to support species similar in nature on an appropriate carrier such as SiO2. A plausible explanation for the catalytic activity enhancement by the introduction of metal halides into TiCI 3 crystals 25 or other transition metal halides is difficult. In fact, it is part of a more general theory of the coordinative catalysis, which is still not clearly understood. It has been suggested, that, by replacing a certain amount of titanium with other metal ions, in the surface of the crystal, active titanium ions may be formed as a result of their conditioning by the neighbouring metal ions through chloride atom bridges. A subtle balance between both metals is necessary in order to achieve the desired increase in the activity. It is worth noting that the amount of supported Ba influences the form of the e.s.r, signal, giving rise to additional peaks, when the percentage of Ba increases (Figure 2). This observation may be explained by changes in the nature, symmetry and distortion of the crystal field surrounding the active titanium centre, due to their interactions with Ba atoms. This fact, however, complicates the possible correlation that might exist between the catalytic activity and the e.s.r, signal intensity at g = 1.97, as reported by other authors 22.

Polymerization rate At the preliminary experimental conditions (n-heptane = 1200 cm3; IAII/ITiI=20; PC2H = 5 bar; T=50°C; catalytic concentration=0.04 g dm -3 and t i m e = l h,) the overall observed catalyst activity was about 12 kg of polymer per g of Ti. This high yield is within the range reported by Greco et al. 9 for mixed TiCI 3 containing metal chlorides. Due to the high catalyst efficiency the initial polymerization rate (Ro) was difficult to measure with sufficient accuracy. A large reaction took place immediately after the catalyst was introduced into the reactor. Due to the reaction exotherm a large amount of heat was evolved which made it difficult to maintain the reaction temperature constant at a fixed value inside the reactor. Consequently the solubility of the ethylene gas and its mass transfer rate into the solvent changes during the first few minutes (10 to 15) of the reaction. However, during the build up period of the reaction, the polymer yield is so high that the polymerization must be stopped since the polymer mass prevents the stirrer from functioning. The general way to avoid these difficulties is by carrying out the polymerization with a reduced amount of catalyst 6. This procedure, however, may give rise to catalyst loss due to the impurity of the reagents, therefore, they must be extremely pure. After the build up period the reaction decays and continues at a constant rate for several hours. Therefore, due to the previous difficulties, only the stationary polymerization rates (R~) could be measured by using low concentrations of catalyst in the range of 5 x 10- 3 g Ti

Kinetics of ethylene polymerization. A. Murioz-Escalona 40

..121 x

20

OI

d

I

0

I

I

1

2b

I0

I

3b

4'0

[AI]/[Ti]

Figure3

DependenceofthestationawrateonthelAII/ITil ratio

15 x 10 - 3 g. Ti/dm3; T = 3 5 ° C ; p = 4 bar; 700 cm 3 n-heptane)

8c x ~_ 6C (3 x

•

•

•

•

A

•

•

u

n

;2"---8

[]

u

u

u

I

5b

I

• []

& []

4C {J 6~ 2 0 v

u

d ©

SiO 2 as catalyst and also with the same cocatalyst for activation. This rather high concentration of cocatalyst, when compared with the conventional Ziegler-Natta catalyst used to obtain satisfactory results, has been related to the necessity of using a more effective scavenger for impurities. The influence of polymerization time, temperature and pressure on the polymerization rate was also investigated. Figure 4 shows a typical result of the effect of polymerization time and temperature on the polymerization rate. The stationary polymerization rate could only be obtained, 15 to 20 min after the polymerization had started, as previously mentioned. The rate increases as the temperature rises and from this temperature dependence the overall activation energy for this period was calculated by using Arrhenius plots (Figure 5). Values between 22.6 and 31 kJ tool - 1 were found, These values agree with those reported in the literature for conventional catalytic systems based on TiCI313. The smaller values correspond to catalysts containing lower ratios of ITil/IBal supported on the surface of the carrier. This fact suggests differences in the nature of the active titanium centre, as pointed out using e.s.r, techniques. The dependence of the stationary rates with pressure can be expressed by a first order rate law and is shown in Figure 6. The same result has also been established for ethylene polymerization using conventional TiCl 3 Ziegler- Natta catalysts 26. on

x u_

L)

e t al.

'0

'

~

2b

'

310 L 410

0---0

+b

J

"70

Time(min)

Figure 4

Kinetic curves at various temperatures: O, 32°C; e, 40°C; E],60°C; &,Co4°C. ( 5 x 1 0 - 3 g T i / d m 3 ; I A I I / I T i l =10;p =4bar; 700 cm 3 n-heptane)

dm -3 (3.2x 10 -5 mol dm-3), i.e. 100 times lower than those generally employed when using conventional Ziegler- Natta catalysts. It should be recalled that the true polymerization rates are much higher than the reported average specific rates, due to the reasons previously mentioned. Haward et al. 4"6 pointed out similar results when studying the polymerization kinetics of ethylene with highly active catalysts, which results in the reduction of TiCI 3 or TiCI 4 with organomagnesium compounds. As the polymerization rate obtained with ZieglerNatta catalysts depends on the concentration of the cocatalyst, the effect of EtzAICI on the catalytic activity was, therefore, studied. The results obtained by plotting the stationary polymerization rate (R++) against the IAli/ITit ratio is given in Figure 3. It can be concluded that using this type of supported Ziegler Natta catalyst a rather high concentration of co-catalyst must be used. Therefore, IAII/ITil ratios between 10-30 were employed. Using these amounts of Et2AICI, stable polymerization rates were obtained, avoiding a large reduction of the Ti to lower and less active oxidation valences. The same value has also been reported by Damyanov et a1.18 for ethylene polymerization using TiCi,+ supported

Rideal and lanqmuir Hinshelwood rate laws The validity of the Rideal and Langmuir Hinshelwood rate laws were tested by studying the dependence of the stationary rate on AIEt/CI concentration. The Riedeal kinetic model is derived from the assumption that the reaction on the catalyst surface takes place between an adsorbed molecule (AIEt2CI) and the ethylene gas in the reaction medium; while the LangmuirHinshelwood model is derived from the consideration that the reaction occurs between the two adsorbed species, AIEteCI and ethylene.

A

c

4

x x

q

'

3! 0

'

3!I

'

312 I

'

3!3

'

3',4 ' 3! 5

'

3'6

( K-Ix 10-~) T

Figure 5

Arrhenius plots of the stationary rate for different I A I I / I T i l ratios: &, 10;O, 15;O, 23; A, 30. Conditions as given for Figure 4

POLYMER,

1981,Vol

22, August

1121

Kinetics of ethylene polymerization." A. Munoz-Escalona et al.

/

Ziegler-Natta catalysts.

Stationary state molecular weight of polymers

200

The molecular weight of polyethylene formed during stationary state polymerization has been given as26:

A t-

g: 160 x

.,.,, d" I

1 kl + K u P KAIk]/2IAIz/2 M~ kv~ + kvP

120

~8 8o 40

0

/ I

I

2

I

4

I

I

6

I 8 ' Pressure (bor)

1E)

'

12

I

I

14

Figure 6 Pressure dependence of the stationary rate (5 x 10- 3 g T i/din3; IAII/ITil = 30; T = 45 ° C; 700 cm 3 n-heptane)

The linear forms for the equations of both rate laws can be given as26:

(8)

where k, K Mand KAj a r e the rate constants for the scission of the catalyst-polymer bond, scission by ethylene monomer and due to the A1Et2Cl. The term [AI1/2 is due to the fact that the AlEt2Cl exists in form of dimers, therefore, IAIEt2C! I= (kdlAI2E4CI21)1/2 = (kalAI/2)1/2. The existence of the chain transfer processes involving the organo-aluminium compound and the monomer are shown in Figures 9 and 10. The slope of the lines suggests that the monomer has a stronger chain transfer than the AIEt2CI. ACKNOWLEDGEMENT The authors wish to thank the Consejo Nacionai de I

IAI 1 IAI Ro~ - k K AH ¢ Hk '

(4)

1-5

Rideal and

p~-j=(kK.KMH),/2 Ail/21

I+KnHP

+ [ KA 1112

Lk--Eyfinj IAI,

g

(5)

x

1,0 Langmuir-Hinshelwood. Bearing in mind that the Roo is given as g C2H4/g TiCI3 x bar x min and where KA and KM are the adsorption equilibrium constants for the cocatalyst and monomer on the catalyst surface respectively. H is the Henry's constant. The plots of the above equations are given in Figure 7. The results show the applicability of both types of rate laws, although the kinetic model of LangmuirHinshelwood agrees better with the experimental results, indicating that the olefin is adsorbed on the catalyst before it reacts with the active centres. Therefore, by increasing the electropositive character of the catalyst a larger concentration of the adsorbed olefin may result, giving rise to an increase in the polymerization rate. Similar results have been reported for the catalyst system based on TiC13/AIEtzC126. Furthermore, the adsorption heat of AIEt2CI on the catalyst surface can be determined by the plot of IA I/R~ or ([Al/Roo)1/2 against 1/T as shown in Figure 8. As can be seen from equations 4 and 5, from the intercepts and the slopes, the temperature dependence of K A c a n be expressed as:

T_

o

' lb'

I

#o

3b'4o

t

I

I

so

I

io'

o

[AI]/[Ti] ~O

1"5

b

x A E

O

b 1-O

O

g

K~ = KA.oeS'°kJ mol - 1 RT- 1

(6)

for Rideal and

(A = KA.Oe9"6kJ mol - 1 RT- l

T

(7)

for Langmuir-Hinshelwood. These values for the heat of adsorption between 5.0-9.6 kJ mo1-1 are lower relative to those reported in the literature for the catalyst TiCI3/A1EtzCI of 46-54.4 kJ mol-1 2T. This difference may account for the higher concentration of A1Et2CI used in the case of the supported

1122

POLYMER, 1981, Vo122, August

? 05

-6 E

0

' ,8'

I

2o

I

3b' 4b'

5b' 6 b ' 7b

IAI ] / [ T i ] Figure 7 Applicability of: (a) Rideal rate law (equation 4) and (b) Langmuir rate law (equation 5): &, 60°C; O, 50°C; A, 40°C; 0, 35°C. Other conditions as given for Figure 3

Kinetics of ethylene polymerization." A. Muhoz-Escalona et al. -6

a

0"4 0'3 x

_10

-10

----1"

-I=~ 8 o.~ 01

Q.

o

03 E

-7 c

I

D

02

I

O13 [Al(mol I - l ) ] ~/2

04

Figure 9

Relationship between the reciprocals of the viscosityaverage molecular weight and AIEt2CI concentration. Conditions as given for Figure 3

-11

1.0 -8

3-1 (K-Ix10-3)

30

32 -4

b

x -Iz~ 8 o s

-2 5

o -5

C~.

~n

¢_

c_

r

-

o'5

1-3

l's

2!o

2's

1__ P (bar~-I Figure 10 Relationship between the reciprocals of the viscosityaverage molecular weight and the pressure. Conditions as given in Figure 3

-3

7 8 9 10 11

-6 --35;[

I

3! 0

I

31.1

I

31.2

I

I

1

Figure 8

T (K-Ixld3) Arrhenius plots of the slope and intercept for: (a) R ideal

rate law (equation 4) and (b) Langmuir rate law (equation 5). Conditions as given for Figure 3

I n v e s t i g a c i o n e s Cientificas y T e c n o l o g i c a s , C O N I C I T " , V e n e z u e l a , for the p a r t i a l financial s u p p o r t of this w o r k .

12 13 14 15 16 17 18 19

REFERENCES 1 2 3 4 5 6

Stauffer Chemical Company, 'Titanium Trichlorides', Tech. Bull., Stauffer Chem. Co., Westport, Conn. 1962 Wilchinsky, Z. W., Looney, R. W. and Tornqvist, E. G. M. J. Catal. 1973, 28, 352 Tornqvist, E. G. M. Ann. N Y Acad. Sci. 1969, 155, 447 Boucher, D. G., Parsons, 1. W. and Haward, R. N. Makromol. Chem. 1974, 175, 3461 Duck, E. W., Grant, D., Butcher, A. V. and Timms, D. G. Eur. Polym. J. 1974, 10, 77 Haward, R. N., Roper, A. N. and Fletcher, K. L. Polymer 1973,14, 365

20 21 22 23 24 25 26 27

Mitsui Petrochemical, Chem. Eng. 1970, 77, 28; Duck, E. W., Grant, D. and Kronfli, E. Eur. Polym. J. 1979, 15, 625 Greco, A., Perego, G., Cesari, M. and Cesca, S. J. Appl. Polym. Sci. 1979, 23, 1319 Greco, A., Bertolini, G., Bruzzone, M. and Cesca, S. J. Appl. Polym. Sci. 1979, 23, 1333 Mufioz-Escalona, A. and Villalba, J. Polymer 1977, lg, 179 Mufioz-Escalona, A. and Hidalgo, J., to be published Mufioz-Escalona, A., Martin, A. and Hidalgo, J., Proceedings of the 5th International Conference in Cationic and Other Ionic Polymerizations, Kyoto, Japan, April 1980 Mufioz-Escalona, A. and Parada, A. Polymer 1979, 20, 474 Chiang, R. J. Phys. Chem. 1965, 69, 1945 Murray, J., Sharp, J. J. and Hockey, J. A. J. Catal. 1970, 18, 52 Maksimov, N. G., Kushnaseva, E. G., Zakharov, V. A., Anufrienko, V. F., Zhdan, P. A. and Ermakov, Ju. I. Kinet. Kat. (English) 1974, 15, 656 Baulin, A. A., Semenova, A. S., Stefanovich, L. G., Chirkov, N. M. and Stafeyev, A. V. J. polym. Sci. USSR 1974, 16, 3130 Damyanov, D., Velikova, M. and Petrov, L. Eur. Polym. J. 1979, 15, 233 Susuki, T., Izuka, Sh-I., Kondo, S. and Takegami, Y. J. Macromol. Sci. Chem. 1977, All(3), 633 Boem, H. P. Adv. Catal. 1966, 16, 179 Ahrland, S., Grenthe, I. and Noren, B. Acta Chem. Scand. 1960, 14, 1059 Kazauski, V. B. and Turkevich, J. J. Catal. 1967, 8, 231 Short, G. A. and Shokal, E. C. J. Polym. Sci. 1965, B3, 859 Hewett, W. A. J. Polym. Sci. 1965, B3, 855 Tornqvist, E. G. M., Richardson, J. T., Wilchinsky, Z. W. and Looney, R. W. J. Catal. 1967, 8, 189 Keii, T. 'Kinetics of Ziegler Natta Polymerization', Chapman and Hall, London, 1972 Schnecko, H., Reinmoeller, M., Weirauch, K. and Kern, W. J. Polym. Sci. 1963, C2, 71

P O L Y M E R , 1981, Vo122, A u g u s t

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