Makromol. Chem. 194, 1397-1401 (1993) 1397

The role of additives for the improvement of &Tic13 catalyst performance

Luiz Claudio de Santa Maria

Departamento de Fisica e Quimica, Instituto de Ciencias, Escola Federal de Engenharia de Itajubft - MG - Brasil

Fernanda M. B. Coutinho? Claudia Alencar

Instituto de Macromolkculas, Universidade Federal do Rio de Janeiro, P. 0. Box 68525 - Rio de Janeiro, 21945 - RJ - Brasil

(Received May 7, 1992; revised mansucript of June 26, 1992)

SUMMARY: The influence of ethylaluminium compounds (AlEt, and AICIEt,) modified by 2,6-di-

tert-butyl-4-methylphenol (BHT), tributylamine (TBA) and triphenylphosphine (TPP) on the polymerization of propene was investigated using catalysts based on TiCl, modified by internal Lewis bases. W o catalysts were employed in this study, one (Cat. A) was prepared by the reduction of TiCl, complexed with dibutyl ether (DBE) - mole ratio DBE/TiCI, = 0,67 - with AlClEt, and the other one (Cat. B) was prepared in the same way, but a second internal base (ethyl benzoate (EB)) was added in a mole ratio EB/TiCI, = 0,M. Activity and stereospecificity of the catalyst systems were strongly affected by these modified cocatalysts. The role of the modifiers is discussed.

Introduction

After the discovery of the Ziegler-Natta catalyst and its uses in the first commercial poly(propy1ene) plants, enormous advances have been made in terms of engineering innovation, process development and, most important, catalyst technology. The essential catalyst characteristics which allowed the use of these new processes are high selectivity, avoiding removal of atactic polymer and extremely high polymerization activity so that the catalyst residues have not to be removed from the polymer.

Although more than thirty years have passed after the discovery of the Ziegler-Natta catalyst, many aspects concerning these systems are still only poorly explained.

The activity of the TiCl, catalyst for the polymerization of propene is greatly improved when triethylaluminium (AlEt,) is used instead of AlClEt,. So an obvious choice for the cocatalyst in this respect would be AlEt,, but although this cocatalyst presents high power to activate the catalytic centres, its stereoselectivity is invariably poor '). Many research groups have studied the effect of electron donors as external bases, such as ethers and esters, with the aim of improving the stereoselectivity of these systems and some good results have been achieved2).

The objective of this work was to investigate the effects of modified cocatalysts with sterically hindered Lewis bases on stereospecificity and catalyst activity and to try to elucidate their role in the polymerization of propene.

0 1993, Hiithig & Wepf Verlag, Basel CCC 0025-1 16)3/93/$05.00

1398 L. C. de Santa Maria, F. M. B. Coutinho, C. Alencar

Experimental part

inert atmosphere techniques. All manipulations of catalyst components were carried out under dry nitrogen using standard

The sources of materials and their purification procedures were reported in earlier papers 3s ').

Tho catalysts (Cat. A and Cat. B) were synthesized to carry out this study. Cat. A was prepared through the reduction of the TiCI, * DBE complex (mole ratio dibutyl ether (DBE)/TiCI, = 0,67) with diethylaluminium chloride (AICIEt,) in isooctane (2,2,4-trimethylpentane; C,) solution. Cat. B was prepared also through the reduction of the TiCI, * DBE complex (mole ratio DBE/TiCI, = 0,67) with AICIEt, in C, solution but ethyl benzoate (EB) was added as a second internal base before thermal treatment of the catalyst. The details of this synthesis are described in a preview paper ').

The polyn~rrization of propene was carried out in a 1 OOO mL glass reactor equipped with a mechanical stirrer. Determined amounts of co-catalyst solution in C, (AlEt, or AlCIEtd and modfier solution in C, were added to the reactor containing 500 mL of C, under dry N, and the resulting solution was kept for 5 min at 50 "C. Then catalyst suspension (3 mmol of Ti) was added to the reactor and propene was continuously fed, in order to maintain a pressure of 1,13 bar for one hour at 50 "C. The reaction mixture was cooled and the poly(propy1ene) filtered off, washed with hexane and dried. For each polymer sample, the isotacticity index I. I. (070 weight of polymer insoluble in boiling C,) was determined through Soxhlet extraction with C, for 6 h.

Results and discussion

Numerous investigations in the Ziegler-Natta-catalyst field have been conducted with the objective of obtaining high stereospecificity and activity. The patents describe the catalyst synthesis in a general way and although almost all conceivable Lewis bases have been claimed as effective, it is well known that this is not the case.

It is seen in Tab. 1 that in a general way AlEt, modified with 2,6-di-tert-butyl-4- methylphenol (BHT) or tributylamine (TBA) enhanced the stereospecificity of the Cat. A/AlEt, system. It is possible that these compounds have complexed the AlEt,, diminishing the content of dimer (Al,Et,) and thus increasing the selectivity of the catalyst system. It is also observed (Tab. 1) that in some cases the catalyst stereospecifi- city decreases when the mole ratio modifier/AIEt, increased (above a mole ratio BHT/AlEt, = 1,5 with a mole ratio Al/Ti = 2 and above a mole ratio TBA/AlEt, =

1,O). In this case, possibly the free modifier (non-complexed with AlEt,) has exposed non-selective catalyst sites, thereby increasing the atactic polymer content. As can also be seen in Tab. 1 , the catalyst activity at the determined mole ratio modifier/AlEt, was higher than without the modifier (PO1 Tab. 1). This increase is probably due to the decrease in the reduction power of AlEt,. Above that value of the mole ratio, the free modifier has poisoned the catalyst centres, diminishing thereby the catalyst activity.

Tab. 2 shows that the cocatalyst modified with BHT up to a mole ratio BHT/ AlClEt, = 1 ,O increased the catalyst activity in case a mole ratio Al/Ti = 2 was used and decreased it for a mole ratio AVTi equal to 5. It is possible that at a mole ratio AVTi = 5 , a greater amount of free modifier was present which poisoned the catalyst sites, consequently decreasing the catalyst activity. It was also observed that the use of the modified cocatalyst resulted in a decrease of the stereospecificity of the catalyst.

As can be seen from Tab. 3, the modifier (BHT and triphenylphosphine (TPP)) had a strong influence on the catalyst activity. The use of a modified cocatalyst enhanced the activity of the Cat. B/AlEt, catalyst system. This behaviour is probably due to the

The role of additives for the improvement of &-Tic], catalyst performance 1399

Tab. 1. Influence of the modifier on activity and stereospecificity of the catalyst system Cat. A/AIEt,

Polymerization w p e ofa) Mole ratio Activity b, I. I. c) number modifier modifier/AIEt,

PO 1 - - 321 95

PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9

BHT i 0,5 d,

1 3 d, 0,5 e,

lJe)

3,0e)

1,Od)

1 ,oe)

2,oe)

357 278 142 344 359 277 178 168

95 94 96 96 97 95 95 92

P10 0,5 e, 396 96 P11 } TBA 1,oe) 536 96

a) BHT 2,6-Di-tert-butyl-4-methylphenol; TBA: tributylamine. b, Measured in g of poly(propy1ene) per g of Ti per hour. ') I. I.: Isotacticity index (wt.-Yo), determined by means of extraction with boiling isooctane

(2,2,4-trimethylpentane). d, Mole ratio AI/Ti = 1. e, Mole ratio AI/Ti = 2.

P12 1,5e) 134 93

Tab. 2. catalyst system Cat. A/AICIEt,

Influence of mole ratio modifier/cocatalyst on the activity and stereospecificity of the

Polymerization Mole ratio Activity b, I. I. '1 number BHT/AICIEt, a) (YO)

P13 P14 P15 P16 P17 P18 P19 P20 P2 1 P22 P23

129 224 130 138 166 181 101 96 95 88 83

97 97 96 96 97 98 96 96 96 96 96

a) BHT 2,6-Di-tert-butyl-4-methylphenol. b, Measured in g of poly(propy1ene) per g of Ti per hour. ') I. I.: Isotacticity index (wt.-Yo), determined by means of extraction with boiling isooctane

(2,2,4-trimethylpentaane). d, Mole ratio AI/Ti = 2.

Mole ratio AI/Ti = 5 .

1400 L. C. de Santa Maria, F. M. B. Coutinho, C. Alencar

decrease in the reduction power of AIEt, and the dimer content (A1,Etd. The best values of the mole ratios TPP/AIEt, and BHTIAIEt, were different, probably due to their different basicity. Stereocontrol was only little affected by the modified co- catalyst. Generally the stereospecificity increased when modified cocatalysts were

Tab. 3. Influence of the modifier on activity and stereospecificity of the catalyst system Cat. B/AlEt,

~~~ ~

Polymerization Type of a) Mole ratio Activity b, I. I. number modifier modifier/AlEt,

- d) 342 95 P24 - - e) 312 96 P25 -

P26 0,5 d, 376 91 1,Od) 410 96 1 3 d, 504 97 P27 P28 1 TPP

P29 2,Od) 360 91

} BHT 1,oe) 468 96 P30 P31 P32 1 3 e, 229 96

0,5 ') 41 1 94

TPP: ltiphenylphosphine; BH'I! 2,6-di-tert-butyl-4-methylphenol. Measured in g of poly(propy1ene) per g of Ti per hour. I. I.: Isotacticity index (wt.-To), determined by means of extraction with boiling isooctane (2,2,4-trimethylpentane).

d, Mole ratio AVTi = 2. ') Mole ratio Al/Ti = 1.

Tab. 4. Influence of the modifier on activity and stereospecificity of the catalyst system Cat. B/AlCIEt,

Polymerization Type of a) Mole ratio Activity b, I. I. c,

number modifier modifier/AlClEt,

P33 - - d) 214 91 P34 - - e) 233 97

0,5 e, 234 97 1 ,oe) 268 97 1 3 e, 202 97

P35 P36 P37

P38 0,5 d, 259 97 P39 } BHT 1,Od) 260 98 P40 1,5d) 181 91

a) T P P Triphenylphosphine; BHT 2,6-di-tert-butyl-4-methylphenol. b, Measured in g of poly(propy1ene) per g of Ti per hour. ') I. I.: Isotacticity index (wt.-To), determined by means of extraction with boiling isooctane

(2,2,4-trimethylpentane). d, Mole ratio Al/Ti = 5. e, Mole ratio AVTi = 2.

} TPP

The role of additives for the improvement of 6-TiC13 catalyst performance 1401

employed; thus it seems that the modifiers have made the system more selective than without them (P24 and P25, Thb. 3).

'Rib. 4 shows that catalyst activity increased when TPP was used as modifier of the cocatalyst at a mole ratio AI/Ti = 2, but stereospecificity stayed the same. Probably TPP has exposed more active centres, but its action has not been selective, activating also the atactic sites. It can still be seen that BHT diminished the activity of the catalyst system Cat. B/AlCIEt,; it is possible that this modifier poisoned the catalyst centres.

Conclusion

It was observed that triphenylphosphine, tributylamine and 2,6-di-tert-butyl-4- methylphenol employed as sterically hindered modifiers of alkylaluminium improved the performance of Ziegler-Natta catalysts in the polymerization of propene. The influence of these modifiers on the activity and stereospecificity of the catalysts was more pronounced with AIEt, as cocatalyst than with AICIEt,. The activity of the catalyst was enhanced by these modifiers, but there are optimum values AVTi and modifier/Al in each case.

The authors thank the Conselho Nacional de Desenvolvimento Cientrj%co e Teenoldgieo (CNPq), Conselho de Ensino para Graduados e Pesquisa (CEPG/UFRJ), Coordena@o de Aperfeicoamento de Pessoal de Nivel Superior (CAPES) and Polibrasil S. A. for financial support.

') A. D. Caunt, .l Polym. Sci., Part C: Polym. Symp. 4, 49 (1963) *) K. Veselg, J. Ambrbz, R. Vilim, 0. Hamiik, .l Polym. Sci. 55, 25 (1961) 3, F. M. B. Coutinho, L. C. Santa Maria, Eur. Polym. .l 27, 987 (1991) 4, F. M. B. Coutinho, L. C. Santa Maria, Polym. Bull. (Berlin) 26, 535 (1991)