Macromolecules zyxwvut 1990,23, zyxwvu 383-386 383 Stereochemical Investigation of the Initiation Step in MgC1,-Supported Ziegler-Natta Catalysts. The Lewis Base Activation Effect Maria Carmela Sacchi,' Chengji Shan, Paolo Locatelli, and Incoronata Tritto Zstituto di Chimica delle Macromolecole del CNR, via E. Bassini, 15, 20133 Milano, Italy. Received February 15, 1989; Revised Manuscript Received April 28, 1989 ABSTRACT: The 13C NMR analysis of the initiation step is applied to the study of the effect of the Lewis bases on high-yield supported Ziegler-Natta catalysts for isotactic propene polymerization. Catalytic sys- tems containing ethyl benzoate (EB) and bis(2-ethylhexyl) phthalate (BEHP) zyxw as internal bases are studied in the absence of external base and in the presence of 2,2,6,64etramethylpiperidine (TMPip) and phenyl- triethoxysilane (PTS),as external bases. The fact that the increase of isotactic productivity here observed, due to the presence of either internal or the external base, is accompanied by a change of the first step stereoregularity gives evidence of the direct participation of the Lewis base on the formation of the isospe- cific centers. Introduction It is generally accepted that the stereoregulating effect of the Lewis bases in high-yield supported Ziegler- Natta catalysts cannot be reduced to a selective poison- ing of the nonstereospecific centers, since the increase of isotactic productivity, observed in some cases, indicates that there is also an activation of the stereospecific centers.'-' However, up to now, the actual mechanism of this activation and the participation, or nonparticipa- tion, of the Lewis base in the formation of the active cen- ters are still open problems. The purpose of this paper is to report our contribu- tion to the understanding of the mechanism of the Lewis base activation and to give some evidence of the actual presence of the base in the isospecific site environment. Our approach to this problem consists in the study of the effect of the Lewis base, used both as an internal and external base, on the steric structure of isospecific centers of different catalytic systems. The method we use to obtain structural information on the active cen- ters is the investigation, by 13C NMR, of the initiation step in propene polymerization in the presence of the selectively 13C-enriched cocatalyst A1(13CH,CH3)3. Tak- ing into account only monomer insertion on the isotac- tic-specific centers, when polymerization zyxwvuts starts on a metal- ethyl bond, we have two possible stereoisomers of chain end groups: c c c c c I I .-C-CCC-C-C-CC I l l * .-C-C-CC-C-C-CC I C eryl hro threo Erythro (or isotactic) is the stereoisomer in which the two first monomeric units have the same configuration and threo (or syndiotactic) that one in which they have the opposite configuration. The use of l3C-enriched ethyl group (coming from the cocatalyst A1(13CH,CH3)3) makes it possible to detect and distinguish the two stereoiso- mers by I3C NMR.*y9 If e (erythro) and t (threo) are the integrated peak areas of the enriched methylene reso- nances assigned, respectively, to the isotactic and syn- diotactic placements of the first propene unit, the e/t 0024-9297 zyxwvutsr f 90 f 2223-0383$02.50/0 ratio represents the extent of the first-step stereoregu- larity. A previous analysis of the stereochemistry of the ethyl chain end groups obtained in the presence of dif- ferent conventional nonsupported catalytic systems has shown the following: (i) The stereospecificity of the first monomer insertion on the titanium-ethyl bond is always lower than the stereospecificity of the following propa- gation (ii) The extent of the first-step stereo- regularity depends on the bulkiness of the halide and alkyl ligands of the catalytically active titanium and on their mutual interactions.lO~ll(iii) While the stereoreg- ularity within the polymer chains is rather independent on the catalytic system, the extent of the first-step ste- reoregularity varies from one catalyst to another and is a reproducible characteristic of each catalytic system.', On the basis of these findings, we applied this method to the study of the effect of the Lewis bases on the fea- tures of the active centers in supported catalysts. Results and Discussion We studied the effect of different internal bases in the absence of external base (see Table I). Catalyst a is a simple catalyst without any base, cata- lyst b contains ethyl benzoate (EB), and catalyst c con- tains bis(Zethylhexy1) phthalate (BEHP) as internal base. The addition of ZnEt, to the cocatalyst, due to its high chain-transfer ability, reduces the molecular weight of the polymers and increases the concentration of poly- mer chain end groups. In the presence of ZnEt,, due to their lower molecular weight, some of the isotactic chains become heptane soluble. This fact, which produces a change of the fraction distribution (in particular the per- cent of heptane insoluble fraction, usually considered the isotacticity index, appears sensibly lower than in the absence of ZnEt,) and of the chain and chain end stere- oregularity of the ether and heptane soluble fractions, does not sensibly affect the e/t ratio of the heptane insol- uble fractions. Table I shows that in catalysts b and c the presence of the internal base produces a noticeable increase of the isotactic fraction with respect to sample a. We can also see that this increase of the isotactic fraction is accom- panied by a noticeable enhancement of the first-step ste- reoregularity. It is worthwhile to remind that, while both 0 1990 American Chemical Society