Propylene Polymerization and Deactivation Processes with Isoselective {Cp/Flu} Zirconocene Catalysts

Xavier Desert,Alexandre Welle,Jean-François Carpentier,Evgueni Kirillov,Thierry Roisnel,Aurélien Vantomme,Vincent Dorcet,Katty Den Dauw

doi:10.3390/catal11080959

Abstract

Industrially relevant single-site precatalysts used to produce isotactic polypropylene (iPP) include C2-symmetric {SBI} and C1-symmetric {Cp/Flu} complexes of group 4 metals. While the latter can produce iPPs with a higher degree of isotacticity, they also suffer from poor productivity compared to their {SBI} counterparts. Several causes for this trend have been suggested—2,1-Regioinsertions are frequently pointed out, as they are suspected to drive the catalyst into a dormant state. While this event does not seem to significantly impact the productivity of {SBI} systems, the influence of these regioerror is poorly documented for isoselective {Cp/Flu} precatalysts. To address this issue, new Ph2X(Cp)(Flu) (Ph2X = Ph2C, FluC, Ph2Si) proligands (2a–k) and some of the corresponding dichlorozirconocenes (3a–h,k) were synthesized. These new compounds were characterized and tested in homogeneous propylene polymerization at 60 °C and the amounts of regioerrors in the resulting polymers were examined by 13C NMR spectroscopy. A possible correlation between poor productivity and a high number of regioerrors was investigated and is discussed. Furthermore, a C-H activation process in the bulky nBu3C substituent upon activation of 4c (the dimethylated analog of 3c) by B(C6F5)3 has been evidenced by NMR; DFT calculations support this C-H activation as a deactivation mechanism.

Highlights

{SBI}-metallocene catalysts, significant breakthroughs have been achieved in the last decade, and a number of new structures have emerged that exhibit significantly improved performances (Scheme 1)
The complete inactivity of the 3-nBu3 C-Cp substituted 3c and 3d in propylene polymerization was rationalized from NMR spectroscopic studies on a model system incorporating the [4c]+ [MeB(C6 F5 )3 ] ion pair and from DFT calculations
No distinct correlation could be identified far between them nor between the observed productivities and the content of the regioirregular sequences in the obtained polymers

Summary

Introduction

Since the development of isoselective polymerization of propylene with C1 -symmetric cyclopentadienyl-fluorenyl {Cp/Flu}-metallocene catalysts [1,2,3,4], many academic groups have set out to find “the” ideal structure that would produce polypropylene with the highest degree of isotacticity and regiocontrol, well-controlled molecular weight characteristics, and high productivity This performance race has resulted in the synthesis of a vast number of structures with various substitution patterns [5,6,7,8,9], for the purpose of studying the influence of ligand architecture on the above parameters [10,11,12]. Of note are metallocene catalysts A [13,14,15,16] and

Objectives

Results

Conclusion