1. A theoretical investigation to evaluate structural and conformational effects of ligands in transition metal complexes useful for the preparation of ultra-high molecular weight poly(ethylene)s
K. Patela, K. Vankab, S. Chikkalia*, S. Sivaramc
aPolymer Science & Engineering Division, bPhysical & Materials Chemistry Division, CSIR-National Chemical Laboratory,
cIndian Institute of Science Education Dr. Homi Bhabha Road, Pune India , * id:
Background
Results
UHMW-PE can be defined as linear polyethylenes with molecular weight greater than ~2-3 million g/mol.
Difficult to prepare such PE’s
Ring inversion (axial/equatorial as function of substituent)
Energy difference between axial and equatorial
conformers vary with the nature of substituent group,
and with sterically more demanding substituent
enforce the conformation exclusively to the equatorial
position in case of ligand and complexes .
Kinetic chain length
kp>>>>>>>>>>>> ktr or kt
Inherent transfer reaction limit Mw
Catalyst (β-H) (Nature of Transition metal)
Co-catalyst (Organoaluminium)
Features of known catalyst
Metallocene
Ti >>Zr >>Hf
Fluxional behavior (Possible structural isomers)
Di Cp > mono Cp
Bulky substituent on cyclohexyl ring restrict fluxional
behaviour, and stabilized Oh complexes in C2 cis-imine
symmetry
di Cp
mono Cp
Bite angle in di Cp > mono Cp
Increased bite angle can reduces β-H transfer
Di Cp produces higher MW than mono Cp (open framework)
Schiff Base Complexes
Ligand effect (Steric and electronic)
Buried volume (%Vbur)
Objectives
%VBur gives a measure of the space occupied by the ligand in the first coordination sphere of the metal.
To investigate the role of steric and/or conformational effect of the ligand on kp/ktr for ethylene
polymerization in a Schiff base metal complexes.
The following structural variants were chosen for study.
Complex (substituent X on Cy) H Me
tBu Ph
Buried volume (% VBar)
69.1
70.8
71.3
70.6
Steric mapping H Me
tBu Ph
3.00
1.50
Steric bulk
Increase Ti Ti Ti Ti
0.00
-1.50 H Me Et Pr
iPr Bu
tBu Ph
-3.00
To estimate ground state energies, symmetry and geometry, electronic and steric effects of given ligand framework and their metal complexes.
Conclusions
Rank of metal complexes in the order of increasing steric crowding around metal centre
Methods
The gometry optimizations were conducted employing DFT with the Turbomole 7.0 (basis set TZVP)
suite of programs with Perdew, Burke, and Ernzerhof (PBE)9 function The resolution of identity (RI)
along with the multipole accelerated resolution of identity (marij) approximations was employed for an
accurate and efficient treatment of the electronic Coulomb term.
Buried volume (%VBur) was calculated using SambVca 2. (A Web Tool for Analyzing Catalytic Pockets
with Topographic Steric Maps) H Ph Me Et Pr
iPr
tBu
Increasing steric crowding
Increasing rate of kp/ktr
Experimentally Zr complexes with phenoxy-imine ligande framework having 2-methyl cyclohexyl group found advantageous in obtaining higher MW as compared to cyclohexyl. group. .
Acknowledgement KP gratefully acknowledges financial support from DST India Young Scientist Scheme. SHC acknowledges DST
References: 1. S. M. Kurtz, The UHMWPE Handbook.; Elsevier Academic Press: New York, 2004
2. H. H. Brintzinger, et al, Angew. Chem. Int. Ed. Engl. 34, 1143, 1995.
3. H. Makio, H. Terao, A. Iwashita, T. Fujita., Chem. Rev. 111, 2363, 2011