Theoretical Studies on the Regioselectivity and Stereoselectivity of the Olefin Polymerization Catalyzed by Bridged-metallocene Complexes†

doi:10.7503/cjcu20160271

Abstract

Abstract:

The reaction selectivity of olefin polymerization catalyzed by three different bridged-metallocene complexes[Me₂SiN(Me₄Cp)TiCl₂, Me₂SiCpFluTiCl₂ and Me₂SiInd₂TiCl₂] were studied with density functional theory. The calculation results show that the ligand of the metalloence complex creates special stereo environment which leads to unique catalytic properties. There are two ways for olefin insertion, primary and secondary insertion, in the chain initiation and chain propagation steps of propylene polymerization catalyzed by bridged-metallocene catalyst. The calculation results suggest that bridged-metallocene catalysts show a preference of primary insertion way in chain initiation step of propylene polymerization, and we believe this regioselectivity is resulted from the rigid structure of catalyst. In the subsequent chain growth step, the catalysts start to show their stereoregularity, and the product of polymerization is atac-, syn-, iso-PP, respectively. By studying the interactions of the monomer, the ligand and the growing chain, we revealed the origin of the regioselectivity and stereospecificity of bridged-metallocene catalyst and provide theoretical support for design of tailor made metallocene catalyst.

Key words: Bridged-metallocene complex, Olefin polymerization, Regioselectivity and stereoregularity, Reaction mechanism, Theoretical study

CLC Number:

O643.31

TrendMD:

TANG Yanhui, WANG Zhidong. Theoretical Studies on the Regioselectivity and Stereoselectivity of the Olefin Polymerization Catalyzed by Bridged-metallocene Complexes^†[J]. Chem. J. Chinese Universities, 2016, 37(10): 1849.

Figures/Tables 9

Scheme 1 Three types of bridged-metallocene catalysts and their polypropylene products

Scheme 2 Four different propylene coordination and insertion ways

Scheme 3 Mechanism of propylene polymerization catalyzed by titanium complexes

Fig.1 Sideview of A1(1 of catalyst A) and four olefin coordination modes[(1,2)si, (1,2)re, (2,1)re and (1,2)si]

Fig.2 Energy profiles of the chain initiation step of propylene polymerization catalyzed by catalyst AValues in parentheses are zero-point energies.

Fig.3 Key parameters of geometrical structures and free energies information of transition states(TSs) of four olefin insertion modes in the chain initiation process of propylene polymerization catalyzed by catalyst ABond lengths are in nm, bond angles and dihedral angles in degree, energies are in kJ/mol, values in parentheses are zero-point energies.

Table 1 Free energy information of the chain initiation process of propylene polymerization catalyzed by catalyst A, B and C

Catalyst	Pathway	Free energy/(kJ·mol^-1)			Energy barrier/(kJ·mol^-1)
Catalyst	Pathway	2		TS2-3	3
A	(1,2)_si	-17.6	49.8	-68.2	67.4
	(1,2)_re	-20.1	55.7	-69.9	75.8
	(2,1)_re	-4.6	70.3	-50.6	74.9
	(2,1)_si	-22.2	62.4	-64.9	84.6
B	(1,2)_si	8.8	74.5	-70.3	74.5
	(1,2)_re	11.3	69.1	-64.9	69.1
	(2,1)_re	8.0	98.8	-51.1	98.8
	(2,1)_si	-1.3	82.9	-44.4	84.2
C	(1,2)_si	13.4	73.7	-64.5	73.7
	(1,2)_re	8.8	75.8	-74.9	75.8
	(2,1)_re	NA^*	NA	NA	NA
	(2,1)_si	-7.1	91.3	-46.5	98.4

Fig.4 Free energy profiles of the 2nd and 3rd monomer insertion processes of propylene polymerization catalyzed by catalyst BThe values in parentheses are zero-point energies.

Table 2 Free energy information of the chain propagation process of propylene polymerization catalyzed by catalysts A, B and C

Catalyst	Pathway	ΔG_{2nd propylene}/(kJ·mol^-1)	ΔG_{3rd propylene}/(kJ·mol^-1)
A	re	93.8	75.3
	si	107.6	86.2
B	re	79.1	90.0
	si	85.4	74.9
C	re	78.3	93.3
	si	97.9	110.9

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