chain transfer rate. The polymerization activity is primarily determined by the
feasibility of the chain propagation reaction. We considered chain propagation
through the classical Cossee–Arlman mechanism through alkylated trivalent
Cr(III)-alkyl active sites, as depicted in Schemes 18 and 19. The energy barriers
for the chain propagation and chain transfer are listed in Table 5. The calculations
showed that the Cr active sites in Ti-modified models exhibited an increased
electron deficiency, and the corresponding energy barriers of the first ethylene
insertion were 19.2, 18.4, and 17.7 kcal mol
À1 for cluster models 4g, 12g, and
15g, respectively. This indicated that Ti-promotional effects could enhance the
polymerization activity of the Phillips catalyst. The MW and MWD of the polyethylene are known to be determined by the relative rate between chain propagation
and chain transfer. β-H elimination to monomer was believed to be the
predominating chain transfer mode for Phillips catalysts (reaction b in Scheme 18).
Scheme 18 (a) Chain propagation through classical Cossee–Arlman mechanism, (b) Chain
transfer by β-H elimination to monomer
Scheme 19 Regiospecific insertion of 1-butene as comonomer in 1,2-orientation and
2,1-orientation for the Phillips catalyst
Table 5 Energy barriers
a for chain initiation, chain propagation, and chain transfer reactions on
various models of the Phillips catalyst
Model
Chain propagation
b
Chain transfer
c
Δ
d
C 2 H 4 1-C 4 H 8 1-C 6 H 12 C 2 H 4 1-C 4 H 8 1-C 6 H 12 C 2 H 4 1-C 4 H 8 1-C 6 H 12
Ti:Cr ¼ 0:1 19.2 21.7
21.8
25.9 26.1
24.3
6.7
4.4
2.5
Ti:Cr ¼ 1:1 18.4 20.2
21.2
24.7 23.5
23.3
6.3
3.3
2.1
Ti:Cr ¼ 2:1 17.7 19.6
20.3
24.1 21.7
22.1
6.4
2.1
1.8
a
Energy barrier is given in kcal mol
À1
b
Based on primary 1,2-insertion
c
Based on β-H elimination to monomer after 1,2-insertion of the corresponding monomer or
comonomer
d
Energy barrier differences between ethylene insertion of chain propagation and chain transfer
steps, in kcal mol
À1
188
R. Cheng et al.