Macromolecules 2003, 36, 9067-9074
9067
Butadiene Insertion and Constitutional Units in Ethene Copolymerizations by C2-Symmetric Metallocenes Pasquale Longo,* Mariagrazia Napoli, Stefania Pragliola, Chiara Costabile, Giuseppe Milano, and Gaetano Guerra Dipartimento di Chimica, Universita` di Salerno, I-84081 Baronissi (SA), Italy Received June 18, 2003; Revised Manuscript Received September 22, 2003
ABSTRACT: Copolymerizations of ethylene with 1,3-butadiene in the presence of catalytic systems based on some C2-symmetric zirconocenes (rac-[CH2(3-R-1-indenyl)2]ZrCl2 where R ) -C(CH3)3 (1), -CH(CH3)2 (2), -CH2CH3 (3), -CH3 (4), and -H (5)) are compared. The chemical nature and the relative amount of constitutional comonomer units from butadiene are strongly affected by the bulkiness of the substituent in positions 3 and 3′. DFT calculations indicate that the observed dependence of constitutional units from butadiene on the bulkiness of the alkyl ligand substituent can be easily rationalized by the occurrence of an unusual insertion mechanism for butadiene. This mechanism, involving butadiene η2 primary coordination followed by its primary vinyl insertion, becomes largely predominant for catalytic systems based on the zirconocenes (1-4), which present R g CH3. This kind of butadiene insertion mechanism is also able to account for the high stereoregularity which is observed for adjacent methylene-1,2-cyclopropane units.
Introduction
Scheme 1
It is well-known that, by copolymerization of 1,3butadiene with ethene, by suitable metallocene or constrained geometry catalysts,1-3 together with the usual 1,4- and/or 1,2-butadiene constitutional units, methylene-1,2-cyclopentane units can be obtained. Recently we have shown that copolymerization of 1,3butadiene with ethene, catalyzed by the C2-symmetric metallocene rac-[CH2(3-tert-butyl-1-indenyl)2]ZrCl2 (1), discovered by Resconi and co-workers,4 which presents a large gap aperture associated with large substituents, generally leads to ethene copolymers only containing unprecedented methylene-1,2-cyclopropane units together with methylene-1,2-cyclopentane units, both with high trans selectivity.5 More recently we have also shown that copolymerization of the same monomers with the same catalytic system, when conducted at high temperature and low ethene concentration, can lead to ethene copolymers only containing unprecedented 1,1 and 1,3 constitutional units from 1,3-butadiene, both presenting only the E configuration of the double bond.6 It is generally accepted that the formation of 1,4- and 1,2-butadiene constitutional units would be obtained by reactions involving a η3-allyl intermediate. This would be generally produced by the insertion into the growing polymer chain of an η4 s-cis coordinated butadiene (A in Scheme 1).7 In particular, cis-1,4-butadiene units (C) would be achieved from the anti-η3-coordinated growing chain (B) kinetic product of the insertion of the η4 s-cis coordinated butadiene (A), while trans-1,4-butadiene units (F) would be generally achieved by isomerization reactions from the anti-η3-coordinated growing chain (B), toward the thermodinamically more stable syn-η3coordinated growing chain (E).8 On the basis of experimental and modeling studies,5,6 the formation of 1,2-cyclopentane, 1,2-cyclopropane, and 1,1 and 1,3 constitutional units has been rationalized * Corresponding author. E-mail:
[email protected].
by the reactions sketched in Scheme 2.6 According to this reaction scheme, all these new kinds of butadiene constitutional units would be obtained by rearrangements of a same intermediate (B in Scheme 2) which would be achieved by an unusual primary vinyl insertion of a η2-coordinated butadiene into a growing polymeric chain (σ-bonded to the metal) (A in Scheme 2). It is worth recalling that metallocene based polymerization catalytic systems are generally highly regioselective in favor of primary (or 1,2) propene insertion.9 In particular, the regioselectivity of syndiospecific or aspecific systems is nearly complete,10 while substantial amounts of regioirregularities are obtained for isospe-
10.1021/ma034830j CCC: $25.00 © 2003 American Chemical Society Published on Web 11/06/2003
9068 Longo et al.
Macromolecules, Vol. 36, No. 24, 2003
Table 1. Copolymerizations of Ethylene-1,3-Butadiene in the Presence of rac-[CH2(3-R-1-indenyl)2]ZrCl2
runa
catalyst
[B]
[E]/[B]
1 2 3 4 5
1 2 3 4 5
0.52 0.51 0.51 0.52 0.50
0.31 0.32 0.32 0.31 0.32
6 7 8 9 10
1 2 3 4 5
0.51 0.52 0.51 0.52 0.53
0.21 0.20 0.21 0.20 0.20
time (h) 16 5 17 16 16 1.5 1.8 1.5 3.0 1.5
yield (g)
T (°C)
XBb
fB∆c
fB0.99 >0.99 0.39
2.0 1.5 1.5 3.1 1.6
50 50 50 50 50
0.04 0.06 0.08 0.05 0.06
0.14 0.03 0.02 0.03
0.04 0.26 0.17 0.89 0.53
fB1,1c
fB1,3c
0.08
0.10
0.61 0.42 0.50 0.02
0.20 0.28 0.28 0.02
fB1,4c
Tmd (°C)
0.61
112 81 117 118 119
0.01 0.03 0.03 0.47
114 83 80 112 118
a Polymerizations were carried out by using 1 × 10-5 mol of zirconocene and 1 × 10-2 mol of MAO (based on Al). b Molar fraction of butadiene units in the copolymer chains. c Fraction of butadiene leading to cyclopropane ring units (fB∆), cyclopentane ring units (fB
