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Ethylene Polymerization by Palladium/Phosphine Sulfonate Catalysts in the Presence and Absence of Protic Solvents: Structural and Mechanistic Differences Masafumi Kanazawa, Shingo Ito, and Kyoko Nozaki* Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
bS Supporting Information ABSTRACT: Ethylene polymerization catalyzed by palladium(II)/ phosphine sulfonate complexes was performed in the presence and absence of CH3OH or CH3OD. The addition of the protic solvents resulted in a significant decrease of molecular weight and a slight increase of internal alkene chain ends, suggesting that β-H elimination from alkylpalladium intermediates was accelerated, which led to the following alkene dissociation from the Pd center or alkene isomerization. In the presence of CH3OD, linear polyethylenes possessing deuteriums in the main chain were obtained presumably via a β-H elimination, H/D exchange, and reinsertion sequence.
C
oordination insertion copolymerization of ethylene with polar vinyl monomers by a Pd/phosphine sulfonate catalyst has recently been the target of intense research since it enables the synthesis of highly linear polyethylenes possessing functional groups in their main chain.1,2 One interesting feature of the catalytic system is high tolerance toward polar functional groups, including those containing active protons, such as hydroxymethyl group substituted norbornene3 and acrylic acid.2n The Pd/phosphine sulfonate catalysts were also shown to be used for emulsion polymerization of ethylene in water.4 In this regard, we developed the copolymerization of ethylene with functionalized allyl monomers, including allyl alcohol and allylamines.5 Especially when protic allyl alcohol or N-Boc-protected allylamine was used as a comonomer, the copolymerization gave different polymer structures compared with other aprotic allyl monomers.5,6 These findings motivated us to investigate the effect of protic solvents onto ethylene polymerization. We previously reported the mechanistic investigations for the ethylene polymerization by a Pd/phosphine sulfonate catalyst in toluene, revealing that β-H elimination is reasonably suppressed under sufficient ethylene pressure to promote chain propagation.7 In the present study, ethylene polymerization by Pd/phosphine sulfonate was investigated in the presence and absence of protic solvents, CH3OH and CH3OD (Scheme 1). The polymerization of ethylene (0.5 MPa) was performed in the presence of Pd complex 1 or 2 (0.10 mmol) at 80 °C for 5 h in toluene (entries 1 and 4 in Table 1), toluene/CH3OH (entry 2), or toluene/CH3OD (entries 3 and 5). In all the entries, almost all the ethylene in the feed was consumed to produce ca. 0.4 g of polyethylene with the number-average molecular weight (Mn) of 4000 5000 (in toluene) and 1400 1900 (in toluene/methanol), respectively. The lower molecular weights of the polyethylenes obtained in the presence of CH3OH and CH3OD indicated that r 2011 American Chemical Society
Scheme 1. Ethylene Polymerization by 1 or 2 in the Presence and Absence of CH3OH or CH3OD
the protic solvent accelerated β-H elimination from alkylpalladium intermediate I to III (Scheme 2), which leads to the following alkene dissociation (route III f E). It is worth noting that a similar decrease of molecular weights in the presence of polar solvents2a,4a,8 or comonomers2n,5 was reported in the literature. The structures of the obtained polymers were fully assigned by quantitative 13C NMR analyses (Figure 1 and the Supporting Information).9 The polymers obtained by using 1 were essentially linear polyethylenes that have ca. 3 branches per 1000 carbons. The polymer in entry 1 possesses more methyl branches (2.5/1000C) than longer alkyl branches (0.5/1000C). On the other hand, in entries 2 and 3, the amount of longer alkyl branches was increased to ca. 2.5/1000C along with the decrease of methyl branches. Considering that the sum of the number of branches (B + C) was almost equal in all the entries, the lower B/ C ratio in entries 2 and 3 would be attributed to relatively accelerated β-H elimination and the following isomerization (e.g., I f III f IV f VI f VII f VIII) by the presence of Received: May 19, 2011 Published: October 10, 2011 6049
dx.doi.org/10.1021/om2004207 | Organometallics 2011, 30, 6049–6052
Organometallics
NOTE
Table 1. Ethylene Polymerization by Complex 1 and 2 in the Presence or Absence of CH3OH or CH3ODa toluene
cosolvent
entry
catalyst
(mL)
(mL)
1
1
15.0
2
1
12.0
CH3OH (3.0 mL)
3
1
12.0
CH3OD (3.0 mL)
4
2
15.0
5
2
12.0
CH3OD (3.0 mL)
Mw/
methyl branch
longer alkyl branch
2-alkene chain end
(F + G + H)/
yield (g)b
Mn (103)c
Mn
(B) (/1000C)d
(C) (/1000C)d
(F + G) (/1000C)d
Ee
0.37
4.1
2.0
2.5
0.5
1.5
0.42
1.4
3.5
0.6
2.5
2.9
5.2
0.41
1.6
3.5
0.8
2.1
4.3
6.2
0.30
4.9
3.0
6.2
10
9.8
Polymerization of ethylene (0.5 MPa) by 1 or 2 (0.10 mmol) was performed at 80 °C for 5 h in a 50 mL autoclave. b Yield determined after precipitation with CH3OH. c Molecular weight measured by SEC using polystyrene as an internal standard and corrected by universal calibration. d Determined by 13 C NMR analysis. e Determined by 1H NMR analysis. a
Scheme 2. Proposed Mechanism for Ethylene Polymerization by 1 and 2 in the Presence of Methanol
the protic solvent compared with ethylene insertion (e.g., I f II, IV f V). Notably, it was reported that methyl branch B is formed through isomerization by chain-walking (I f III f IV), followed by ethylene insertion (IV f V f B), but longer alkyl branch C was not detected in the absence of the protic solvent.8
Figure 1. 13C NMR spectrum and the assignments of the polyethylene obtained in (a) entry 1 and (b) entry 2 in Table 1 (Cl2CHCHCl2, 120 °C).
Another possible mechanism for the formation of C is the copolymerization of ethylene with in situ formed linear 6050
dx.doi.org/10.1021/om2004207 |Organometallics 2011, 30, 6049–6052
Organometallics
NOTE
Scheme 3. Possible Formation of Pd Deuteride Complex by Methanolysis of Alkylpalladium I by CD3OH
Scheme 4. Proposed Mechanism for Incorporation of Deuteriums into the Methylene Units of the Polyethylene Figure 2. 2H NMR spectrum of the polyethylene obtained in entry 3 in Table 1 (Cl2CHCHCl2, 120 °C).
Table 2. D/H Ratio of the Obtained Polyethylenes as the Number of D per 1000 Hydrogen Atomsa D/1000 hydrogen atoms i
ii
iii
iv
v
polyethylene by 1 (entry 3 in Table 1)
40
5
51
21
4
polyethylene by 2 (entry 5 in Table 1)
2
