C O M M U N I C A T I O N S
Scheme 2
(Scheme 2). (PO-OMe)PdMe(CH2dCHOEt) undergoes 1,2-inser-
tion to give (PO-OMe)PdCH2CH(OEt)Me as the major product
(52%; kobs for consumption of (PO-OMe)PdMe(CH2dCHOEt) )
3.01(4) × 10-4 s-1, 24 °C). NMR data for the PdCH2CH(OEt)Me
group are very similar to data for (R-diimine)PdCH2CH(OEt)Me+.
Neither the possible chain walk isomer (PO-OMe)PdCMe2OEt nor
the 2,1-insertion product (PO-OMe)PdCH(OEt)CH2Me were
observed.4 The ethylene/vinyl ether copolymerization behavior of
(PO-OMe)PdMe is similar to that of 1.
Catalyst 1 also copolymerizes ethylene and CH2dCHOPh to a
copolymer that contains -CH2CH(OPh)CH2- (I) and CH3CH-
(OPh)CH2- (II) units. However, 1 does not incorporate CH2d
CHOSiPh3 in ethylene polymerization.
Figure 1. 1H-13C HMQC NMR spectrum (CDCl2CDCl2, 70 °C) of
ethylene/2b copolymer (4.2 mol % of 2b, Table 1, entry 4).
respectively. NMR spectra of the resulting hydroxy-polyethylene
(i.e., ethylene/CH2dCHOH copolymer) contain resonances for CH3-
CH(OH)CH2- and -CH2CH(OH)CH2- units.8a Some isomeriza-
tion of the vinyl end groups to 2-olefins occurs during thermolysis,
and partial hydrochlorination of the vinyl ends to CH3CHClCH2-
groups occurs in the HCl reaction. The reaction of the ethylene/2b
copolymer with HBr (CDCl2CDCl2, 80 °C, 48 h) yields the
corresponding bromo-polyethylene (86% conv. of -OEt to -Br
groups). Isomerization (to 2-olefins) and hydrobromination of the
vinyl ends are observed in this case. The characterization of these
hydroxy- and bromo-polyethylenes confirms the NMR assignments
for the ethylene/CH2dCHOR copolymers.
In summary, the neutral catalyst (PO-OMe)PdMe(py) (1)
copolymerizes ethylene and alkyl vinyl ethers to linear copolymers
with in-chain and chain-end vinyl ether units. The key features of
1 that enable this chemistry are the neutral charge, which disfavors
initiation of competing cationic polymerization of the vinyl ether,
and the high barrier to chain walking,13 which results in linear
polymers. We are investigating the mechanisms of the reactions of
1 with vinyl ethers and the performance of modified (PO)PdR
catalysts to enhance the scope, activity, and polymer molecular
weights in olefin/vinyl ether copolymerization.
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG-02-00ER15036) and the National
Science Foundation (CHE-0516950).
Control experiments provide strong evidence against radical and
cationic mechanisms for Scheme 1. The reaction of ethylene/2b
-
mixtures with H(OEt2)2+, Ph3C+, or Li(OEt2)2.8+ (as the B(C6F5)4
salts) in CH2Cl2 at 80 °C results in cationic homopolymerization
of 2b but no ethylene incorporation. The NMR spectra of the
products are essentially identical to those of -[CH2CH(OEt)]n-
generated by other cationic initiators.9 The reaction of ethylene/2b
mixtures with AIBN in toluene at 80 °C gives only trace quantities
of -[CH2CH(OEt)]n- oligomers with no ethylene incorporation.10
In contrast, under these conditions, 1 copolymerizes ethylene and
2b and the reactivity of ethylene is much higher than that of 2b.
In 1-catalyzed ethylene/vinyl ether copolymerization, the in-chain
units I can form by 1,2- or 2,1-insertion of CH2dCHOR into active
(PO-OMe)PdCH2CH2P species, followed by ethylene insertion.
Plausible pathways to the chain-end units II include (i) 1,2-insertion
of CH2dCHOR followed by chain walking and growth, and (ii)
chain transfer from (PO-OMe)PdCH2CH2P to CH2dCHOR fol-
lowed by ethylene insertion.11 The total level of vinyl ether
incorporation and the ratio of in-chain to chain-end placements (I/
II) is higher for 2b,c than for 2a under similar polymerization
conditions. Steric crowding may inhibit CH2dCHOtBu insertion
of (PO-OMe)PdCH2CH2P species and ethylene insertion of (PO-
OMe)PdCH2CH(OtBu)P or (PO-OMe)PdCH(OtBu)CH2P species.
The base-free species (PO-OMe)PdMe was generated by the
reaction of 1 and B(C6F5)3, and its reactivity with vinyl ethers was
explored.12 NMR monitoring experiments show that (PO-OMe)-
PdMe reacts with excess (16 equiv) 2b in CD2Cl2 to form (PO-
OMe)PdMe(CH2dCHOEt), which undergoes fast exchange of free
and coordinated CH2dCHOEt on the NMR time scale at 24 °C
Supporting Information Available: Experimental procedures and
characterization data for polymers. This material is available free of
References
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bridged dimer in the solid state.
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