Olefin Polymerizations Catalyzed by Late Transition Metal Complexes Maurice Brookhart University of...
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Olefin Polymerizations Catalyzed by Late Transition Metal Complexes
Maurice BrookhartUniversity of North Carolina
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Polyolefins
Total : 100 billions / year
16lbs / person on Earth / year !
• Inexpensive monomers
• Little waste in production
• Attractive physical properties, long term stabilities
CH CH2
CH3CH CH2
CH2 CH2Polyethylene,n
6 x 1010 lbs/yr
Polypropylene, n 3 x 1010 lbs/yr
1 x 1010
lbs/yr
nPolystyrene,
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Polymer Microstructure — Key to Properties
isotactic
syndiotactic
atactic
H3C H3C H3C H3C H3C
H H H H H Tm = 160°C
R R
Polypropylene
Polyethylene
Tm = 165°C
Stereoregular
Completely amorphous
High Density PE (HDPE) Tm= 136°C
Linear Low Density PE (LLDPE)
Tm = 115~130°C
Low Density PE (LDPE) Tm= 105~115°C
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Polyolefins Primarily Produced via Metal-Catalyzed Processes
Catalyst Structures Control:
— polymer microstructures
— polymer molecular weights, molecular weight distributions
— comonomer incorporation
Early Metal Catalysts (Ti, Zr, Cr) Late Metal Catalysts (Pd, Ni, Co)
Ph Ph Ph
Ph Ph Ph
isotactic polypropylene
syndiotactic polypropylene
atactic polypropylene
syndiotactic polystyrene
O
O
O
O
alternating CO / copolymer
high % crystallinity
Tm ~ 250oC
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General Mechanism for Polymer FormationInitiation
Chain Growth (RP)
Chain transfer (RCT)
RP >> RCT => High Polymer
RP ~ RCT => Short Chains (oligomers)
RP < RCT => Only C4
LnM H LnM H migratory
insertionLnM
LnM LnMmigratory
insertionLnM
LnMetc LnM
LnMP
H H
-H
elim.LnM
P
H
LnM
H
+ P
etc. LnMnew chain starts
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Olefin Polymerizations Using Late Metal Catalysts (Ni, Pd)
Why Late Metals ?
1. Potentially different enchainment mechanisms =>
new microstructures
2. Less oxophilic — functional group compatible
But…
1. Normally lower insertion barriers
2. Chain transfer competitive with propagation =>
dimers, short chain oligomers
GG G
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α–Diimine Based Catalysts
■ High molecular weight polymers with unique microstructures from:
● ethylene
● α – olefins
● cyclopentene
● trans-1,2-disubstituted olefins
■ Copolymers of ethylene with certain polar vinyl monomers
N N
RR
H3C solv.
R'
R'R'
R' M
R = H, Me, acenaphthyl
R' = -iPr, -Me, aryl, halogen
A
M = Ni, PdA =
CF3
CF3
B4
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Catalysts Modeled on α–Diimine Systems
M
NN
R
R
R
R
M
PN
R
R
Ar
Ar
Ph
N
NN
R
R
R
R
M
X X MMAO
Ni
O N
R
R
O NNi
R
R
N
R
R
N
R
R
N N
M
M = Ni, Pd
Daugulis, BrookhartBennettSmall, Brookhart Gibson
M = Fe, Co
G
GrubbsJohnson (DuPont)
Hicks, Jenkins, Brookhart
M = Ni, Pd
Killian (Eastman)
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Polyethylene
N N
R' Solv
R
RR
R Pd
+
N N
Br Br
R
RR
R Ni
/ Et2AlCl
30 °C~500 TO/hr
30 °C
~1-3 x 106 TO/hr
Mn > 105
amorphous PE
hyperbranched,
~100 branches / 1000 C's
Mn 105 - 106
Tm: 25° - 135° C
5 - 80 branches / 1000 C's
increasing [C2H4] decreases branching
increasing T increases branching
Early Metal CatalystsTi (IV), Zr (IV), Cr/SiO2
n
linear PEsemicrystallineTm ~ 136 °C
High Density PE, HDPE
R R
1-10% incorporation, LLDPE
Tm = ~ 115 - 130 oCR
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Poly (α–Olefins)
N N ArArM
+R R
Early Metal CatalystsR
R RRR
"chain-straightened" (1, 3 enchainment)
chain-straightened, primarily C1, C4 branches(1,6 enchainment)
1,2-insertion
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1,2–Disubstituted Olefins
N N
RR
X Y
ArArM
+ A
cis-1,3-enchainment
chainstraightening
-
1,3 insertion
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Mechanistic StudiesGeneration of Cationic Alkyl Complexes
N
NM
RR
RR
X
X
N
NM
CH3
CH3
2 R'MgX
N
NM
CH3
CH3
N
NM
CH3
OEt2
R' = -CH3 -CH2CH3 -CH2CH2CH3 -CH2CH(CH3)2
M = Pd stable at 25 °C
M = Ni stable only below ca. -20 °C
H(OEt2)2+ BAr'4
-
Et20
+ BAr'4-
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1H, 13C NMR Studies – Pd(II)
N
N
PdMe
Ar
Ar
OEt2
N
N
Pdpoly
Ar
Ar
N
N
Pd
Ar
Ar
N
N
PdMe
Ar
Ar
+
C2H4 (excess)
+
+
kp, -30 °C
+
-30 °Ck1
catalystresting state
CD2Cl2-80 °C
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Insertion Kinetics – Ni(II)
N
N
NiMe
Ar
Ar
OEt2
N
N
NiPr
Ar
Ar
CDCl2F
N
N
NiR
Ar
Ar
N
N
NiMe
Ar
Ar
+ 1. 20 eq C2H4 -130 °C2. -110 °C
+
-80 °C
k1st insertion
+
-70 °C
ksub. insert.
+
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Activation Barriers to Insertion (ethylene)
N
N
Pd
Me
N
N
Ni
Me
13.6 kcal/mol (-81 oC) 14.0 kcal/mol (-72 oC)
G (1st insertion) G (subseq. insertions)
G‡ (Pd-Ni) ca. 5 kcal/mol
+
18.4 kcal/mol (-20 oC) 18.6 kcal/mol (-20 oC)
+
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Mechanistic Model
MN
N
RM
N
N
R
MN
N
R
MN
N
R
H
MN
N
R
insertion
methyl branchresting state
MN
N
R'
turnover-limiting
"chain running"
MN
N
insertion
ethyl branch
R'
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Blocking of Axial Coordination Sites
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Chain Transfer Mechanisms
NN
HR
R+NN
HM M
(1) Associative Displacement (retarded by blocking axial postions)
(2) Chain Transfer to Monomer (suggested by Ziegler calculations)
N
N
H3C
H3CNi Ni
N
N
H3C
H3CNi
N
NH
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Mechanistic Model
MN
N
RM
N
N
R
MN
N
R
MN
N
R
H
MN
N
R
insertion
methyl branchresting state
MN
N
R'
turnover-limiting
"chain running"
MN
N
insertion
ethyl branch
R'
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Formation of Agostic Ethyl Complex
N
NPd
N
NPd
H
Pd
CC
Ha
Hc Hc
Hb
Hb
CH3CH3
H(OiPr2)2BAr'4
CDCl2F, -80 oC
BAr'4
-8.9 ppm
t, 2JHH = 16 Hz1JCH = 67 Hz
1H 2.2 ppm13C 38.5 ppm1JCH = 153 Hz
1H 1.4 ppm13C 19.3 ppm1JCH = 155 Hz
(-130 oC)
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Dynamics of Agostic Ethyl Complex
Pd
N
N Ha
Hc Hc
Hb
Hb Pd
N
N Ha
Hb Hb
Hc
HcPd
N
N Ha
k = 1450 s-1, -108 oC
G‡ = 7.1 kcal/mol
**
*
Ni
N
N H
H H
H
H Ni
N
N H
H H
H
HNi
N
N H
k = 170 s-1, 16 °C
G‡ = 14.0 kcal/mol
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Cationic Metal Alkyl Intermediates –Ethylene Trapping Experiments
Pd
N
N
Pd
N
N
H(OEt2)2+ BAr'4
-
Pd
N
N H H
1 20
Pd
N
N-80 °C
-80 °C
Pd
N
N
-65 °C
-25 °C
insertion
(several 100 1,2 shifts prior to insertion)
(via reversibleloss of C2H4)
1 20
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Cationic Metal Alkyl Intermediates –Ethylene Trapping Experiments
Ni
N
N
Ni
N
NX
Ni
N
N
Ni
N
N
-80 °C -80 °C
etc. etc.
no equilibrationprior to insertion
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Mechanistic Model
MN
N
R
HM
N
N H
R
MN
N
R
MN
N
R
H
MN
N
R
insertion
methyl branchresting state
MN
N H
R'
turnover-limiting
"chain running"
MN
N
insertion
ethyl branch
R'
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Commercial Copolymers of Ethylene and Polar Vinyl Monomers
CO2Me
CO2Bu
CO2H
CO2H
OAc
CN
Si(OMe)3
● Radical Initiation
● High temperatures, very high ethylene pressure
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Examination of Pd and Ni Diimine Catalysts for Copolymerizations of Ethylene and:
O R
O
OR
O
SiOR
OROR
1.
2.
3.
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Problems Connected with Copolymerization
1. Monomer Binding through the Functional Group
G
2. β-Elimination of G
ML
L
R
+ G ML
L
R
G
ML
L
R
G
ML
L
R
GM
L
L G
R
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3. Weak Competitive Binding of
4. Strong Chelate Formation Following Insertion
G
ML
L
R
G
insertion isomerizationM
L
L G
ML
L
G
K << 1
ML
L
R
G
+ ML
L
R
+G
K >> 1
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5. High Barrier to Insertion of Open Chelate
ML
L
insertionR
G
ML
L
R
G
ML
L
insertionR ML
L
R
G1‡
G2‡
G1‡ G2
‡>
slow
fast
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Examples: G = -CN ; -Br, -Cl
PdN
N
CH3
PdN
N
CH3
OEt2
CN
X
PdN
N
CH3
PdN
N X
NC
PdN
N X
X
PdN
N
PdN
N X
X = Br, Cl elim
2+Sen, et. al. 2002Jordan, et. al. 2003
Ittel, Johnson, Brookhart, Chem. Rev. 2000
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Ethylene / Acrylate Copolymerization - Pd
N
NPd
CH3
NCCH3
N
NPd
CH3 N
NPd
O
OCH3 N
NPd
O OCH3
N
NPd
OOCH3
CO2CH3
OCH3
O
CO2CH3
CH2Cl2, T = 35 oC
P(C2H4) = 2 atm
MA = 25 vol%
TOF = ca. 100 TO/h (slow!)Branched Copolymer6 mol% MA incorporation103 branches/1000 C
Methyl Acrylate Insertion
-80 oC
2,1-insertion
-60 oC
-30 oC
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Mechanism of Copolymerization
N
NPd
rearrangementP
OCH3
O2,1-ins.
G‡ = 16 kcal/mol
N
NPd
P
OOCH3
N
NPd
P OCH3
O
N
NPd
P OCH3
Ochain growth
chain running
resting state
+C2H4
-C2H4
K ~ 0.02 M-1
25 °C
insertion
G‡ ~ 18 kcal/mol
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Examination of Pd and Ni Diimine Catalysts for Copolymerizations of Ethylene and:
O R
O
OR
O
SiOR
OROR
1.
2.
3.
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Ethylene / Alkoxy Vinyl Silane CopolymersVersipol Group - DuPont
N N
R'
R'R'
R'M
R''R''
R L
Si(OR)xR'y
N NNi
SiMe3Me3Si
Si(OEt)3
/
25 - 120 °C
random copolymer
linear to highly branched
up to 32 mol% comonomer incorporation
600 psi C2H4, 60 °C
5 vol%toluene
5 eq. B(C6F5)35 eq. LiB(C6F5)4
PE copolymer0.42 mol% silane10 Me branches / 1000 CTm = 121 °CMn = 25.5 K, Mw/Mn = 2.9110 kg PE / gm Ni
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Vinyl Alkoxy Silane Insertion Chemistry -
N NNi
Me OEt2
Si(OEt)(Me)2
CD2Cl2-60 °C
N NNi
SiO
Et
N NNi
Si
O Et
no 2-alkene
complexes
observed
15%2,1 insertion
85%1,2 insertion
Si(OEt)(Me)2
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Evidence for Reversible C2H4 Coordination
N NNi
SiO Et
N NNi
SiEtO
+ C2H4 4.68 4.59 3.96 3.73
CDCl2F
Keq ca. 0.035 M-1, -120 °C
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Advantages of Vinyl Alkoxy Silane Comonomers
1. Insertion barriers of vinyl alkoxy silanes into Pd-R and Ni-R bonds are similar to ethylene insertion barriers.
2. Chelates resulting from vinyl alkoxy silane insertions are readily opened with ethylene.
3. Open chelates readily insert ethylene.
4. Relative binding affinities favor ethylene, but not to a prohibitive extent.