Making Plastics from Carbon Dioxide:
Copolymerization of Epoxides and CO2

Contents:
I Global warming
II Utilization of CO2
III Copolymerization of epoxides and CO2
a Background of copolymerization of epoxides and CO2
b First example of copolymerization of epoxides and CO2
c Salen metal complexe as catalysts
1 Cr salen complex
2 Co salen complex
d Zinc catalysts
1 Coates 's β-diiminate zinc catalysts
2 Asymmetric copolymerization using zinc catalysts
IV Outlook

Review:
1 Chem. Rew. 2001, 101, 953.
2 Angew. Chem. Int. Et. 2004, 43, 6618.
3 Chem. Rew. 2007, 107, 2388.

2008/3/12, Z. Chen (D1)I Global warming!
Fig 1 sea-level rise caused by global warming

Fig 2: Desertification

Fig 3 The major reason of global warning

On the other hand,since petroleum resources are predicted
to be exhausted within the next century
at the current rate of consumption, there
is a growing effort to develop new chemical
processes using biorenewable resources.
CO2 is an abundant, inexpensive, andnontoxic biorenewable resource.
CO2 might be an attrative raw material!

II Utilization of CO2
a Situation in Japan: 1997 Kyoto Protocol; 2008 TOYAKO Summit Conference.
Fig 4 Immobilization of CO2 in Japan

Let's concentrant on immobilization of CO2 by chemical methods!

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Scheme 2 Example of metal catalyzed:
Cu/ZrO2 heterogeneous catalytic hydrogenationof CO2 to methanolb Reduction of CO2Methods:
Electrochemical reduction;
Metal-catalyzed hydrogenation;
Photochemical reduction and so on
Scheme 1 Example of electrochemical reduction:

Example of chemical transfromation of CO2:

c Chemical transformation of CO2
Scheme 3 Chemical transformations of CO2

Mori miwako group, JACS 2002, 124, 10008.
Novel Catalytic CO2 Incorporation Reaction:
Nickel-Catalyzed Regio- and StereoselectiveRing-Closing Carboxylation of Bis-1,3-dienes
Scheme 4 Ni-catalyzed ring-closing carboxylation

Scheme 5 Possible reaction mechanism

One of the most efficient way toimmobilize CO2 is polymerzation of CO2

CO2MORM

OO

OR

Eq 1O n OorOOCO2OnO

O

Eq 2O

So far it is very difficult!

Today let me introduce copolymeriztion of epoxides and CO2 to you.

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III Copolymerization of epoxides and CO2orCHO: cyclohexene oxide

PO: propylene oxidea Background of copolymerization of epoxides and CO2
Scheme 6 The basic mechanism of epoxide and CO2
copolymerization and the formation of cycli carbonates

backbiting

Major by-product(Thermodynamically morestable than polycarbonates)

Scheme 7 Qualitative, ideal free-energy profile depicting alternating
copolymerization of propylene oxide and CO2,
as well as potential side-reactions.

Scheme 8 Regiochemistry of PPC

low temperature is better!
Chiral center!

Other key point of this reaction:
1 TOF
2 Pressure(CO2)
3 temperature
4 Mn
5 Mw /Mn

least hinderedC-O bond

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b First example: Inoue group (aluminum catalysts)
Makromol. Chem. 1978, 179, 1377.
Macromolecules 1982, 15, 682.
Scheme 9 Alumium porphyrins for copolymerizationof epoxide and CO2

Scheme 10 Other alumium catalysts:
3: Aluminuim calix-4-arene (Kuran, 1998)
4,5,6: Aluminum alkoxides complex (Beckman, 1999, 2000)

Cocatalysts:
phosphonium salts
20 oC, 8 atm CO2,0.3 h-1 TOF

C Salen metal complex:
1 Cr Salen complex
i Jacobsen group: JACS 1995, 117, 5897.
1First example of copolymerization of Salen complex: Jacobsen PCTInt. Appl. WO00/09463, 2000.

ii Other groups developed the salen complex system with cocatalysts:

2001, Nguyen: Cat 9d/MeIm

2002, Darensbourg: Cat 10/PtBu3

2003, Rieger: Cat 11/DMAP

80oC, 55 atm CO2, 12 h-1 TOF

80oC, 60 atm CO2, 32.2 h-1 TOF,
100% carbonate linkages.

75oC, 35 atm CO2, 226 h-1 TOF,
98% carbonate linkages.

iii Proposed mechanism with cocatalyst in the initiation step:(intermolecular)(intarmolecular)

Nuc: cocatalyst

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iv Intermolecular bimetallic pathway:

four-membered transition statev Intarmolecular monometallic pathway:
Darensbourg; Chem. Rew. 2007, 107, 2388.
Scheme 11: X-ray structure of intermediate 4
Kim group: Angew. Chem. Int. Ed. 2000, 39, 4096.

Scheme 12: The case in Cr-salen/DMAP system
N3N3OCrCrNNON

N
ring openedby DMAP

dimeric pyridinium alkoxyzinc derivatives

Table 1 Effect of various phosphines on the rate ofcopolymerization of cyclohexene oxide and CO2
catalyzed by Cr Salen complex:

Scheme 13 Activation of phosphines by formation ofphosphonium zwitterious

Sterical phosphines are effective cocatalysts
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vi Cocatalyst as a turning tools:

Table 2 Catalytic activity in the presence of
PPNX cocatalyst.

PPNCl: (C6H5)3P=N(Cl)=P(C6H5)3
N-MeIm: N-methylimidazole
Fig 4: Cocatalyst loading
Reason:PolymerO

OBackbiting

OPolymerO

OO

NuCrX

NuCr
X+OOOOO

Nu = cocatalyst

2 Co (III) Salen complex:
Competition of data in carbonate linkages, TOF, pressure, temperature and so on.

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i 2003: Coates group (Co Salen complex only), Angew. Chem. Int. Ed. 2003, 42, 5484.
* 1mPa = 145 psi

ii 2004: Lu group (with quaternary amminium salts), Angew. Chem. Int. Ed. 2004, 43, 3574.

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iii 2006: Nozaki group (with a piperidinium end-capping arm)
Angew. Chem. Int. Ed. 2006, 45, 7274.
Scheme 14 Catalyst design to suppress the productionof cyclic carbonate

Table 3: Copolymerization of epoxides with CO2
catalyzed by cobalt complex 1

Scheme 15: Synthesis of a block terpolymer

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d Zinc catalysts:
1 Coates 's β-diiminate zinc catalysts
i High-activity Zn(II)-based catalysts for the copolymerization ofCO2 and cyclohexene oxide.
Scheme 16 X-ray crystal structure of the dimer ofβ-diiminate zinc catalyst 1JACS 1998, 120, 11018.
JACS 2001, 123, 8738.

Scheme 17 Synthesis of β-diiminate zinc catalyst

Table 4 Results of copolymerization of CO2 and cyclohexene oxide

ii Proposed copolymerization mechanism using β-diiminate zinc catalyst:
Scheme 18 Insertion reaction of CO2

JACS 2003, 125, 11911.

Scheme 19 Insertion reaction of CHO

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Scheme 20 Proposed copolymerization mechanism and epoxide ring-opening transition state:

Scheme 21 Unsymmetrical, electron-deficient β-di-zinc complex for the copolymerizaiton of propylene oxide and CO2JACS 2002, 124, 14284.

Scheme 22 Alternating copolymerization of limonene oxide
and carbon CO2
JACS 2004, 126, 11404.

Scheme 23 Ring opening of 1a and 1b during copolymerizationand hydrolytic cleavage to give diaxial diol 13

iii Fromation of nanoparticles by intramoleucar cross-linking:following the reaction progess of single polymer chains

JACS 2007, 129, 11350.

Scheme 24 Synthesis of alkene crossing-linked
polycarbonate nanoparticles

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2 Zinc catalysts for asymmetric CHO-CO2 copolymerizaitoni Nozaki Group: JACS 1998, 121, 11008.
Scheme 26 Asymmetric alternatingcopolymerization of CHO and CO2

Scheme 25 Synthesis of complex 2a and it's structurerecrystallization

Fig 5 MALDI-TOF mass specrtum to determine
the end group

Scheme 27 Proposed structure of copolymerization

Each signal =142.2n (repeating unit) + 252.3 (1a) + 1.0 (H) + 23.0 (Na+ ion)
Scheme 28 Proposed mechanism

ii Coates group: Chem. Commun. 2000, 2007.

IV Outlook:*

Remained task:
1 Completely controlled asymmetric
CHO-CO2 copolymerizaiton
2 Polymerzation of CO2NtBu

OMORMOOOnORONOOtBuNM1OM2tBuCO2

How about?

NM

But

Metal Salen complex(Efficient catalysts for copolymerizationof epoxides and CO2)OO

Bimetallic Schiffbase complex

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