Folding-Driven Reversible Polymerization of Oligo

(m-phenylene ethynylene) Imines: Solvent and Starter Sequence Studies

D. Zhao, J. S. Moore,

Macromolecules 2003, 36, 2712-2720

Tobe Lab. Yui Yamaguchi

What is “Folding”?

Folding

Unfolding

Driving Force isinteraction

andSolvophobic interactio

n(in Polar Solvent)

Folding – Driven Reversible Polymerization

►External Conditions 　 (Solvent, Temperature, Reaction time)

►Molecular design (Starter Sequences)

Folding of the chains can drive the polymerization to generate high polymers.

What is the “Imine Metathesis” ?

The sum of the bond energies on one side of the equilibrium distribution will not be biased to a particular product by bond energy changes.

The formation of high molecular weight products can reasonably be attributed to the energy gained by folding or collapsing of the polymer chains.

Solvent Studies: Polymerization of 1 and 2

Image of SEC (Size Exclusion Chromatography)

Retention time

Molecular weighthigher lower

Solvent Studies: Polymerization of 1 and 2

The molecular weight depends on polarity of solvent.

Solvent Studies: Polymerization of 1 and 2

A consistent increase in the number- and weight - average molecular weight of the products was observed as the A313 / A295 absorbance ratio decreased.

Mn=MiNiNi

Mw =Mi2NiMiNi

Mn; Number- average molecular weight

Mw; Weight- average molecular weight

Ni; the number of molecules whose molecular weight is Mi.

The folding was responsible for shifting the equilibrium, driving the chain to elongate into high polymers.

Macrocyclization of Oligomer 5 and 6

Macrocyclization of Oligomer 5 and 6

Figure 2.

CHCl3

CH3CN

CHCl3

CH3CN

5 and 6

Macrocyclization of Oligomer 5 and 6

Results of Metathesis 1 and 2, 5 and 6

3.

Metathesis Polymerization of 2 and 5

2 and 5

CHCl3

CH3CN

4.

Starter Sequences with a Larger Polymerization Driving Force

Starter Sequences with a Larger Polymerization Driving Force

Starter Sequences with a Larger Polymerization Driving Force

The speed of imine metathesisby methyl – substituted sequences

is much slower.

5.

8 and 9

CHCl3

THF

dioxane

MeOAc

EtOAc

CH3CN

Starter Sequences with a Larger Polymerization Driving Force

6.

3

10

3

10

Given sufficient reaction time, much higher molecular weights are achieved by the methyl - substituted sequences.

Starter Sequences with a LargerPolymerization Driving Force

►The metathesis of the Imine bond may require at least partial unfolding the mPE chain. Under conditions that strongly stabilize the helical conformation, the unfolded state is considerably disfavored. Thus, a larger energy barrier must be overcome before incorporation of more monomer units.

►Intermolecular association, which becomes significant for the more solvophobic backbone in polar media.

Slow kinetics of metathesis likely resulted from disfavored

dissociation and the imine bonds buried within stacked helix.

The reason of longer equilibration time required by the methyl substituted sequences

Two hypothesis

Starter Sequences with a LargerPolymerization Driving Force

7.

23 ºC

33 ºC

40 ºC

48 ºC

The molecular weight increase with the reaction temperature up to 30 ºC and it decreased at even higher equilibrium temperature.

The observed lower molecular weight ofthe more stabilized polymer is a kinetic rather than thermodynamic limitation.

Conclusion

The solvent and sequence effect on the reversible imine metathesis polymerization of mPE oligomers have investigated.

By means of tuning the solvent quality and temperature, the folding propensity can be modulated, and as a result, the control over the molecular weight of the resulting polymers can be achieved.

By varing the chain length of the starter sequences, macrocyclization can be either favored or circumvented. When the chain growth proceeds via an oligomeric intermediate containing six m – phenylene units, macrocycles form exclusively.

Besides the chain length, the structural characteristics of the starter sequence strongly influence the kinetics and equilibrium state of the final product.

Improving the folding capability of mPE chain resulted in higher molecular weight polymers. This agrees with the folding-driven nature of the polymerization.

Mechanism of Imine Metathesis

Starter Sequences with a LargerPolymerization Driving Force

8.

CHCl3

CHCl3 / CH3CN = 0.75

CHCl3 / CH3CN = 0.50

CHCl3 / CH3CN = 0.25

CHCl3 / CH3CN = 0.22

CHCl3 / CH3CN = 0.125

CHCl3 / CH3CN = 0.05

CH3CN

Starter Sequences with a LargerPolymerization Driving Force

9.

3

10

3 10