Folding-Driven Reversible Polymerization of Oligo (m-phenylene ethynylene) Imines: Solvent and Starter Sequence Studies D. Zhao, J. S. Moore, Macromolecules 2003, 36, Tobe Lab. Yui Yamaguchi

What is “Folding”? Folding Unfolding Driving Force is  interaction and Solvophobic interaction (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 weight higherlower

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 A 313 / A 295 absorbance ratio decreased. Mn=  MiNi  Ni Mw =  Mi 2 Ni  MiNi 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

Figure 2. CHCl 3 CH 3 CN CHCl 3 CH 3 CN 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 CHCl 3 CH 3 CN 4.

Starter Sequences with a Larger Polymerization Driving Force

The speed of imine metathesis by methyl – substituted sequences is much slower and 9 CHCl 3 THF dioxane MeOAc EtOAc CH 3 CN

Starter Sequences with a Larger Polymerization Driving Force Given sufficient reaction time, much higher molecular weights are achieved by the methyl - substituted sequences.

Starter Sequences with a Larger Polymerization 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 Larger Polymerization Driving Force 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 of the 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 Larger Polymerization Driving Force 8. CHCl 3 CHCl 3 / CH 3 CN = 0.75 CHCl 3 / CH 3 CN = 0.50 CHCl 3 / CH 3 CN = 0.25 CHCl 3 / CH 3 CN = 0.22 CHCl 3 / CH 3 CN = CHCl 3 / CH 3 CN = 0.05 CH 3 CN

Starter Sequences with a Larger Polymerization Driving Force