Solvent and Starter Sequence Studies

disgustedtukwilaInternet και Εφαρμογές Web

14 Δεκ 2013 (πριν από 3 χρόνια και 7 μήνες)

61 εμφανίσεις

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 is

p
-

p
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

higher

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 A
313

/ A
295

absorbance ratio
decreased.

Mn=

S
䵩乩

S


Mw =

S
Mi
2
Ni

S
䵩乩

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.

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

Starter Sequences with a Larger

Polymerization Driving Force


Starter Sequences with a Larger

Polymerization Driving Force


The speed of imine metathesis

by methyl


substituted sequences

is much slower.

5.

8 and 9

CHCl
3

THF

dioxane

MeOAc

EtOAc

CH
3
CN

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 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


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 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
m
PE 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
m
PE 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 = 0.125

CHCl
3
/ CH
3
CN = 0.05

CH
3
CN

Starter Sequences with a Larger

Polymerization Driving Force



9.

3

10

3

10