reaction of two molecules of 2-bromo-5-chlorozincio-3-hexylthiophene or
2-bromo-5-chloromagnesio-3-hexylthiophene with a Ni(dppp)Cl 2 catalyst molecule and (2) reductive elimination to form TT coupled 5,5
0 -dibromo-4,
4
0 -dihexylbithiophene and a Ni(0) complex. The Ni(0) and the dimer form an
associated pair, which is a π-complex between the Ni catalyst and the growing
polymer chain. The associated pair is involved in further oxidative addition reaction
and monomer insertion, followed by another reductive elimination. This catalytic
cycle continues by the addition of one monomeric unit each time until all the
monomer is consumed. The main difference between the KCTP mechanism
suggested by Yokozawa and McCullough’s mechanism is that, in McCullough’s
method, the nickel group breaks off from the chain by reductive elimination and
forms an associated pair with the polymer chain only to undergo further oxidative
addition, whereas KCTP mechanism claims that the nickel group undergoes successive reductive elimination and oxidative addition, without the formation of an
associated pair (by catalyst transfer). Both the mechanisms agree on the
transmetallation step, thereby producing at least one TT coupling defect in the
polymer chain. They both also predict that formation of the HH coupling product is
not likely due to the steric hindrances involved. Boyd et al. prepared the reverse
monomer (5-bromo-3-hexyl-2-iodothiophene) but did not observe polymerization
or even dimer formation (even with 0.5 equivalents of Ni(dppp)Cl 2 ) [112], which
supports the steric arguments of McCullough. The Kiriy group observed that the
transmetallation step is crucial in the polymerization process and that further chain
propagation is not favored if sterically hindered compounds are formed during the
transmetallation step [113]. McCullough’s group further established that GRIM
polymerization was a quasi-living reaction [111] and that the polymerization was a
first-order reaction with respect to Ni(dppp)Cl 2 catalyst. It was also demonstrated
that the degree of polymerization could be precisely controlled by adjusting the
ratio of monomer to initiator concentrations. The quasi-living nature of the polymerization meant that the polymer chains were still active towards the end of
reaction and, hence, end-capping of the polymer chains or synthesis of block
copolymers could be performed. End-capping of the polymer chains was carried
out using an active reagent such as a Grignard reagent. The end-capping method
allowed estimation of the average number of repeating units in the polymer chain
by
1 H NMR peak integration.
(Note that gel permeation chromatography (GPC) overestimates the molecular
weight of rod-like polymers such as P3HT. GPC generally measures the relative
molecular weight of the polymers with respect to polystyrene standards. GPC
measurements are based on the correlation of hydrodynamic volume of the polymer
chains with their molecular weights and, hence, give an overestimated molecular
weight for rod-like polymers such as P3HT [114]. A systematic comparison of
molecular weights from
1 H NMR and GPC measurements by the Seferos group
revealed that GPC overestimates the M n values of P3HT as compared to
1 H NMR
linearly by a factor of 1.3 for M n of 6.5-23 kDa and nonlinearly for higher molecular
weights [115]. On the other hand, MALDI-TOF MS can be used obtain the absolute
molecular weights [116]). However, end-capping reactions are generally not
Progress in the Synthesis of Poly(3-hexylthiophene)
15
2-bromo-5-chloromagnesio-3-hexylthiophene with a Ni(dppp)Cl 2 catalyst molecule and (2) reductive elimination to form TT coupled 5,5
0 -dibromo-4,
4
0 -dihexylbithiophene and a Ni(0) complex. The Ni(0) and the dimer form an
associated pair, which is a π-complex between the Ni catalyst and the growing
polymer chain. The associated pair is involved in further oxidative addition reaction
and monomer insertion, followed by another reductive elimination. This catalytic
cycle continues by the addition of one monomeric unit each time until all the
monomer is consumed. The main difference between the KCTP mechanism
suggested by Yokozawa and McCullough’s mechanism is that, in McCullough’s
method, the nickel group breaks off from the chain by reductive elimination and
forms an associated pair with the polymer chain only to undergo further oxidative
addition, whereas KCTP mechanism claims that the nickel group undergoes successive reductive elimination and oxidative addition, without the formation of an
associated pair (by catalyst transfer). Both the mechanisms agree on the
transmetallation step, thereby producing at least one TT coupling defect in the
polymer chain. They both also predict that formation of the HH coupling product is
not likely due to the steric hindrances involved. Boyd et al. prepared the reverse
monomer (5-bromo-3-hexyl-2-iodothiophene) but did not observe polymerization
or even dimer formation (even with 0.5 equivalents of Ni(dppp)Cl 2 ) [112], which
supports the steric arguments of McCullough. The Kiriy group observed that the
transmetallation step is crucial in the polymerization process and that further chain
propagation is not favored if sterically hindered compounds are formed during the
transmetallation step [113]. McCullough’s group further established that GRIM
polymerization was a quasi-living reaction [111] and that the polymerization was a
first-order reaction with respect to Ni(dppp)Cl 2 catalyst. It was also demonstrated
that the degree of polymerization could be precisely controlled by adjusting the
ratio of monomer to initiator concentrations. The quasi-living nature of the polymerization meant that the polymer chains were still active towards the end of
reaction and, hence, end-capping of the polymer chains or synthesis of block
copolymers could be performed. End-capping of the polymer chains was carried
out using an active reagent such as a Grignard reagent. The end-capping method
allowed estimation of the average number of repeating units in the polymer chain
by
1 H NMR peak integration.
(Note that gel permeation chromatography (GPC) overestimates the molecular
weight of rod-like polymers such as P3HT. GPC generally measures the relative
molecular weight of the polymers with respect to polystyrene standards. GPC
measurements are based on the correlation of hydrodynamic volume of the polymer
chains with their molecular weights and, hence, give an overestimated molecular
weight for rod-like polymers such as P3HT [114]. A systematic comparison of
molecular weights from
1 H NMR and GPC measurements by the Seferos group
revealed that GPC overestimates the M n values of P3HT as compared to
1 H NMR
linearly by a factor of 1.3 for M n of 6.5-23 kDa and nonlinearly for higher molecular
weights [115]. On the other hand, MALDI-TOF MS can be used obtain the absolute
molecular weights [116]). However, end-capping reactions are generally not
Progress in the Synthesis of Poly(3-hexylthiophene)
15