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School of Natural Sciences and Mathematics 2011-11-28

Grignard Metathesis (GRIM) Polymerization for the Synthesis of Conjugated Block Copolymers Containing Regioregular Poly(3-Hexylthiophene

Mihaela C. Stefan, et al.

© 2012 The Royal Society of Chemistry. This copy may not be distributed.

Fig. 1 Regiochemical couplings of 3-alkylthiophenes.

Scheme 1 Grignard metathesis (GRIM) polymerization of 3-

hexylthiophene.

The McCullough method uses the 2-bromo-3-hexylthiophene

monomer which is subjected to lithiation and transmetallation

with MgBr2 to generate 2-bromo-3-alkyl-5-thienyl magnesium

chloride.5,6 The addition of Ni(dppp)Cl2 catalyst generates

regioregular poly(3-hexylthiophene). An improvement over the

original McCullough method was achieved by using ZnCl2 in the

transmetallation step.7 At the same time as McCullough, Rieke

reported a polymerization method in which 2,5-dibromo-3-hex-

ylthiophene was reacted with activated Rieke zinc to generate the

organozinc derivative which was further reacted with Ni(dppp)

Cl2 to generate regioregular poly(3-hexylthiophene).8,9 Both

organomagnesium and organozinc-mediated McCullough and

Rieke methods require cryogenic temperature during the metal

halogen exchange step.

Grignard metathesis (GRIM) polymerization was developed

in 1999 by McCullough’s group and allows the synthesis of

P3HT in large scale at room temperature.10,11 The GRIM poly-

merization employs 2,5-dibromo-3-hexylthiophene which is

reacted with t-butyl magnesium chloride to generate a mixture of

regioisomers (2 and 3 in Scheme 1) in a ratio of 80 : 20.10–12 Upon

addition of Ni(dppp)Cl2 the regioisomer 2 is incorporated into

the polymer while the more sterically hindered 3 is not consumed

during the polymerization.12 GRIM polymerization generates

regioregular polymers.12 Development of living nickel mediated

cross-coupling polymerizations (McCullough and GRIM

methods) has provided a versatile tool for the synthesis of

regioregular P3HT with well-defined molecular weights and

Prakash Sista

Prakash Sista received an inte-

grated MSc in chemistry (BSc

and MSc) from the Indian

Institute of Technology Bombay

in 2003. He received an MS in

chemistry from the University of

San Francisco in 2008 working

on electron transfer reactions of

ruthenium complexes. He is

currently working on his PhD in

Dr Mihaela Stefan’s research

group on the synthesis of novel

semiconducting polymers con-

taining benzodithiophene for

organic electronics applications.

1694 | Polym. Chem., 2012, 3, 1693–1701

narrowmolecular weight distribution.7,12 Yokozawa’s group also

reported the living nature of nickel catalyzed cross-coupling

polymerization of 2-bromo-5-iodo-3-hexylthiophene.13,14 GRIM

polymerization is also referred to as Kumada catalyst-transfer

polycondensation (KCTP) as proposed by Kiriy.15–17 The

purpose of this review is to describe the synthesis of conjugated

block copolymers containing regioregular poly(3-hexylth-

iophene) by GRIM polymerization. McCullough published

a comprehensive review describing the synthesis of regioregular

polythiophenes.18 A feature article on conjugated rod–coil and

rod–rod block copolymers was published by Lin.19 KCTP

polymerization was recently reviewed by Kiriy17 and

Luscombe.20

Due to its living nature, the GRIM polymerization allows the

synthesis of end-functionalized P3HT.21,22 A versatile method for

in situ end-group functionalization of regioregular P3HT

synthesized by GRIM has been reported.21,22For example, P3HT

with allyl end groups can be synthesized by addition of allyl

magnesium bromide to the nickel-terminated P3HT synthesized

by GRIM (Scheme 2).21,22 The facile end group functionalization

of P3HT synthesized by GRIM triggered the development of

various block copolymers containing regioregular P3HT.

Rod–coil block copolymers containing conjugated poly(3-hexylthiophene)

The first synthesis of diblock and triblock copolymers containing

conjugated rod-like P3HT was reported by McCullough.23

Harsha D: Magurudeniya

Harsha D. Magurudeniya

received his BSc in chemistry

from the University of Per-

adeniya in Sri Lanka in 2008.

He joined Prof. Mihaela Ste-

fan’s group in 2009 as a PhD

student at the University of

Texas at Dallas. He is currently

a third-year graduate student

and his research includes the

synthesis of novel nickel cata-

lysts for the polymerization of

bulky conjugated monomers

using Grignard metathesis

(GRIM) polymerization.

This journal is ª The Royal Society of Chemistry 2012

Scheme 2 In situ end-capping of regioregular P3HT.

Scheme 4 Synthesis of triblock copolymers containing P3HT by ATRP.

Polystyrene and poly(methyl acrylate) were incorporated in

diblock copolymers with conjugated P3HT by using a combina-

tion of the McCullough method and atom transfer polymeriza-

tion (ATRP) (Scheme 3).23 This method requires multiple steps

and uses cryogenic temperature during the synthesis of P3HT

precursor.23 The synthesis of triblock copolymers containing

P3HT was also reported by McCullough and employs a H/H-

terminated P3HT which was subjected to a Vilsmeier reaction to

generate the dialdehyde-terminated P3HT, which was converted

to an ATRP macroinitiator and used for polymerization of

styrene and methyl acrylate (Scheme 4).23

A simpler method which contains fewer steps was reported for

the synthesis of diblock copolymers containing P3HT. In this

method vinyl or allyl-terminated P3HT was synthesized by in situ

end capping using the corresponding Grignard reagent, followed

by hydroboration/oxidation to generate the hydroxy-terminated

P3HT (Scheme 5).24

Controlled radical polymerization (CRP) methods, such as

ATRP, reversible-addition fragmentation chain transfer poly-

merization (RAFT), and nitroxide mediated polymerization

(NMP), have been employed for the synthesis of P3HT diblock

copolymers (Scheme 6).24–26 The advantage of using CRP tech-

niques is that the molecular weight of the second block is

determined by the molar ratio of the polymerized monomer

relative to the P3HT macroinitiator. The opto-electronic prop-

erties and morphologies of copolymers depend on the molar

Scheme 3 Synthesis of P3HT-PS and P3HT-PMA diblock copolymers

by the McCullough method and ATRP.

This journal is ª The Royal Society of Chemistry 2012

ratio between the semiconducting P3HT segment and the coil-

like insulating block.

Coil-like polystyrene, poly(methyl acrylate), poly(methyl

methacrylate), poly(t-butyl acrylate), poly(t-butyl methacrylate),

poly(isobornyl methacrylate), poly(2-(dimethylamino)ethyl

methacrylate), poly(fluorooctyl methacrylate), poly(4-vinyl-

pyridine), and polyisoprene were incorporated into diblock

copolymer structures with P3HT.24–31 The synthesized block

copolymers display nanofibrillar morphology similar to that

observed for the P3HT homopolymer. A P3HT diblock copol-

ymer containing C60 was reported by Jo.32 The allyl-terminated

P3HT was converted to a bromoester macroinitiator which was

used for ATRP copolymerization of methyl methacrylate

(MMA) and 2-hydroxyethyl methacrylate (HEMA). The HEMA

units of the copolymer were reacted with the carboxylic acid

derivative of C60 to generate a diblock copolymer containing

P3HT and an electron acceptor C60 (Scheme 7).32 The incorpo-

ration of C60 in block copolymers with P3HT was also achieved

by a combination of GRIM and RAFT polymerization.33 Bie-

lawski reported the synthesis of a diblock copolymer containing

P3HT and poly(acrylic acid) by a combination of ATRP, GRIM,

and copper-catalyzed [3 + 2] cycloaddition.34 Lin reported the

synthesis of an amphiphilic 21-arm star-like diblock copolymer

containing conjugated P3HT and poly(acrylic acid) (Scheme 8).35

The hydroxyl groups of b-cyclodextrin were converted to bro-

moester to initiate the ATRP of t-butyl acrylate. The copper-

catalyzed [3 + 2] cycloaddition of azido-terminated 21-arm poly

(t-butyl acrylate) with ethynyl-terminated P3HT generated the

star-like diblock copolymer which was subjected to hydrolysis to

generate the poly(acrylic acid) block.35 The ethynyl-terminated

P3HT was synthesized by GRIM.

A diblock copolymer of P3HT and poly(2-phenyl-5-(4-vinyl-

phenyl)-1,3,4-oxadiazole) was also synthesized by a combination

of GRIM and ATRP.36

Yang reported the synthesis of a diblock copolymer of P3HT

and N-vinyl carbazole by RAFT polymerization (Scheme 9).

Diblock and triblock copolymers containing P3HT were

synthesized by a combination of GRIM and anionic

Scheme 5 Synthesis of hydroxypropyl-terminated P3HT.

Polym. Chem., 2012, 3, 1693–1701 | 1695

Scheme 6 Synthesis of diblock copolymers containing regioregular P3HT by a combination of GRIM and the CRP technique.

Scheme 7 Synthesis of a diblock copolymer containing P3HT and C60

by a combination of GRIM and ATRP.

Scheme 8 Synthesis of a 21-arm star-like amphiphilic block copolymer

containing P3HT and poly(acrylic acid).

Scheme 9 Diblock copolymer containing P3HT and poly(2-vinyl

carbazole) synthesized by GRIM and RAFT.

polymerization. The synthesis of poly(3-hexylthiophene)-b-

polystyrene (P3HT-b-PS) by coupling of living poly(styryl)

lithium with allyl-terminated P3HT was reported in 2006

(Scheme 10).37 A diblock copolymer of P3HT and poly(methyl

methacrylate) (P3HT-b-PMMA) was synthesized by anionic

1696 | Polym. Chem., 2012, 3, 1693–1701

coupling reaction of a-phenyl acrylate-terminated P3HT with

living PMMA anions (Scheme 11).38

A diblock copolymer of P3HT and poly(2-vinylpyridine)

(P3HT-b-P2VP) was synthesized by reacting the vinyl-termi-

nated P3HT with sec-BuLi, followed by the anionic polymeri-

zation of 2-vinylpyridine (Scheme 11).39 The synthesis of

This journal is ª The Royal Society of Chemistry 2012

Scheme 10 Synthesis of P3HT-b-PS by GRIM and anionic

polymerization.

Scheme 11 P3HT diblock copolymers with PMMA and P2VP synthe-

sized by GRIM and anionic polymerization.

a triblock copolymer of P3HT and PMMA (PMMA-b-P3HT-b-

PMMA) by a combination of GRIM, living anionic polymeri-

zation, and 1,1-diphenylene (DPE) chemistry was reported by

Scheme 12 Synthesis of a triblock copolymer containing P3HT and

PMMA by a combination of GRIM and anionic polymerization.

This journal is ª The Royal Society of Chemistry 2012

Ueda (Scheme 12).40 Ueda’s reported method is a two-step

reaction which involves DPE a,u-end functionalization of

P3HT, lithiation with sec-BuLi and anionic polymerization of

MMA. The most critical step in this procedure is the selective a,

u-difunctionalization.40 Anionic coupling of a dialdehyde

terminated P3HT also is used for the synthesis of a triblock

copolymer of P3HT and polyisoprene.41

Ring-opening cationic polymerization of tetrahydrofuran and

2-ethyl-2-oxazoline using a triflate ester-terminated P3HT mac-

roinitiator was reported by Stefan’s group (Scheme 13).42,43 The

diblock copolymer containing P3HT and elastomeric poly

(tetrahydrofuran) (P3HT-b-PTHF) displayed nanofibrillar

morphology similar to that of the P3HT homopolymer.42

Nanofibrillar morphology was also observed for the diblock

copolymers of P3HT with poly(2-ethyl-2-oxazoline) (P3HT-b-

PEOXA), where the density of the nanofibrils was found to be

dependent on the content of the PEOXA coil segment.43 Both

P3HT-b-PTHF and P3HT-b-PEOXA diblock copolymers dis-

played solvatochromism and opto-electronic properties compa-

rable to those of the P3HT precursor.

Meijer reported the synthesis of the P3HT-poly(cyclooctene)

diblock copolymer by the ruthenium-mediated ring-opening

metathesis polymerization (ROMP) of cyclooctene using allyl-

terminated P3HT as a chain transfer agent.44 The poly(cyclo-

octene) blockwas hydrogenatedwith p-toluene sulfonylhydrazide

to generate the P3HT-PE diblock copolymer (Scheme 14).44

The formation of crystalline domains of P3HT and PE was

demonstrated for this diblock copolymer. The copolymer was

found to be resistant to solvents at temperatures below 70 �C but

soluble in xylene and chlorobenzenes at elevated temperatures.44

Hillmyer reported the synthesis of diblock and triblock

copolymers containing both regioregular and regiorandom poly

(3-alkylthiophenes) and polylactide.45,46 The polylactide block

was synthesized by the ring-opening polymerization of D,L-lac-

tide using a hydroxy-terminated polythiophene macroinitiator

(Scheme 15).45,46

Scheme 13 Synthesis of diblock copolymers containing P3HT by

a combination of GRIM and cationic ring-opening polymerization.

Polym. Chem., 2012, 3, 1693–1701 | 1697

Scheme 14 Synthesis of P3HT-PE diblock copolymer by a combination

of GRIM, ROMP, and hydrogenation.

Scheme 15 Synthesis of PLA-PT-PLA by ring-opening polymerization

of D,L-lactide using hydroxyl-terminated polythiophenes.

Scheme 16 Synthesis of a rod–rod diblock copolymer containing P3HT

and PBLG by a combination of GRIM and ring-opening polymerization.

Scheme 17 Synthesis of a rod–rod diblock copolymer containing P3HT

and PBLG by a combination of GRIM, copper-catalyzed [3 + 2] cyclo-

addition, and ring-opening polymerization.

Rod–rod block copolymers containing conjugated poly(3-hexylthiophene)

The synthesis of rod–rod diblock copolymers containing rod-like

conjugated P3HT and liquid crystalline poly(g-benzyl-L-gluta-

mate) (P3HT-b-PBLG) was reported by Stefan’s group.47 The

H/Br terminated P3HT was reacted with N-(p-bromobenzyl)

phthalimide, followed by the deprotection of the amine group to

generate the benzylamine-terminated P3HT which was used as

a macroinitiator for the ring-opening polymerization of g-

benzyl-L-glutamate N-carboxyanhydride (Scheme 16).47 The

morphology of thin films of P3HT-b-PBLG diblock copolymer

was found to be dependent on the casting solvents and annealing

conditions.47 Bielawski also reported the synthesis of P3HT-b-

PBLG but using a different synthetic procedure.48 In this method

the ethynyl-terminated P3HT synthesized by GRIM was reacted

with the azido-terminated PBLG via a copper-catalyzed [3 + 2]

cycloaddition to generate P3HT-b-PBLG diblock copolymer

(Scheme 17).48

A rod–rod diblock copolymer containing regioregular P3HT

and poly(arylisocyanate) was reported by Bielawski.49 The

copolymer was synthesized in a one-pot reaction by polymeri-

zation of n-decyl-4-isocyanobenzoate initiated from the nickel-

terminated P3HT polymer synthesized by GRIM (Scheme 18).49

1698 | Polym. Chem., 2012, 3, 1693–1701

TMAFM analysis of the diblock copolymer demonstrated

a nanofibrillar morphology similar to other block copolymers

containing P3HT.49

A diblock copolymer containing P3HT and a side-chain liquid

crystalline polymer was reported by Zhao.50 A bromoester-termi-

nated P3HTwas used as amacroinitiator for theATRPof 4-{4-(6-

methacryloyloxyhexyloxy)benzoate]-40-hexyloxyazo benzene

(Scheme 19).50

The living nature of the GRIM polymerization allowed the

synthesis of rod–rod block copolymers containing various

3-alkylthiophene blocks (Scheme 20).12,51–54

The addition of LiCl to the isopropylmagnesium chloride used

during the magnesium halogen exchange step made the GRIM

method amenable to the polymerization of less reactive conju-

gated monomers.55,56 This improvement over the conventional

GRIM triggered the synthesis of various rod–rod polymers in

which both blocks are conjugated polymers.56–65

This journal is ª The Royal Society of Chemistry 2012

Scheme 18 Synthesis of diblock copolymer containing P3HT and poly

(arylisocyanate).

Scheme 19 Diblock copolymer of P3HT with a side-chain liquid crys-

talline polymer.

Scheme 20 Diblock copolymers containing 3-alkylthiophene synthe-

sized by living GRIM polymerization.

Scheme 21 Rod–rod block copolymers synthesized by living GRIM

polymerization.

Scheme 22 Synthesis of PPP-b-P3HT by GRIM.

The general procedure for the synthesis of diblock copolymers

containing conjugated blocks involves the metal halogen

exchange of the dibromo or iodo-bromo monomer with i-Pr-

MgCl–LiCl, followed by the addition of Ni(dppp)Cl2 or Ni

(dppe)Cl2 catalyst. The conjugated polymer generated in this step

is a living polymer that contains the nickel moiety incorporated

into the polymer as an end-group.56 The sequential addition of

the organomagnesium derivative of the other conjugated

monomer allows the growth of the second conjugated block.

Scheme 21 shows a few diblock copolymers containing regiore-

gular poly(3-hexylthiophene) and other conjugated blocks. For

example, poly(9,9-dioctyl-2,7-fluorene), poly(3-phenox-

ymethylenethiophene), poly(3-triethylene glycol thiophene), and

poly(2,5-dihexyloxy-p-phenylene) have been incorporated in

diblock copolymers with P3HT.52–62 The order of addition of

conjugated monomers in GRIM polymerization is very impor-

tant. As Yokozawa demonstrated for the synthesis of a diblock

copolymer of P3HT and poly(2,5-dihexyloxy-p-phenylene) (PPP)

to conserve the livingness of polymerization during the

This journal is ª The Royal Society of Chemistry 2012

cross-propagation step the polymerization should be started

from the monomer with the low p-donator ability (Scheme 22).57

The field-effect mobility of some of the rod–coil and rod–rod

block copolymers containing regioregular P3HT was measured

in thin-film transistor devices. Table 1 shows the field-effect

mobility values measured for P3HT block copolymers. The field-

effect mobilities of P3HT block copolymers are 1–3 orders of

magnitude lower as compared to P3HT homopolymers, which is

most probably due to the presence of the insulating block.

Polym. Chem., 2012, 3, 1693–1701 | 1699

Table 1 Field-effect mobilities of P3HT diblock copolymersa

Polymer Hole mobilitya/cm2 V�1 s�1

P3HT-b-PCHT51 1.9 � 10�3

P3HT-b-PS66 8.0 � 10�2

P3HT-b-P4VP28 2.8 � 10�4

P3HT-b-PTHF42 9.0 � 10�3

P3HT-b-PEOXA43 1.9 � 10�5

P3HT-b-PBLG47 7.0 � 10�4

a Determined in bottom-contact, bottom-gate thin film transistors withgold source and drain electrodes.

Conclusions

The living GRIM polymerization was successfully employed for

the synthesis of a plethora of rod–coil and rod–rod block

copolymers containing regioregular poly(3-hexylthiophene). For

many of the reported rod–rod block copolymers the opto-elec-

tronic properties are comparable to those of P3HT. However, the

introduction of insulating coil blocks has been shown to affect

the opto-electronic properties. The phase separation of the block

copolymers containing P3HT is usually dictated by the crystal-

line P3HT block. Most of the reported rod–coil and rod–rod

block copolymers display nanofibrillar morphology similar to

that of the regioregular P3HT homopolymer.

Acknowledgements

We thank The Welch Foundation (AT 1740) and the National

Science Foundation (DMR-0956116) for financial support of our

research programs.

Notes and references

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