High performance poly(styrene-b-diene-b-styrene) triblock copolymers from a hydrocarbon-soluble and additive-free dicarbanionic initiator

Article abstract of DOI:10.1021/ja062695v

A new hydrocarbon-soluble (additive-free) dicarbanionic organolithium initiator, obtained by a simple halogen-lithium exchange reaction (Gilman's reaction) from a diarylhalide containing a side C₁₅ alkyl chain, has been designed and used to ini

Full text of DOI:10.1021/ja062695v

Published on Web 06/01/2006
High Performance Poly(styrene-b-diene-b-styrene) Triblock Copolymers from
a Hydrocarbon-Soluble and Additive-Free Dicarbanionic Initiator
Rachid Matmour,† Arvind S. More,‡ Prakash P. Wadgaonkar,‡ and Yves Gnanou*,†
Laboratoire de Chimie des Polyme`res Organiques, ENSCPB, UniVersite´ Bordeaux 1, 16 AVenue Pey Berland,
33607 Pessac Cedex, France, and Polymer Science and Engineering DiVision, National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune-411 008, India
Received April 26, 2006; E-mail: gnanou@enscpb.fr
Scheme 1. Synthesis of SBS Triblock Copolymers
Anionic polymerization is certainly the most reliable and versatile
technique for the synthesis of block copolymers due to the absence
of transfer and termination reactions.1 For this reason, it occupies
a key position in the industrial production of block copolymers,
the most emblematic being SBS triblock copolymers comprising a
low Tg polybutadiene (B) block flanked by two glassy polystyrene
(S) end blocks. Such triblocks are industrially obtained by butyl-
lithium(BuLi)-initiated sequential polymerization of styrene and
butadiene, followed by the deactivation/coupling of growing
carbanionic chains by a difunctional electrophilic reagent.2 Utilized
for the last 40 years, this method has, however, a drawback, which
is its extreme sensitivity to the stoichiometry of the final dichain
coupling; as a result, the triblock copolymers obtained are generally
contaminated with diblocks, which is detrimental to excellent
stress-strain properties. The use of a dicarbanionic initiator that
could trigger polymerization in two directions thus appears as the
only viable alternative.2 One major difficulty met in the latter case
is the limited solubility of dicarbanionic initiators in apolar solvents,
media that are required for the preparation of a polybutadiene central
block with a high content in 1,4-units and elastomers with optimal
properties. Whether obtained by reaction of BuLi with appropriate
divinylic species3-8 in 2:1 ratio or by generation of ion radical
species which then couple (e-transfer from lithium to R-substituted
vinyl monomers),9 organolithium diinitiators indeed require the
presence of active polar additives to efficiently initiate polymeri-
zation, which in turn modifies the stereochemistry of the polydiene
block and increases its content in 1,2 unsaturations. A dilithiated
carbanionic initiator that would be entirely soluble in apolar medium
in the absence of any additives or ligands and yet reactive enough
is thus very much in demand.
dilithiated initiator. 1 was obtained in excellent yields (84%) after
bromination in the para position of 1-pentadecyl-3-phenoxy ben-
zene. The structure of the dibromo compound (1) was confirmed
1
by H and 13C NMR spectroscopy and mass spectrometry (see
Figures S1 and S2 in Supporting Information). Next, 1 was treated
with stoichiometric amounts of sec-butyllithium (s-BuLi) in cy-
clohexane which generated 1′ and 2-bromobutane. To prevent the
subsequent deactivation of the growing carbanionic chains by
2-bromobutane, another equivalent of s-BuLi was added to trans-
form 2-bromobutane into 3,4-dimethylhexane (2) (Scheme 1).10 The
product (1′) of the reaction of 1 with s-BuLi in cyclohexane became
a yellow color and a gelly aspect at the concentration of [Li+] )
5.3 × 10-2 M. The presence of a C15 alkyl side chain in 1′ is
responsible for the formation of such a physical gel instead of the
irreversible precipitate observed with the dilithiated species formed
upon reaction of 4,4′-dibromobenzene with s-BuLi.10
The structure of the dilithium diadduct 1′ was first confirmed
by 1H and 13C NMR spectroscopy by analysis of the product isolated
after methanol quenching (see Figure S3 in Supporting Information).
Mass spectrometry also showed the total disappearance of the peak
of the dibromo precursor 1 (M ) 538 g/mol) in favor of the
protonated version of 1′ with a principal peak at M ) 381 g/mol
after methanolysis (see Figure S4 in Supporting Information). 1′
was then used ([Li+] ) 5.3 × 10-2 M) to initiate the polymerization
of styrene or butadiene (Scheme 1). After introduction of a few
monomer units, the dilithiated species formed appeared totally
soluble in cyclohexane even in the absence of stirring and of any
polar additives. On deactivation of the living carbanionic chains
by ethylene oxide after complete monomer consumption, R,ω-
dihydroxy telechelic polymers were obtained. Analysis by size
exclusion chromatography (SEC) indicated the complete consump-
tion of 1′, the only trace seen being a narrow and monomodal peak
corresponding to the expected linear polymer in the high molar
mass region (see Figure S5 in Supporting Information). Such an
excellent agreement between experimental and targeted values of
molar masses and narrow molar mass distributions (Mw/Mn typically
We previously reported the preparation of tri- and tetrafunctional
polylithium organic compounds by lithium-halogen exchange and
their use as initiators for the synthesis of polystyrene and poly-
butadiene star polymers.10 However, the latter polylithiated species
formed were found soluble only in the presence of σ/µ-coordinating
ligands. Using the same lithium-halogen exchange chemistry, we
describe here a new dilithiated initiator that is totally soluble in
apolar media in the absence of any additive and is efficient enough
to generate well-defined polybutadiene telechelics and SBS triblock
copolymers with a high content in 1,4-units and excellent mechan-
ical properties.11
3-Pentadecyl phenol, obtained from cashew nut shell liquid,12
served as the precursor for the dibromo compound, 1-bromo-4-
(4′-bromophenoxy)-2-pentadecyl benzene13 (1) (see Scheme S1 in
Supporting Information); the latter was subsequently dilithiated and
eventually used for difunctional initiation. In this chemistry,
advantage is taken of the saturated C15 side chain to obtain a soluble
† Laboratoire de Chimie des Polyme`res Organiques.
‡ Polymer Science and Engineering Division, National Chemical Laboratory.
9
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J. AM. CHEM. SOC. 2006, 128, 8158-8159
10.1021/ja062695v CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
glassy end blocks occurred under living conditions. Excellent
mechanical properties were measured on these SBS triblock
samplessespecially on those containing small polystyrene end
blocks (wt % (PS) < 35%)swith ultimate tensile strength higher
than 30 MPa and elongation at a break of 1000% (see Table 2 in
Supporting Information).
In summary, the halogen-lithium exchange reaction has been
successfully applied to generate a new dicarbanionic initiator from
a dibromoaryl compound. The presence of a C15 side alkyl chain
in the dibromo precursor is essential to the solubility of the
dilithiated initiator in apolar solvent. This is the first example of
dilithiated species initiating efficiently anionic polymerization in
absence of additive and affording well-defined polybutadiene
telechelics with a high percentage of 1,4-units (91%). Accordingly,
SBS triblock copolymers with remarkable ultimate tensile strength
and elongation at break could eventually be obtained, and in this
way, a long-standing issue faced by the industry of styrenic
thermoplastic elastomers could be sorted out.
Figure 1. 1H NMR spectrum (CD2Cl2, 400 MHz) of a R,ω-dihydroxy-
terminated polybutadiene.
Table 1. Characterization of Polystyrene, Polybutadiene, and SBS
Samples Synthesized from 1′ as Difunctional Initiatora
Microstructure (%)d
Mn
(g/mol)b
Mn(expected)
(g/mol)c
b
sample
Mw/Mn
1,4-cis
1,4-trans
1,2
Acknowledgment. The authors thank CNRS, the French
S
S
B
B
B
B
B
SBS
11400
43800
2100
1.15
1.13
1.2
1.13
1.07
1.08
1.1
12000
44000
2000
Ministry of Research, and CSIR for support of this research.
Supporting Information Available: Experimental details for the
synthesis and the characterization of the initiator, homopolymers, and
block copolymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
38
39
39
40
41
41
47
47
47
50
50
50
15
14
14
10
9
6200
7000
16600
51900
67400
110900
17000
52000
70000
110000
1.2
9
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d Calculated by 1H and 13C NMR (see Figure S8 in Supporting Information).
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(Scheme 1). The initiation of styrene by polybutadienyllithium being
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1
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Information). In all cases, narrow molar mass distributions were
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