Preparation of polystyrene-polyolefin multiblock copolymers by sequential coordination and anionic polymerization

Article abstract of DOI:10.1039/c6ra25848d

Block copolymers of polyolefins (PO) and polystyrene (PS) are attractive materials that are not synthesized directly from the olefin and styrene monomers. A strategy for construction of PS-b-PO-b-PS triblock units directly from the olefin and styrene monomers is disclosed herein. PO chains (ethylene/1-octene or ethylene/1-pentene copolymers) were grown from dialkylzinc species bearing the α-methylstyrene moiety (i.e., [4-(isopropenyl)benzyl]₂Zn) by 'coordinative chain transfer polymerization (CCTP)' using a typical ansa-metallocene catalyst, rac-[Me₂Si(2-methylindenyl)₂]ZrCl₂ activated with modified-methylaluminoxane (MMAO). PS chains were subsequently grown from the Zn-alkyl sites and from the α-methylstyrene moieties of the resulting PO chains by switching to anionic polymerization. When nBuLi(tmeda)₂ was fed into the system as an initiator in a quantity fulfilling the criterion [Li] > [Zn] + [Al in MMAO], nBuLi(tmeda)₂ successfully attacked the α-methylstyrene moieties to initiate the anionic styrene polymerization at both ends of the PO chains generated in the CCTP process. However, in model studies, the attack of nBuLi(tmeda)₂ on α-methylstyrene in the presence of (hexyl)₂Zn consumed two molecules of α-methylstyrene per nBuLi to afford mainly R-CH₂C(Ph)(Me)-CH₂C(Ph)(Me)Li, where R is either an nbutyl or hexyl group originating from nBuLi or (hexyl)₂Zn, respectively. This observation suggests that the block copolymer does not simply comprise the PS-b-PO-b-PS triblock, but instead comprises a multiblock containing PS-b-PO-b-PS units. The molecular weight of the polymer increased after performing anionic polymerization. Even though the phase separation of the PS and PO blocks observed in the TEM images is less regular in the multiblock copolymers, the elastomeric property of the multiblock copolymers observed in the hysteresis testing is better than that of the diblock analogue.

Full text of DOI:10.1039/c6ra25848d

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PAPER
Preparation of polystyrene–polyolefin multiblock
copolymers by sequential coordination and anionic
polymerization†
Cite this: RSC Adv., 2017, 7, 5948
Dong Hyun Kim, Seung Soo Park, Su Hyun Park, Jong Yeob Jeon, Hyo Bo Kim
*
and Bun Yeoul Lee
Block copolymers of polyolefins (PO) and polystyrene (PS) are attractive materials that are not synthesized
directly from the olefin and styrene monomers. A strategy for construction of PS-b-PO-b-PS triblock units
directly from the olefin and styrene monomers is disclosed herein. PO chains (ethylene/1-octene or
ethylene/1-pentene copolymers) were grown from dialkylzinc species bearing the a-methylstyrene
moiety (i.e., [4-(isopropenyl)benzyl]2Zn) by ‘coordinative chain transfer polymerization (CCTP)’ using
a
typical ansa-metallocene catalyst, rac-[Me2Si(2-methylindenyl)2]ZrCl2 activated with modified-
methylaluminoxane (MMAO). PS chains were subsequently grown from the Zn-alkyl sites and from the
a-methylstyrene moieties of the resulting PO chains by switching to anionic polymerization. When
nBuLi(tmeda)2 was fed into the system as an initiator in a quantity fulfilling the criterion [Li] > [Zn] + [Al in
MMAO], nBuLi(tmeda)2 successfully attacked the a-methylstyrene moieties to initiate the anionic styrene
polymerization at both ends of the PO chains generated in the CCTP process. However, in model
studies, the attack of nBuLi(tmeda)2 on a-methylstyrene in the presence of (hexyl)2Zn consumed two
molecules of a-methylstyrene per nBuLi to afford mainly R–CH2C(Ph)(Me)–CH2C(Ph)(Me)Li, where R is
either an nbutyl or hexyl group originating from nBuLi or (hexyl)2Zn, respectively. This observation
suggests that the block copolymer does not simply comprise the PS-b-PO-b-PS triblock, but instead
comprises a multiblock containing PS-b-PO-b-PS units. The molecular weight of the polymer increased
after performing anionic polymerization. Even though the phase separation of the PS and PO blocks
observed in the TEM images is less regular in the multiblock copolymers, the elastomeric property of the
multiblock copolymers observed in the hysteresis testing is better than that of the diblock analogue.
Received 26th October 2016
Accepted 9th January 2017
DOI: 10.1039/c6ra25848d
www.rsc.org/advances
the high manufacturing cost. In industry, SEBS is not produced
directly from olen and styrene monomers, but, instead, is
Introduction
generated via hydrogenation of PS-block-poly(1,4-butadiene)-block-
PS (SBS), which is produced by anionic living polymerization of
1,4-butadiene and styrene (Scheme 1(a)).7–10 The hydrogenation
process is not an easy task and is a signicant contributor to the
manufacturing cost in the production of SEBS. Hence, prepara-
tion of PO–PS block copolymers directly from olen and styrene
monomers is a worthwhile and formidable pursuit.11–14
Recently, we disclosed a novel method for direct preparation
of PO–PS diblock copolymers using olen and styrene mono-
mers, where coordination and anionic polymerizations were
sequentially performed in one-pot (Scheme 1(b)).15 The poly-
olen chains were grown from dialkylzinc sites by the so-called
‘coordinative chain transfer polymerization (CCTP)’,16–20 which
is a useful tool for precise architectural design of polyolen
chains.21–32 The PS chains were subsequently grown from the
zinc sites attached to the PO chains by feeding styrene mono-
mer and a sub-stoichiometric amount of nBuLi(tmeda) (tmeda:
N,N,N0,N0-tetramethylethylenediamine) initiator (0.2 eq./Zn)
into the system. Reversible formation of zincate species via
Block copolymers comprised of polyolens (PO) and polystyrene
(PS) are attractive materials that have drawn signicant attention
in industry and academia.1–6 The triblock copolymer, PS-block-
poly(ethylene-co-1-butene)-block-PS (SEBS) is a thermoplastic
elastomer. Glassy PS blocks form spherical domains that are
uniformly distributed in a rubbery poly(ethylene-co-1-butene)
matrix. SEBS is commercially produced on a scale of several
hundred thousand ton per year and advertised to be the material
of choice for production of so and strong compounds for
handles and grips, elastic components in diapers, oil gels for
telecommunications and medical applications, impact modiers
of engineering thermoplastics, and exibilizers and tougheners
for clear polypropylene. The unique and attractive properties have
led to a massive increase in its demand, but its use is limited by
Department of Molecular Science and Technology, Ajou University, Suwon 443-749,
South Korea. E-mail: bunyeoul@ajou.ac.kr; Tel: +82-31-219-1844
† Electronic supplementary information (ESI) available: NMR spectra, mass data,
GPC curves, and DSC thermogram. See DOI: 10.1039/c6ra25848d
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Scheme 1 Preparation of PO–PS block copolymers.
Scheme 2 Strategy for direct preparation of PS-block-PO-block-PS using olefin and styrene monomers.
the reaction between the dialkylzinc species and the chain- polymerization behaviour did not change in the presence or
growing alkyllithium sites may be responsible for the growth absence of a-methylstyrene.
of PS chain from the zinc sites. Coordination of tmeda enhances
The key material in the strategy, [4-(isopropenyl)benzyl]2Zn,
the reactivity of the alkyllithium species, playing a key role in was prepared in a straightforward manner from 4-(chlor-
the formation of the zincate species in the aliphatic hydro- omethyl)benzoyl chloride (Scheme 3). Thus, addition of two
carbon solvent in which the anionic polymerization is per- equivalents of MeMgBr to 4-(chloromethyl)benzoyl chloride (p-
formed. In this work, we report the preparation of PS–PO ClC(O)C6H4CH2Cl) and subsequent treatment of the resulting
multiblock copolymers that mimic the chain structure of SEBS tert-benzylic alcohol (4-(Me2C(OH))C6H4CH2Cl) with the weak
(Scheme 2). Preparation of PO-based block copolymers is acid KHSO4 at 140 ꢀC afforded 4-(isopropenyl)benzyl chloride in
currently highly topical.33–43
good yield (76%).44 Generation of the Grignard reagent using
the prepared 4-(isopropenyl)benzyl chloride and subsequent
addition of half equivalent of ZnCl2 in diethyl ether afforded the
desired [4-(isopropenyl)benzyl]2Zn. Purication of dibenzylzinc
derivatives is sometimes problematic, but in this case,
extremely pure [4-(isopropenyl)benzyl]2Zn was obtained
through recrystallization in hexane.45–47
Results and discussion
Strategy for preparation of PS-block-PO-block-PS
The present strategy for preparation of the PS-block-PO-block-PS
copolymer is summarized in Scheme 2. The PO chains were
grown from dialkylzinc species bearing an a-methylstyrene
moiety, i.e., [4-(isopropenyl)benzyl]2Zn, by the established CCTP
method, and the PS chains were subsequently grown from both
the zinc sites and the a-methylstyrene moieties by switching to
anionic polymerization. The prerequisite for success of the
strategy is that the a-methylstyrene moiety should remain intact
during the coordination polymerization process. The styrene
monomer readily acts as a chain terminating agent or becomes
incorporated into polymer chains during the coordination
polymerization,11,12 but, a-methylstyrene does not interfere in
the ethylene/1-octene copolymerization carried out using the
typical ansa-metallocene catalyst rac-[Me2Si(2-methylindenyl)2]
In the previous studies focusing on the preparation of PO–PS
diblock copolymers, a minimal amount of nBuLi(tmeda) ([Li]/
ZrCl2 activated with modied-methylaluminoxane (MMAO); the Scheme 3 Synthesis of [4-(isopropenyl)benzyl]2Zn.
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[Zn] ¼ 0.2) was used to minimize the generation of PS homo-
polymers (Scheme 1(b)); however, in the present strategy to
prepare PS-b-PO-b-PS triblock copolymers (Scheme 2), a signi-
cant amount of RLi(tmeda) ([Li]/[Zn] ¼ 2) should be fed into the
system to attack the a-methylstyrene moieties. Gratifyingly, PS
chains were effectively grown from almost all the Zn-alkyl sites,
even when the [Li]/[Zn] ratio was as high as 1.0, 1.5, and 2.0. The
([PS growth sites] ꢁ [Li])/[Zn] values determined from the
measured PS-Mn values (i.e., ([PS growth sites] ꢁ [Li])/[Zn] ¼
{[styrene]/DP ꢁ [nBuLi(tmeda)]}/[Zn], where DP ¼ PS-Mn/104)
were close to the value of 2.0 that is expected when PS chain
growth occurs from all the Zn-alkyl sites (1.89, 1.97, and 1.82 for
[Li]/[Zn] feed ratios of 1.0, 1.5, and 2.0, respectively) (2–4 in
Table 1). Additionally, the molecular weight distributions were
narrower at high feed ratios of nBuLi(tmeda) (Mw/Mn, 1.26, 1.27,
and 1.23 for [Li]/[Zn], 1.0, 1.5, and 2.0, respectively) compared
with the Mw/Mn value (1.52) observed when the [Li]/[Zn] ratio
was low at 0.20 (entry 1). Addition of two equivalents tmeda per
nBuLi afforded similar results at [Li]/[Zn] ratios of 1.0 and 1.5;
PS chains were effectively grown from most of the Zn-alkyl sites
with narrow molecular weight distributions (([PS growth sites]
ꢁ [Li])/[Zn], 2.12 and 1.73; Mw/Mn, 1.32 and 1.29; entries 5 and
6). However, growth of the PS chain from the zinc sites was not
very effective when the [Li]/[Zn] ratio was high at 2.0 (([PS growth
sites] ꢁ [Li])/[Zn] ¼ 1.23) (entry 7).
Fig. 1 1H NMR spectra for the reactions of a-methylstyrene with
nBuLi(tmeda) (a) and nBuLi(tmeda)2 in the presence of (hexyl)2Zn (b).
adduct, etc., were not generated; that is, a-methylstyrene can
only be inserted successively twice, but further insertion of
three or more units is not allowed. These observations agree
with the fact that homopolymerization of a-methylstyrene is not
feasible, especially at a high temperature due to its low ceiling
temperature (Tc, 66 ꢀC),49–51 and that the maximum a-methyl-
styrene content attainable in a-methylstyrene/styrene anionic
copolymerization is limited to ca. 70 wt% (i.e., 2 : 1 a-
methylstyrene/styrene mole ratio).52
Initiation of a-methylstyrene
For success of the present strategy, the alkyllithium species
should be able to attack the a-methylstyrene moieties in the
presence of dialkylzinc species. Disappointingly, the reactivity
of tert-BuLi, nBuLi(tmeda), and p-MeC6H4CH2Li(tmeda) toward
a-methylstyrene was extinguished in the presence of an equiv-
alent amount of (hexyl)2Zn; formation of Li–Zn aggregates may
reduce the reactivity. However, when [Li] > [Zn], consumption of
a-methylstyrene was observed, with generation of the charac-
teristic orange-red colour of the styryl anion. However, the
reaction rate was rather slow; only ꢂ50% consumption of a-
methylstyrene was observed at the reaction time of 30 min using
the feed ratio of [nBuLi(tmeda)] : [(hexyl)2Zn] : [a-methylstyr-
ene] ¼ 1.5 : 1 : 2. Addition of nBuLi(tmeda)2 instead of nBu-
Li(tmeda) leads to acceleration of the reaction rate and almost
all the a-methylstyrene was consumed within 30 min. In the 1H
It was reported that tert-BuLi reacts with a-methylstyrene in
aliphatic hydrocarbon solvent to form a 1 : 1 adduct (tBu–
CH2C(Ph)(Me)Li).48 Less reactive nBuLi does not react with a-
methylstyrene, but nBuLi(tmeda) rapidly attacks a-methylstyr-
ene; 1H NMR studies in deuterated cyclohexane indicate that
the 1 : 1 adduct (nBu–CH2C(Ph)(Me)Li) and the 1 : 2 adduct
(nBu–CH2C(Ph)(Me)–CH2C(Ph)(Me)Li)
were
generated
(Fig. 1(a)). The xylyllithium species p-MeC6H4CH2Li(tmeda),
which is generated cleanly through a-lithiation of p-xylene using
nBuLi(tmeda), also attacks a-methylstyrene. Analyses of the
generated organolithium species itself and the protonated
organic compounds (two spots in TLC studies) by 1D and 2D
NMR also indicated formation of the 1 : 1 and 1 : 2 adducts
(MeC6H4CH2–CH2C(Ph)(Me)Li
and
MeC6H4CH2–CH2-
C(Ph)(Me)–CH2C(Ph)(Me)Li) (see ESI†). Higher adducts, 1 : 3
Table 1 Anionic styrene polymerization in the presence of (hexyl)2Zna
Entry
Initiator
[Li]/[Zn]
Timeb (h)
Mnc (Da)
Mw/Mn
([PS growth sites] ꢁ [Li])/[Zn]d
1
2
3
4
5
6
7
nBuLi(tmeda)
nBuLi(tmeda)
nBuLi(tmeda)
nBuLi(tmeda)
nBuLi(tmeda)2
nBuLi(tmeda)2
nBuLi(tmeda)2
0.20
1.0
1.5
2.0
1.0
1.5
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
25 100
18 000
15 000
13 600
16 700
16 100
16 100
1.52
1.26
1.27
1.23
1.32
1.29
1.18
1.87
1.89
1.97
1.82
2.12
1.73
1.23
a
Polymerization conditions: styrene in methylcyclohexane (25 wt%, 2.5 g, 24 mmol), (hexyl)2Zn (11.3 mg, 0.048 mmol, [styrene]/[Zn] ¼ 500), 90 ꢀC.
b
c
d
Time not optimized for full conversion. Measured by GPC at 40 ꢀC using THF eluent and PS-standards. Calculated as ([styrene]/[Zn]/DP ꢁ
[nBuLi(tmeda)])/[Zn], where DP ¼ Mn/104.
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Scheme 4 Reaction of nBuLi(tmeda)2 with a-methylstyrene.
NMR spectrum, only signals assignable to the 1 : 2 adduct (R– Attack of nBuLi(tmeda)2 on the a-methylstyrene moieties
CH2C(Ph)(Me)–CH2C(Ph)(Me)Li) were observed, with no that are attached to the end of PO chains results in formation
appearance of the signals assignable to the 1 : 1 adduct (R– of PS-b-PO-b-PS triblock units. In this one-pot process,
CH2C(Ph)(Me)Li) (Fig. 1(b) and Scheme 4). Aer quenching with a substantial amount of nBuLi(tmeda)2 is needed for the
acidic water, full analyses of the protonated organic compound reaction between nBuLi(tmeda)2 and a-methylstyrene (i.e., [Li]
(one spot in TLC analysis) using NMR and mass spectral data > [Al] + [Zn]) since some portion of the nBuLi(tmeda)2 fed into
further supported primary generation of 1 : 2 adduct attached the system remains unreacted with a-methylstyrene moieties
via either n-butyl or hexyl group (i.e., R ¼ nBu or hexyl in R– by becoming trapped by aggregation with the Al and Zn sites;
CH2C(Ph)(Me)–CH2C(Ph)(Me)Li) originating from nBuLi or consequently, formation of a substantial amount of PS-
(hexyl)2Zn, respectively (see ESI†). When styrene monomer was homopolymers is inevitable.
added to the generated the R–CH2C(Ph)(Me)–CH2C(Ph)(Me)Li
The PS homopolymer can be selectively extracted from the
species in the NMR cell, rapid and full consumption of styrene generated block copolymers using chloroform and acetone
monomer was observed, indicating the generated lithium (1 : 2 w/w). When the amount of nBuLi(tmeda)2 (300 mmol) was
species acts well as an initiator for the styrene anionic less than ‘[Zn] + [Al]’ (i.e., 200 + 200 ¼ 400 mmol), nBuLi(tmeda)2
polymerization.
did not attack the a-methylstyrene moieties, as indicated by the
lack of appearance of the characteristic orange colour of the
styryl anion. The number of homo-PS growth sites calculated as
‘(homo-PS weight)/PS-Mn’ was 310 mmol, which is consistent
with the amount of nBuLi(tmeda)2 fed into the system. This
agreement indicates that PO-b-PS diblock copolymers were
generated and the a-methylstyrene moieties did not participate
in the anionic styrene polymerization (entry 1). However, when
the amount of nBuLi(tmeda)2 fed into the system was 500 mmol,
which exceeds the sum of [Zn] and [Al] (i.e., [Li] > [Zn] + [Al]), the
colorless solution became red-orange, indicating generation of
the styryl anion via attack of nBuLi(tmeda)2 on the a-methyl-
styrene moieties; the homo-PS growth sites (320 mmol) were less
than the amount of nBuLi(tmeda)2 used as feed (500 mmol)
(entry 3). When the zinc species do not bear a-methylstyrene
moieties, i.e., when (benzyl)2Zn was used instead of [4-(iso-
propenyl)benzyl]2Zn, the quantity of homo-PS growth sites (510
mmol) again agreed well with the amount of nBuLi(tmeda)2 used
as feed (500 mmol) (entry 4). In all other cases where [Li] > [Zn] +
[Al], some portion of the fed lithium species reacted with the a-
methylstyrene moieties, and the quantity of homo-PS growth
sites was less than the amount of the lithium species used as
feed (entries 5, 7, and 12). As the amount of nBuLi(tmeda)2 fed
into the system increased from 300 to 400, and 500 mmol, the
polydispersity (PDI, Mw/Mn) of the extracted homo-PS gradually
decreased from 1.82 to 1.66 and 1.46; however, the PDI
increased (1.66) with a further increase of the nBuLi(tmeda)2
feed to 600 mmol (entries 1–3 and 5). In all cases, the polymer-
ization rate was fairly high and the styrene monomers were
completely consumed within 1.5 h.
Preparation of multiblock copolymers
Based on the aforementioned background studies, preparation
of multiblock copolymers containing PS-b-PO-b-PS units was
attempted (Table 2). The olen polymerization was performed
using
a
typical ansa-metallocene catalyst (rac-[Me2Si(2-
methylindenyl)2]ZrCl2) activated with MMAO, which was re-
ported to be effective in the CCTP with suppression of the
undesirable b-elimination process.15,53,54 Olen polymerization
was performed by feeding ethylene gas into the system con-
taining 1-octene or 1-pentene monomer. The catalyst worked
efficiently, leading to consumption of almost all of the a-olen
monomers, once the zinc species [4-(isopropenyl)benzyl]2Zn is
sufficiently pure. The molecular weights of the generated POs
(Mw, 115–147 kDa) were similar to the Mw (140 kDa) observed for
PO generated in the presence of (benzyl)2Zn, indicating that [4-
(isopropenyl)benzyl]2Zn works as effectively as (benzyl)2Zn in
the CCTP while the a-methylstyrene moieties do not interfere in
the olen polymerization (entry 4 versus others).
Anionic polymerization of styrene was subsequently per-
formed in one-pot by successive feeding of nBuLi(tmeda)2 and
styrene monomer into the system with a lag time of 1 h. The
time lag was introduced to allow nBuLi(tmeda)2 to attack the
a-methylstyrene moieties fully. nBuLi forms 1 : 1 aggregates
with alkylaluminum species (MMAO), preventing initiation of
the anionic polymerization when [Li] < [Al]. However, when
the [Li]/[Al] ratio exceeds 1.0, styrene polymerization is initi-
ated, but the PS-chains grow from all of nBuLi fed into the
system, including nBuLi units trapped by alkylaluminum
species and dialkylzinc sites.55 PS chains grown from nBuLi
form the undesired PS homopolymer (homo-PS), while those
grown from Zn sites form the desired block copolymers.
The fraction of extracted homo-PS declined when a small
amount of divinylbenzene (0.25–1.0 mol% per styrene), which
may connect the homo-PS chains with the block copolymer
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Table 2 Data for preparation of PS-b-PO-b-PS multiblock copolymersa
Consumed monomers
Homo-PS
growth sitese
(mmol)
PO-Mw
(PDI)f
(kDa)
Block
RLi(tmeda)2
1-Alkene
(g)
C2H4
(g)
Styreneb
(g)
Homo-PSc
(g; %)
PS-Mn
copolymer-Mw
Entry
(mmol)
(PDI)d (kDa)
(PDI)f (kDa)
1
2
nBu; 300
nBu; 400
nBu; 500
nBu; 500
nBu; 600
Benzyl; 400
Benzyl; 500
nBu; 500
nBu; 500
nBu; 500
nBu; 500
nBu; 500
nBu; 500
nBu; 500
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C8; 10.0
C5; 10.0
C5; 10.0
C5; 10.0
15.4
19.0
18.4
17.4
18.0
16.7
20.1
19.3
18.3
19.1
19.8
15.8
16.2
19.0
10.4
10.4
10.4
10.4
10.4
10.4
10.4
10.4 (0.25)
10.4 (0.50)
10.4 (0.75)
10.4 (1.0)
10.4
4.4 (42)
4.6 (44)
4.6 (45)
5.5 (53)
5.7 (55)
5.4 (52)
5.0 (48)
3.1 (30)
3.1 (29)
2.0 (19)
1.5 (14)
5.0 (48)
3.7 (36)
2.8 (27)
14.3 (1.82)
11.4 (1.66)
14.5 (1.46)
10.7 (1.65)
11.2 (1.66)
12.0 (1.56)
14.3 (1.65)
16.1 (1.54)
11.3 (1.79)
11.1 (1.56)
10.1 (1.66)
14.5 (1.51)
14.8 (1.74)
11.5 (1.78)
310
400
320
510
510
450
350
129 (4.06)
135 (4.57)
115 (3.02)
140 (3.55)
134 (3.76)
133 (3.33)
142 (3.61)
147 (2.96)
132 (3.15)
136 (3.09)
130 (3.05)
119 (3.65)
126 (3.22)
129 (2.92)
143 (2.60)
150 (2.92)
145 (2.51)
148 (3.37)
155 (2.68)
138 (2.86)
154 (3.19)
162 (3.68)
157 (2.54)
146 (2.42)
155 (3.28)
134 (2.98)
140 (3.20)
153 (3.19)
3
4g
5
6
7
8
9
10
11
12
13
14
340
10.4 (0.25)
10.4 (0.50)
a
Polymerization conditions: rac-[Me2Si(2-methylindenyl)2]ZrCl2 (1.0 mmol), MMAO (200 mmol-Al), initial ethylene charge at 30 bar and then
regulation at 10–20 bar, 120 ꢀC, 20 min for the rst step CCTP; [styrene]/[Zn] ¼ 500, 120–130 ꢀC, 1.5 h (complete conversion of styrene) in the
b
c
second step anionic polymerization. Values in parentheses indicate mol% of additional divinylbenzene per styrene. Filtrate portion extracted
d
with acetone and chloroform (2 : 1 weight ratio); percentage value ¼ (extracted PS weight)/(consumed styrene weight). Measured with GPC at
e
f
40 ꢀC eluting with THF using PS-standards. (Homo-PS weight)/PS-Mn. Measured with GPC at 160 ꢀC eluting with 1,2,4-trichlorobenzene using
PS-standards. g (Benzyl)2Zn was used instead of [4-(isopropenyl)benzyl]2Zn.
chains during the course of anionic polymerization, was used as When the amount of nBuLi was 500 mmol to meet the criterion
the feed. In fact, by feeding divinylbenzene in quantities of 0.25, ‘[Li] > [Al] + [Zn]’, where PO–PS multiblock copolymers are
0.50, 0.75, and 1.0 mol%, the fraction of homo-PS fraction generated by the attack of nBuLi on the a-methylstyrene moie-
gradually declined from 45% to 30%, 29%, 19%, and 14%, ties (entry 3), DMw was signicant, i.e., 30 kDa, which is almost
respectively (entries 8–11). The extracted homo-PS Mn values double of the Mn value obtained for homo-PS (14.5 kDa). In the
gradually decreased from 16.1 to 11.3, 11.1, and 10.1 kDa with GPC curves plotted in the log(molecular weight) scale (Fig. 2(b)),
an increase of the amount of divinylbenzene from 0.25 mol% to the shi is substantial at both the low molecular and the high
0.50, 0.75, and 1.0 mol%.
Formation of the block copolymers was evident from
comparison of the molecular weights of the samples before and
aer anionic polymerization (i.e., PO-Mw and block copolymer-
Mw). The refractive index (RI) detector response of the GPC
instrument is opposite for the PO and PS samples (see ESI†),
which complicates the analyses, but the retention time should
be shortened by attaching the PS-blocks, which leads to
increase of the molecular weight, even though the actual value
of the increment may not be fully reliable. When [Li] was less
than ‘[Al] + [Zn]’, where PO-b-PS diblock copolymers are the
main species generated, the Mw increased from 129 to 143 kDa
aer anionic polymerization (entry 1); the increment of the Mw
values (DMw) was 14 kDa, which is almost similar to the
measured molecular weight of homo-PS (Mn, 14.3 kDa). In the
GPC curves plotted in the log(molecular weight) scale (Fig. 2(a)),
the shi is substantial at the low molecular end while it is
insubstantial at the high molecular weight end, and the
molecular weight distribution is narrowed aer the anion
polymerization (Mw/Mn, 2.60 versus 4.06). In the step of anion
polymerization, the molecular weight distribution of PS fraction
is not as narrow as that in the living polymerization (Mw/Mn
1.82) and some low molecular weight portion below 10 kDa is Fig. 2 GPC curves before and after the anionic polymerization for the
generation of PO–PS diblock copolymer (a, entry 1 in Table 2) and
multiblock copolymer (b, entry 3 in Table 2).
observed in the GPC curve of the generated block copolymer.
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molecular weight ends, which suggests that the generated
copolymer is not a simple triblock but, instead, a multiblock
copolymer. In contrast, DMw was marginal (8 kDa) when
(benzyl)2Zn was used instead of [4-(isopropenyl)benzyl]2Zn
under the identical conditions; the a-methylstyrene moieties
must play a role in obtaining the high DMw value. The DMw
values were relatively small (5 or 12 kDa) when benzyl-
Li(tmeda)2 was fed as an initiator in amounts of 400 and 500
mmol; benzyl-Li(tmeda)2 is not as good an initiator as
nBuLi(tmeda)2 for successful execution of the strategy. When
divinylbenzene is used, much higher DMw values are expected
due to connection of the growing block copolymer chains;
however, the observed DMw values were not so very high (10–25
kDa). The polymerization solution turned turbid at an early
stage of the anionic polymerization, indicating that the gener-
ated block copolymers are insoluble and form micelles. In this
situation, connection between the growing block copolymer
chains may not be feasible. In the case of ethylene/1-pentene
copolymerization, the molecular weight also increased aer
anionic polymerization (DMw, 14–24 entries 12–14). In most
cases, the molecular weight distributions became narrow aer
anionic polymerization.
Fig. 4 Plots of hysteresis experiment for the PO–PS diblock copol-
ymer (a, entry 1 in Table 2) and PO–PS multiblock copolymer (b, entry
3 in Table 2). Ten cycles at 200% strain were performed.
Formation of the block copolymers was evident from inves-
tigation of the TEM images of the thin lms aer staining with
RuO4. Because PS is immiscible with PO, phase separation
occurs and the PS domains selectively stained by RuO4 are
clearly seen as a dark image. In contrast with the image of the
PO–PS blend (Fig. 3(a)), the PO-b-PS diblock copolymers
generated under the condition [Li] < [Al] + [Zn] (entry 1) formed
spherical PS domains with an average 40 nm; these domains
were regularly distributed in the PO matrix (Fig. 3(b)). However,
the shape of the PS domains was rather blurred and irregular for
the multiblock copolymers generated by the attack of
nBuLi(tmeda)2 on the a-methylstyrene moieties under the
condition [Li] > [Al] + [Zn] (Fig. 3(c)). When divinylbenzene was
fed in the system, the shape of the PS domains became much
more blurred and irregular. Specically, large domains were
observed when the amount of divinylbenzene feed was high
(0.50 and 0.75 mol% per styrene) (Fig. 3(e) and (f)). The PS-
domains are very regularly distributed with uniform sizes in
the case of the related triblock copolymer SBS.56 Generation of
multiblock instead of triblock copolymers may deteriorate the
regularity of the phase separation of PS and PO blocks.
The polymer samples were compressed between hot plates at
110 ꢀC to make lms with 400–500 mm thickness and the lms,
aer cutting with 100 ꢃ 10 mm2 size, were subjected to
hysteresis testing where each sample was extended to 200%
strain over 10 cycles to determine elastic recovery. Both the PO–
PS diblock (entry 1 in Table 2) and multiblock copolymers (entry
3 in Table 2) exhibit the elastomeric property; the rst cycle
results in the most signicant amount of permanent deforma-
tion, followed by minimal deformation on subsequent cycles
(Fig. 4). However, the multiblock copolymer shows better elas-
tomeric property than the diblock analogue; the permanent
deformation of the multiblock copolymer is signicantly less
than that of the diblock analogue.
Conclusion
Dialkylzinc bearing an a-methylstyrene moiety (i.e., [4-(iso-
propenyl)benzyl]2Zn) was prepared for the use in so-called ‘coor-
dinative chain transfer polymerization (CCTP)’ using a typical
ansa-metallocene catalyst, rac-[Me2Si(2-methylindenyl)2]ZrCl2
activated with modied-methylaluminoxane (MMAO) to grow PO
Fig. 3 TEM images (1500 nm ꢃ 1500 nm) of thin films prepared using
a blend of poly(ethylene/1-octene) and PS (a) and PO–PS block
copolymers (entry 1 (b), entry 3 (c), entry 8 (d), entry 9 (e), entry 10 (f) in
Table 2).
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chains attached to zinc at one end and bearing the a-methyl- Celite. ZnCl2 (2.70 g, 18.0 mmol) was subsequently added to the
styrene moiety at the other end. Switching to anionic polymeri- ltrate. The white solids that precipitated aer stirring for three
zation by adding nBuLi(tmeda)2 in an amount to meet the days were removed by ltration over Celite. The solvent was
criterion [Li] > [Zn] + [Al in MMAO] with subsequent addition of removed under vacuum to yield a yellowish residue, which was
the styrene monomer enabled growth of PS chains from the Zn- dissolved in methylcyclohexane (200 g). Aer removal of the
alkyl sites and a-methylstyrene moieties, thereby generating PS- residual solid by ltration, the white solid was isolated by
b-PO-b-PS triblock units. A PS growth site was generated from the evaporation of the solvent under vacuum (5.31 g, 90%); the solid
a-methylstyrene moiety by the attack of nBuLi(tmeda)2 and the PS was pure based on 1H and 13C NMR spectral analyses. For use in
growth sites on the other side were generated from the Zn-alkyl polymerization as a chain transfer agent, the isolated product
sites via alkyl exchange between the zinc and lithium species. was recrystallized from hexane at ꢁ30 ꢀC two additional times.
Formation of a substantial amount of PS homopolymers grown 1H NMR (400 MHz, C6D6) : d 7.36 (d, J ¼ 8.0 Hz, 4H), 6.86 (d, J ¼
from nBuLi(tmeda)2 was inevitable (40–50%). The amount of 8.0 Hz, 4H), 5.44 (s, 2H), 5.00 (s, 2H), 2.04 (s, 6H), 1.55 (s,
homo-PS was reduced due to the consumption of some 4H) ppm. 13C{1H} NMR (C6D6): d 144.21, 143.42, 127.24, 126.19,
nBuLi(tmeda)2 by the reaction with a-methylstyrene moieties and 110.63, 23.91, 22.28 ppm.
was further signicantly reduced by up to 14% upon addition of
a small amount of divinylbenzene (0.25–1.0 mol% per styrene).
The molecular weights increased aer anionic polymerization,
supporting formation of the block copolymers. However, in the
model reaction of nBuLi(tmeda)2 with a-methylstyrene in the
presence of (hexyl)2Zn, the 1 : 2 adduct, R–CH2C(Ph)(Me)–CH2-
C(Ph)(Me)Li, where R is either n-butyl originating from nBuLi or
hexyl from (hexyl)2Zn, was generated. This observation suggests
that simple PS-b-PO-b-PS triblock copolymers were not generated,
but, instead, multiblock copolymers containing PS-b-PO-b-PS
units might be formed. Generation of multiblock beyond triblock
deteriorates the regularity of the phase separation of PS and PO
blocks.
Typical procedure for anionic polymerization of styrene in the
presence of (hexyl)2Zn (entry 5 in Table 1)
nBuLi (3.07 mg, 48 mmol) and tmeda (11.2 mg, 96 mmol) were
successfully added to a ask containing (hexyl)2Zn (11.3 mg, 48
mmol) and methylcyclohexane (1.0 g) inside a glove box. Styrene
(2.5 g, 24 mmol) dissolved in methylcyclohexane (6.5 g)ꢀwas added
and the anionic polymerization was performed at 90 C for 2 h.
Complete conversion of the styrene monomer was evident from
the analysis of the 1H NMR spectrum of the sample. Aqueous HCl
(2 N, 0.3 mL) was added and the resulting solution was stirred for
30 min at 90 ꢀC to destroy the zinc species. The solution was
ltered through a short pad of silica gel that was thoroughly
washed with toluene. Toluene was removed by using a rotary
evaporator to isolate PS; the isolated sample was dried in
Experimental section
General remarks
ꢀ
a vacuum oven at 130 C for 1 h. The weight of isolated PS was
identical to that of the styrene monomer fed into the system.
All manipulations were performed under an inert atmosphere
using a standard glove box and Schlenk techniques. Methyl-
cyclohexane and 1-octene were purchased from Sigma-Aldrich
and puried over a Na/K alloy. Ethylene was puried by contact
with molecular sieves and copper at 40 bar pressure. Styrene was
purchased from Sigma-Aldrich. N,N,N0,N0-Tetramethylethylenedi-
amine (tmeda) was purchased from Sigma-Aldrich and puried
over CaH2. The 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were recorded using a Varian Mercury Plus 400 instru-
ment. The gel permeation chromatograms (GPC) were obtained in
Typical procedure for synthesis of PO–PS block copolymers
(entry 3 in Table 2)
A bomb reactor (125 mL) was charged with a solution of MMAO
(AkzoNobel, 7.0 wt%-Al in heptane, 77 mg, 200 mmol-Al) in
methylcyclohexane (17.0 g). Aer stirring for 1 h at 100 ꢀC using
a mantle, the solution was removed using a cannula. This
washing procedure was performed to clean up any catalyst
poisons with the MMAO solution. The reactor was again
charged with a solution of 1-octene (10.0 g) and [(4-isopropenyl)
benzyl]2Zn (65.6 mg, 0.200 mmol) in methylcyclohexane (17.0 g)
under an inert atmosphere. The catalyst solution was prepared
by reacting rac-[Me2Si(2-methylindenyl)2]ZrCl2 (0.48 mg, 1.0
mmol) and MMAO (77 mg, 200 mmol-Al) in methylcyclohexane
(2.0 g) for 40 min. Aer injection of the catalyst solution with
ꢀ
THF at 40 C using a Waters Millennium apparatus or in 1,2,4-
ꢀ
trichlorobenzene at 160 C using a PL-GPC 220 system at Korea
Polymer Testing & Research Institute. Transmission electron
microscopes (TEM) images were recorded on an FEI Tecnai G2
F30 S-Twin instrument. Differential scanning calorimetry (DSC)
data were collected from the second heating, performed at
a heating rate of 10 ꢀC minꢁ1 using a Thermal Analysis Q10
instrument. 1-Chloromethyl-4-isopropenylbenzene was prepared
by the method reported in the literature.44
ꢀ
a syringe at 60 C, ethylene gas was immediately charged into
the system at 30 bar. Even though the reactor was cooled in an
ice bath, the temperature increased to ꢂ130 ꢀC within 5 min
due to the exothermic nature and the pressure gradually
decreased due to the consumption of ethylene. Whenever the
pressure reached 10 bar, ethylene gas was fed to a pressure of 15
Synthesis of [4-(isopropenyl)benzyl]2Zn
1-Chloromethyl-4-isopropenylbenzene (6.00 g, 36.0 mmol) in bar and the temperature was self-controlled within the range of
diethyl ether (20 mL) was added dropwise to a ask containing 120–130 ꢀC. Twenty minutes aer injection of ethylene gas, the
magnesium (1.31 g, 54.0 mmol) in diethyl ether (40 mL). Aer stirring rate was decreased from 300 to 15 rpm by formation of
stirring for 4 h, the remaining Mg was removed by ltration over
a
thick viscose solution. The olen polymerization was
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terminated by venting the remaining ethylene gas and an
aliquot of the sample was taken to measure the molecular
weight of the generated PO. When the ethylene feed was
disconnected, the temperature decreased. When the tempera-
ture reached 90 ꢀC, a solution of nBuLi(tmeda)2 prepared by
mixing nBuLi (32.0 mg, 0.500 mmol) and tmeda (116.2 mg, 1.00
mmol) in methylcyclohexane (2.0 g) was added to the reactor.
The temperature was increased to 120 ꢀC by heating with
a mantle for 1 h under stirring, aer which styrene (10.4 g, 100
mmol) was injected. The temperature was controlled in the
range of 100–130 ꢀC using a mantle. The viscosity gradually
increased to reach an almost unstirrable state aer 1.5 h. Full
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