Thiol-end-functionalized polyethylenes

Article abstract of DOI:10.1021/ma101265t

Synthesis of well-defined and highly thiol-end-functionalized polyethylene (PE-SH) was performed using olefin polymerization by coordination chemistry and robust functionalization chemistries. Using a Nd-based catalyst in combination with n-butyloctylmagnesium (BOMg), dipolyethylenylmagnesium compounds (PE-Mg-PE) were first prepared by catalyzed polyethylene chain growth on magnesium. Taking advantage of the reactivity of the carbon-magnesium bonds so formed, several functionalization strategies were assessed with the main aim of producing the targeted PE-SH in the simplest and most efficient way. In particular, polysulfurs and thiothiocarbonylated-end-functionalized polyethylenes were successfully obtained and reduced by lithium aluminum hydride (LiAlH ₄) to give the desired PE with up to 90% thiol chain end functionality.

Full text of DOI:10.1021/ma101265t

Macromolecules 2010, 43, 7495–7503 7495
DOI: 10.1021/ma101265t
Thiol-End-Functionalized Polyethylenes
†
Jerome Mazzolini, Ilham Mokthari, Remi Briquel, Olivier Boyron, Frederic Delolme,
†
†
†
‡
ꢀ ^
ꢀ
ꢀ ꢀ
Vincent Monteil,† Denis Bertin,§ Didier Gigmes,§ Franck D’Agosto,*,†
and Christophe Boisson*,†
†
ꢁ
ꢀ ꢀ
ꢀ
Laboratoire de Chimie Catalyse Polymeres et Procedes (C2P2), Universite de Lyon, Univ. Lyon 1, CPE Lyon,
CNRS UMR 5265, Equipe LCPP Bat 308F, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne,
‡
ꢀ
ꢀ
France, Institut de Biologie et de Chimie des Proteines, CNRS UMR 5086 Universite Lyon 7,
Passage du Vercors, 69367 Lyon Cedex 7, France, and §Laboratoire Chimie Provence, UMR 6264,
ꢀ
ꢁ
Universites d’Aix-Marseille I, II et III-CNRS, Equipe Chimie Radicalaire Organique et Polymeres de
Speꢀcialiteꢀ, Case 542, Av. Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
Received June 7, 2010; Revised Manuscript Received August 12, 2010
ABSTRACT: Synthesis of well-defined and highly thiol-end-functionalized polyethylene (PE-SH) was
performed using olefin polymerization by coordination chemistry and robust functionalization chemistries.
Using a Nd-based catalyst in combination with n-butyloctylmagnesium (BOMg), dipolyethylenylmagnesium
compounds (PE-Mg-PE) were first prepared by catalyzed polyethylene chain growth on magnesium. Taking
advantage of the reactivity of the carbon-magnesium bonds so formed, several functionalization strategies
were assessed with the main aim of producing the targeted PE-SH in the simplest and most efficient way.
In particular, polysulfurs and thiothiocarbonylated-end-functionalized polyethylenes were successfully
obtained and reduced by lithium aluminum hydride (LiAlH4) to give the desired PE with up to 90% thiol
chain end functionality.
Introduction
participate. Thiol propionate, thiol glycolate, and alkyl thiols
have been reported for this chemistry. The marked and reported
The importance of polyolefins (principally polyethylenes (high
density (PEHD) and linear low density (LLDPE)) and isotactic
polypropylene (i-PP)) in the world of polymer thermoplastics
cannot be underestimated. Many of the properties associated
with this class of polymers cannot be achieved by other polymers
at a reasonable cost. Polyolefins exhibit such unique mechanical
properties that they remain unavoidable for many applications.
In addition to the favorable mechanical properties, the very high
productivity related to the catalytic feature of the polymerization
systems that produce polyolefins are attractive to industry. There
are however areas for which more expensive polymers or cost-
effective post-treatments of polyolefins are required to balance
their lack of polarity, adhesion, or compatibility with other mate-
rials. The possibility of tuning polyolefin properties to a wider
range of applications partially relies on the discovery of new
macromolecular architectures incorporating polyolefin segments.
Polyolefin building blocks in which functionalities have been easily
and efficiently introduced and that can further readily react with
other materials are in this context very promising macromolecules.
Descriptions of the use of thiols in chemical reactions extend
well over 100 years, encompassing a number of fields ranging
from biochemistry to polymer science. As an example, thiol can
advantageously be involved in reactions toward alkene function-
ality through a free radical process. This chemistry, better known
as thiol-ene free radical chemistry, dates from the late 1930s to
the early 1950s.1 In the polymer field, thiol-ene chemistry is
mainly known for the production of networks through thiol-ene
step growth polymerizations involving a reaction between multi-
functional thiol and ene (vinyl) monomers.2 The most salient
feature of thiol-ene chemistry is that almost any type of ene can
insensitivity of this chemistry to oxygen contamination together
with the versatility of free radical processes and their tolerance to
a large variety of functionalities makes this method an efficient
way to produce films and thermoset plastics industrially with
unprecedented physical and mechanical properties. Because of
these unique features, an extension of this chemistry has been
proposed in the particular field of polymer functionalization and
consists of a one-step modification of polymer precursors bearing
a vinyl group (ene) by free radical addition of ω-functional
mercaptans (thiols).3 This cheap and very robust strategy allows
for preparing functionalized commodity materials of added value.
Finally, thiol-ene chemistry is witnessing a renewal of interest
particularly in polymer science in the recent literature where its
power is discussed4 and advantageously used for polymer con-
jugations and macromolecular architecture design.5
The simplicity and the efficacy of this chemistry are two key
advantages when trying to functionalize polyolefin-based mate-
rials. As a result, appropriately functionalized polyolefins could
be perfect candidates as starting materials in the aforementioned
thiol-ene chemistry. As far as we know, only two recent papers
on the functionalization by a thiol containing silane of commer-
cial PP or PE carrying low loadings of ene end functionality have
been reported.6 Considering that the main-chain stopping event
occurring in an olefin polymerization by coordination chemistry
produces end unsaturation, polyolefins with high content of ene
functionalities could be synthesized and efficiently functionalized
using thiol-containing molecules of interest. This strategy is being
researched in our laboratories. In the same vein, the introduction
of a thiol end group on a polyolefin is also an appealing tech-
nique. This, however, requires identification of a catalytic olefin
polymerization system that would allow high thiol functionalization
rates utilizing simple chemistry.
*Corresponding authors. E-mail: dagosto@lcpp.cpe.fr (F.D.); boisson@
lcpp.cpe.fr (C.B.).
r 2010 American Chemical Society
Published on Web 09/01/2010
pubs.acs.org/Macromolecules

7496 Macromolecules, Vol. 43, No. 18, 2010
Mazzolini et al.
Scheme 1. Catalyzed Chain Growth (CCG) Concept in Catalytic Ethylene Polymerization by Coordination Chemistry Using a Main Group (M0)
Organometallic Species as Reversible Chain Transfer Agent (CTA) and a Transition Metal (M) Catalyst
The catalyzed polyolefin chain growth (CCG) on main group
metal is indeed a very powerful tool to end functionalize poly-
olefins. This polymerization system is depicted in Scheme 1 in the
case of ethylene and relies on the use of a catalyst based on a
transition metal (M) in conjunction with an efficient chain transfer
agent based on a main group metal (M0). A rapid and reversible
chain transfer is operating between these two metal centers. In the
absence of any irreversible chain stopping events, the system is
turned into a pseudoliving polymerization of ethylene.7 This is
indeed the only way to control the polymerization of ethylene
under true catalytic conditions and is conceptually similar to
reversible addition-fragmentation chain transfer (RAFT) in the
controlled radical polymerization area.8 The reactivity of poly-
olefins formed via CCG is useful since all the chains are linked to
a metal. One can thus take advantage of the reactivity of the end
metal-carbon bond to introduce a variety of functional or reactive
end groups. We recently showed that catalyzed polyethylene chain
growth on magnesium can be used to very efficiently and easily
introduce end functional groups in polyethylene chains9 such as
iodine, azide, amines, (meth)acrylate, alkoxyamines, and thio-
carbonylthio groups.10
standards, Sequazyme kit). The samples were deposited on a
stainless steel target and dried. Samples were prepared by
dissolving the polymer in freshly distilled tetrahydrofuran
(THF) at a concentration of 10 g L-1. The matrix was used
without further purification and dissolved in distilled THF
(10 g L-1). The samples were prepared by mixing a solution of
polymer in THF (10 mg mL-1) and a solution of matrix in THF
(20 mg mL-1), with a volumetric ratio in the ratio 1/10 (Vsample/
Vmatrix). 1 mL of the resulting mixture was placed onto the
sample plate, and the solvent was evaporated at room temperature.
Synthesis of (C5Me5)2NdCl2Li(OEt2)2. The neodymocene-
complex was synthesized as described in the literature.11
Polymerization Procedure: Synthesis of PE-Mg-PE. Ethylene
polymerization was carried out in toluene in a 500 mL reactor
connected to an ethylene reservoir equipped with a pressure
sensor. The reactor was charged with a mixture of the chlor-
oneodymocene complex (C5Me5)2NdCl2Li(OEt2)2 (8.3 ꢀ 10-5
mol L-1) and butyloctylmagnesium (BOMg) in a 400 mL
toluene solution. The molar ratio Mg/Nd was adjusted with
respect to the targeted molar mass and the concentration of the
polymer in the reactor (Mn = 1000 g mol-1, Mg/Nd = 265;
Mn = 2000 g mol-1, Mg/Nd = 200). The solution was rapidly
set to the polymerization temperature (80 °C) and then to an
ethylene pressure of 3 bar. The consumption rate of ethylene was
measured from the observed drop of pressure in the reservoir.
The reaction was assumed to reach completion when the
amount of ethylene consumed corresponds to the expected
amount of polymer according to the targeted polymerization
degree (DPn). The latter is calculated using the equation DPn =
We report here on our investigations to introduce thiol end
groups on polyethylene chains using CCG.
Experimental Section
Materials. Toluene (Acros Organics, 99%) was dried over
˚
activated 3 A molecular sieve (SDS) under argon bubbling.
Tetrahydrofuran (THF, Fisher) was distilled over sodium/benzo-
phenone. Elemental sulfur (S8, Aldrich), n-butyloctylmagnesium
20 wt % solution in n-heptane (BOMg, Crompton), iodine (I2,
99.5%, Aldrich), N,N- dimethylformamide (DMF, Fisher Scientific,
99,9%), diethyl ether (Aldrich), potassium xanthic acid salt
(KSC(=S)OEt, 97%, ACROS), tetraethylthiuram disulfide
(97%, Aldrich), sodium hydrogenosulfide (NaSH, Aldrich), lithium
aluminum hydride (LiAlH4, 95%, Aldrich), and magnesium sulfate
(MgSO4, 99.5%, Aldrich) were purchased and used as received.
Analytical Techniques. High-resolution liquid 1H and 13C
NMR analyses were performed on a Bruker DRX 400 spectro-
meter. Spectra were obtained with a 5 mm QNP probe at 363 K.
A mixture of tetrachloroethylene (TCE) and perdeuterioben-
zene (C6D6) (2/1 v/v) was used as solvent. Chemical shift values
(δ) are given in ppm in reference to the solvent residual peak
(C6H6, 7.16 ppm).
High-temperature SEC analyses were performed using a Waters
Alliance GPCV 2000 chromatograph equipped with three
columns (two Styragel HT6E and one Styragel HT2). Samples
(1 mg mL-1) were eluted with trichlorobenzene with a flow rate
of 1 mL min-1 at 150 °C. Online detection was performed by
refractometric and viscosimetric measurements using Waters
equipments with polyethylene standards (395, 750, 1110, and
2155 g mol-1).
Nethylene/(2NCTA), where Nethylene is the number of moles of
ethylene consumed and NCTA is the number of moles of chain
transfer agent (CTA) used, i.e., BOMg.
Synthesis of Polysulfur (PE-Sk-PE) (1). Once ethylene poly-
merization was assumed to have reached completion, the reactor
was degassed with argon. A solution containing elemental sulfur
(4-8 mol equiv of S compared to PE-Mg-PE, in 50 mL of THF)
was added, and the resulting mixture was stirred under argon at
80 °C for 1 h. Then, the reactor was cooled to room temperature,
the mixture was poured into a large amount of methanol, and
the polymer was collected by filtration. After washing three
times with 200 mL of methanol, the precipitate was dried under
reduced pressure. A polysulfur PE-Sk-PE was recovered as a fine
light yellow powder. 1H NMR 400 MHz (TCE/C6D6, 363 K), δ:
0.85 (t, CH3-CH2n), 1.30 (br, (-CH2-CH2)n-), 1.45 (quint,
CH2-CH2-S), 1.65 (quint, CH2-CH2-Sk>1), 2.40 (t, CH2-S),
2.60 (t, CH2-S-S), 2.78 (t, CH2-S3), 2.85 (m, CH2-Sk>3).
Synthesis of Bis-O-ethyl Xanthate. An aqueous iodine solu-
tion (18.72 mL, 18.72 mmol) was added dropwise to potassium
xanthic acid salt (5.0 g, 31.2 mmol) dissolved in water (50 mL)
with the immediate formation of a bright yellow precipitate. The
mixture was extracted with diethyl ether (3 ꢀ 100 mL). The
organic extracts were dried with MgSO4, filtered, and concen-
trated to afford 2.83 g (75%) of a light yellow oil.
MALDI-TOF mass spectrometry analyses were recorded in
the linear and the reflectron mode using an Applied Biosystems
Voyager DE-STR equipment using a nitrogen laser (wavelength
337 nm) and an accelerating voltage of 20 kV. The positive ions
were detected in all cases. The spectra were the sum of 200 shots,
and an external mass calibration was used (mixture of peptides
1H NMR (CDCl3, 298 K), δ: 1.45 (6H, t, CH3-CH2); 4.69
(4H, q, CH2-O).
Synthesis of Iodine-End-Functionalized Polyethylene (PE-I)
(2). Once ethylene polymerization was assumed to have reached
completion, the reactor was degassed with argon. After cooling
to room temperature, an iodine solution (4 mol equiv of I2

Article
Macromolecules, Vol. 43, No. 18, 2010 7497
Scheme 2. Various Strategies Employed To Synthesize PE-SH Using Catalyzed Polyethylene Chain Growth on Magnesium
Scheme 3
compared to PE-Mg-PE, in 50 mL of THF) was added, and the
resulting mixture was stirred under argon at 20 °C for 2 h. The
mixture was then poured into a large amount of methanol, and
the polymer was collected by filtration. After washing three
times with 200 mL of methanol, the precipitate was dried under
reduced pressure for 10 h. A fine white powder was then
1
recovered. H NMR 400 MHz (TCE/C6D6, 363 K), δ: 0.85 (t,
CH3-CH2-); 1.30 (br, CH2-CH2)n-); 1.66 (m, CH2-CH2-I);
13C NMR 100.58 MHz (TCE/C6D6, 363 K), δ: 14.1 (CH3-CH2),
22.9 (CH3-CH2), 24.6 (CH2-SH), 28.7 (CH2-CH2-CH2-
SH), 29.4 (CH3-CH2-CH2-CH2), 29.7 (CH2-CH2-CH2-
CH2-SH), 30.0 ((CH2)n), 32.3 (CH3-CH2-CH2), 34.4 (CH2-
CH2-SH). MALDI-ToF MS: m/z(C78H158S, M=1128gmol-1);
[MH]þ = 1128.0.
Synthesis of Thiol-End-Functionalized Polyethylene (PE-SH)
4b. A solution of PE-I in toluene was heated up to 100 °C, and a
solution of NaSH (10 equiv) in DMF (1 mL) was added. After
90 min of reaction at 100 °C, the resulting mixture was cooled
down slowly to room temperature and filtered, and the product
washed with methanol and dried under vacuum.
2.95 (t, CH2-I).
Synthesis of Thiothiocarbonylated End-Functionalized Poly-
ethylene (PE-SC(dS)-X) (3a, 3b). Once ethylene polymeriza-
tion was assumed to have reached completion, the reactor was
degassed with argon. Then, a solution of XC(S)S-S(S)CX
disulfide (molar ratio XC(S)S-S(S)CX/PE-Mg-PE = 2:1) in
toluene was added, and the resulting mixture was stirred at 80 °C
for 2 h. The final polymer was collected by precipitation into
methanol, washed with methanol, and dried under vacuum at
room temperature for 10 h.
3a: PE-SC(dS)-OEt: 1H NMR 400 MHz (TCE/C6D6, 363
K), δ: 0.85 (t, CH3-CH2-); 1.30 (br, CH2-CH2)n-); 1.57 (m,
CH2-CH2-S); 3.05 (t, CH2-S); 4.51 (q, CH2-O).
1
3b: PE-SC(dS)-N(Et)2: H NMR 400 MHz (TCE/C6D6,
Results and Discussion
363 K), δ: 0.85 (t, CH3-CH2-); 1.08 (t, CH3-CH2-N); 1.30
(br, CH2-CH2)n-); 1.66 (m, CH2-CH2-S); 3.28 (t, CH2-S);
3.69 (q, CH2-N).
The introduction of a thiol end group onto polyethylene is an
interesting end functionalization since it has not been reported as
far as we know for polyethylene. In addition, the attractiveness of
the resulting material relies on the reactivity of the introduced end
group that could further be used in various fields.
PE-SC(dS)-OEt (3a) was also obtained by an alternative
strategy. A solution of PE-I in toluene was heated up to 90 °C.
Solution of potassium xanthic salt (2 equiv) in DMF was added
to the previous solution. After 2 h of reaction at 90 °C, the
resulting mixture was slowly cooled down to room temperature
and filtered, and the product was washed with methanol and
dried under vacuum. A fine white powder was then recovered.
Synthesis of Thiol-End-Functionalized Polyethylenes (PE-SH)
4a and 4c. A solution of PE-Sk-PE or PE-SC(dS)-X in toluene
(200 mL) was heated up to 90 °C. A solution of LiAlH4 (10
equiv) in THF (100 mL) was added to the previous solution.
After 4 h of reaction at 90 °C, the resulting mixture was filtrated
at 90 °C in order to remove the excess of LiAlH4 and aluminum
salts formed during the reaction. 300 mL of methanol was then
added to the filtrate in order to precipitate polyethylene. After
filtration and washing twice with methanol, the product was
dried under vacuum. A fine white powder was recovered.
The following analyses are given for PE-SH obtained in
experiment 11, Table 4: 1H NMR 400 MHz (TCE/C6D6, 363 K),
δ:0.85(3H,t,CH3-CH2n), 1.03 (1H, t, -SH),1.30(2nH,br,CH2n),
1.45 (4H, quint, CH2-CH2-SH), 2.33 (4H, t, CH2-SH).
The routes that have been investigated for that purpose are
summarized in Scheme 2.
They are all based on a first step that consists in Nd-catalyzed
polyethylene chain growth on magnesium. The resulting dipo-
lyethylenyl magnesium compounds (PE-Mg-PE) are then em-
ployed in various synthetic pathways as depicted below.
As mentioned in the Introduction, the chemical strategies
dealing with the functionalization of polyethylenes need to be
not only simple and efficient but also as cheap as possible. An old
industrial process of functionalization of ethyleneoligomers isthe
Alfol synthesis which provides fatty primary alcohols.12 In this
process, ethylene is oligomerized using trialkylaluminium under
harsh conditions(120 °C, 120-140bar). An oxidative work-up of
the Al-C bonds followed by hydrolysis provides the correspond-
ing alcohols. Other reported examples of oxidative work up of the
metal-C bonds that are produced during ethylene polymerization
allowed the synthesis of hydroxyl-end-functionalized PE.13,14

7498 Macromolecules, Vol. 43, No. 18, 2010
Mazzolini et al.
Table 1. Molar Mass Characteristics of Polyethylenes (1) Obtained after Addition of Elemental Sulfur on PE-Mg-PE and Corresponding
Functionalities
expt no. S/Mg Mna (g mol-1
)
Mnb (g mol-1
)
PDIb PE-Sat (%)c PE-Vt (%)c PE-S-PE (%)c PE-S-S-PE (%)c PE-Sk>2-PE (%)c
1
2
3
4
4
8
4
8
1000
1000
2000
2000
2280
2070
4420
3950
1.3
1.3
1.3
1.3
6.8
2.2
8.7
7.6
2.1
1.8
6.2
6.1
16.3
20.0
19.8
16.4
28.7
18.9
25.9
20.5
46.1
57.1
39.4
49.4
a Theoretical Mn of polyethylene chains. b Determined by high-temperature (150 °C) SEC with trichlorobenzene as eluent (1 mL min-1) with a
calibration with PE standards. c In mol % determined by 1H NMR analyses.
Figure 1. 1H NMR (C6D6/TCE 1/2 v/v, 400 MHz, 512 scans, 363 K) of a polyethylene obtained by addition of elemental sulfur before (black) (1) and
after reduction (blue) by LiAlH4 (4a). Starred peaks are solvent impurities.
Scheme 4
On the basis of this reaction concept, we envisioned the use of
elemental sulfur (S8) after the catalyzed polyethylene chain
growth on magnesium. Boscato et al.15 reported that the reaction
of a benzene solution of S8 onto carbanions such as sec-butylli-
thium16 was very fast and led to the precipitation of lithium
polysulfides formed by a mechanism shown in Scheme 3 . The
characterizations of the resulting organic products showed struc-
tures corresponding to sec-butyl-Sx-sec-butyl with 1 < x < 5
when the stoichiometry of S8 to the C-Li bonds was less than 1.
Experiments were then carried out (Table 1) in which S8 was
added when the polymerization of ethylene was complete. PE chains
with molar masses around 1000 g mol-1 (entries 1 and 2) and 2000 g
mol-1 (entries 3 and 4) were targeted. In each case two stoichio-
metries of S to PE-Mg-PE (4 and 8) were employed. In all cases,
SEC analyses performed on the precipitated polymers were con-
sistent with the mechanism proposed in Scheme 4 since the addition
of elemental sulfur to PE-Mg-PE clearly leads to PE chains that
exhibit molar masses twice that for a single PE chain (see Table 1).
1H NMR analyses confirmed the SEC measurements (Figure 1).
The spectrum corresponding to the polysulfur products revealed
the presence of the expected signals corresponding to PE such as
the methyl 1S at 0.85 ppm and the methylenes of the repetitive

Article
Macromolecules, Vol. 43, No. 18, 2010 7499
Figure 2. High-temperature SEC (PE standards) chromatograms of polyethylenes obtained by addition of elemental sulfur before (black) (1) and after
reduction (red) by LiAlH4 (4a).
Table 2. Molar Mass Characteristics of End-Functionalized Polyethylene and Thiol-End-Functionalization of Polyethylenes (4a) Obtained after
Reduction of Polysulfurs (1)
expt no.a
Mnb (g mol-1
)
PDIb
PE-Sat (%)c
PE-Vt (%)c
PE-SH (%)c
PE-S-PE (%)c
PE-S-S-PE (%)c
5 (1)
6 (2)
7 (3)
8 (4)
1300
1130
2570
2270
1.5
1.5
1.5
1.4
6.9
5.1
12.0
15.9
1.4
2.2
5.2
4.2
70.6
72.7
56.9
63.4
18.0
18.7
19.5
13.9
3.1
1.3
6.4
2.6
a Experiment number corresponding to the reduction step of the experiment in parentheses (see Table 1). b Determined by high-temperature (150 °C)
SEC with trichlorobenzene as eluent (1 mL min-1) with a calibration with PE standards. c In mol % determined by 1H NMR analyses.
Table 3. Molar Mass Characteristics of End-Functionalized
Polyethylene (4b) Obtained after Reaction of NaSH with PE-I
expt no.
Mna (g mol-1
)
PDIa
PE-Vt (%)b
PE-SH (%)b
PE-S-PE (%)b
PE-S-S-PE (%)b
9 (air)
10 (argon)
1710
1360
1.2
1.4
1.6
1.7
11.0
48.7
2.1
26.4
68.5
15.7
a Determined by high-temperature (150 °C) SEC with trichlorobenzene as eluent (1 mL min-1) and calibration with PE standards. b Determined by of
1H NMR analyses. The remaining chains are saturated PE.
Table 4. Molar Mass Characteristics and End-Functionalization Rates of Starting PE-I (2) or PE-SC(dS)-X (3) and Resulting PE-SH (4c)
a
Mn
(g mol-1
PE-I
(2) (%)b
PE-SC(dS)-OEt
(3a) (%)b
PE-SC(dS)-NEt2
(3b) (%)b
PE-SH
(4c) (%)b
reduction
yield (%)b
expt no.
)
PDIa
11
12
13
14
15
16
1240
1940
1270
2410
1270
1860
1.3
1.2
1.5
1.3
1.3
1.2
95.0
88.0
91.8
88.6
81.8
71.8
88.4
69.3
82.7
71.4
88.0
82.7
96.2
78.2
100
99.6
92.1
90.8
95.5
91.1
a Determined by high-temperature (150 °C) SEC with trichlorobenzene as eluent (1 mL min-1) and a calibration with PE standards. b Determined by
1H NMR analyses. The remaining chains are saturated and unsaturated PE.
unit between 1.05 and 1.45 ppm. Additional signals were showing
up between 2.4 and 3.0 ppm that were all corresponding to the
methylenes adjacent to one sulfur atom in the different poly-
sulfurs formed. The very small fractions (around 6%) of vinyl
chain ends formed by β-H elimination during the catalyzed
polyethylene chain growth process9 were characterized by the
presence of resonances of vinyl protons at 4.90 and 5.70 ppm and
of the adjacent methylene protons at 1.95 ppm.
A proposed mechanism for the formation of polysulfurs PE-
Sk-PE (k < 8) by addition of S8 onto PE-Mg-PE and based on the
previous works of Boscato15 is shown in Scheme 4.
In order to obtain the targeted thiol end groups, a reduction of
these polysulfurs with LiAlH4 was performed.
The resulting products were again analyzed by SEC (Figure 2)
and 1H NMR (Figure 1), and the corresponding data were
compared to the starting polysulfurs. The reduction step seemed

7500 Macromolecules, Vol. 43, No. 18, 2010
Mazzolini et al.
Figure 3. 1H NMR (C6D6/TCE 1/2 v/v, 400 MHz, 512 scans, 363 K) of PE-I (black) and thiol-end-functionalized polyethylenes (4b) obtained by addition
of sodium hydrogenosulfide on PE-I (2) under air (blue) and argon (pink). Starred peaks are solvent impurities.
Figure 4. 1H NMR (C6D6/TCE 1/2 v/v, 400 MHz, 512 scans, 363 K) of PE-I (2) (black), PE-SC(dS)-OEt (3a) (red) and PE-SH (4c) (blue).

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Macromolecules, Vol. 43, No. 18, 2010 7501
Figure 5. MALDI-TOF mass spectrometry spectrum of PE-SH (4c) (entry 11 in Table 4) in reflectron mode and corresponding enlargement between
1089 and 1165 Da. P1 = expected population.
to occur as the SEC analyses of the corresponding polymers
systematically exhibited molar masses half that of the starting
product (Figure 2).
reduction step, respectively. The corresponding values are com-
piled in Tables 1 and 2.
A slightly higher thiol-end-functionalization rate was obtained
for PE of low molar masses (entries 5 and 6 around 70%)
compared to PE of higher molar masses (entry 7 and 8 around
60%). The formed polysulfur compounds were however always
efficiently reduced into thiol-end-functionalized PE. However,
these functionalization rates were minimized by the systematic
and unavoidable formation of thioether compounds that do not
undergo reduction. An attempt to increase the stoichiometry of
S8 did not decrease the thioethers content (entries 6 and 8). Thus,
although this strategy is a useful, simple, and cheap way to
1H NMR analyses showed the complete disappearance of the
signals corresponding to polysulfurs (k > 2). The presence of a
quadruplet at 2.33 ppm and a triplet at 1.03 corresponding to a
methylene adjacent to a thiol group and the proton of the thiol
function at 1.03 ppm confirmed the success of the reduction. The
remaining triplet at 2.40 ppm corresponds to thioether com-
pounds PE-S-PE that were not reduced by LiAlH4. The determi-
nations of the integrals of the different signals allowed to quantify
the polysulfur and thiol end group contents before and after the

7502 Macromolecules, Vol. 43, No. 18, 2010
Mazzolini et al.
synthesize PE-SH, it suffers from the unavoidable formation of
thioethers which limits thiol-end-functionalization rates.
the best candidate to be reduced to thiol since it is obtained in a
one-step reaction using a commercial disulfide (difulfiram) and in
high yields (entries 15 and 16, Table 4). The same reduction
procedure was then applied with LiAlH4 and successfully led to
highly thiol-functionalized polyethylenes (reduction step yields
>90%) in a minimum of chemistry steps, making this last route
the most valuable of the tested ones.
Alternatives were then envisioned, and the reactivity of iodo-
end-functionalized polyethylene (PE-I, 2) was considered. The
preparation of PE-I was previously reported by our group.10a
High functionalities (up to 97% depending on PE molar masses)
were obtained by a simple addition of I2 on PE-Mg-PE right after
the ethylene polymerization step. For example, the reactivity of
PE-I was used to introduce, almost quantitatively, an azide group
at the chain end of polyethylene via reaction with NaN3. Using a
similar procedure, PE-I (95%) was reacted with NaSH in a
toluene/DMF mixture at 100 °C. The resulting products (4b)
were analyzed by 1H NMR (Figure 3), and the proportion of the
different products formed was assessed (Table 3). Disulfide PE
chains were formed together with the expected PE-SH. However,
the yield was poor (11%) when the reaction was performed under
air (entry 9, Table 3). A very small fraction of thioether (2.1%)
was also observed and may correspond to the reaction of the
formed PE-SNa with PE-I. The high proportion of disulfides
observed is directly linked to the well-known oxidation coupling
reaction of thiol compounds particularly favored under air.17
This first experiment indeed showed that almost 80% of PE-SH is
potentially formed by addition of NaSH onto PE-I. Conse-
quently, an additional experiment was carried out under inert
atmosphere (entry 10, Table 3). A higher selectivity toward the
formation of the expected thiol-end-functional polyethylenes
(PE-SH) was observed (48.7%). However, the proportion of
thioethers was higher (26.4%). Indeed, under air, PE-SH was
rapidly transformed into PE-S-S-PE which prevented the formation
of PE-SNa and its further reaction with PE-I to give PE-S-PE.
The reaction between PE-SNa and PE-I is probably more favored
under argon. This results in a higher thioether content in the final
polymer mixture.
Conclusion
Several strategies for the preparation of thiol-end-functiona-
lized polyethylenes were assessed. They all rely on the use of
catalyzed polyethylene chain growth on magnesium that gives
rise to PE-Mg-PE compounds. The reactivity of these com-
pounds is utilized in the direct addition of sulfur at the end of
the polymerization process. PE-SH can be obtained in moderate
yields (70%) after an additional reduction step of the formed
polysulfurs. Alternatively, PE-Mg-PE can be used to synthesize
PE-I. The use of this highly functionalized intermediate in
substitution reactions with NaSH was evaluated. However, a
dipolyethylenyl disulfide and a dipolyethylenyl thioether were
obtained in high yields under air and argon, respectively, due to
side reactions. The synthesis of polyethylenes bearing a dithio-
carbonate end group via reaction of PE-I with the potassium
xanthic acid saltis appealing as reduction of the product provided
PE-SH in high yields. Taking advantage of this last result, the
direct synthesis of end-thiothiocarbonylated (dithiocarbamate
and dithiocarbonate) polyethylenes was re-examined by adding
the corresponding disulfides directly onto PE-Mg-PE. The re-
duction of these functional PEs led, almost quantitatively, to
PE-SH. The best route seems to be the direct preparation of PE-
dithiocarbamate which is obtained in high yield (>90%) using
commercial difulfiram and can efficiently be reduced into PE-SH
with LiAlH4 (reduction yields >90%). The reactivity of those
synthesized PE-SH toward ene-containing compounds is cur-
rently being investigated in our group.
A third strategy to efficiently and easily introduce thiol end
functions on PE was investigated. It relies on our previous results
based on the synthesis of thiothiocarbonylated PE.
In the field of controlled radical polymerization, the introduc-
tion of functionalities at polymer chain ends quantitatively and
selectively is of particular interest. In the case of the RAFT
process, polymers bearing thiothiocarbonylated end groups
(dithioesters, dithiocarbamates, dithiocarbonates, and trithiocar-
bonates) are obtained and readily reduced to thiols.18 The
reduction with a source of hydride has been depicted as a valuable
strategy. With regard to the double objective of preparing macro-
RAFT agents based on polyethylene10e and precursors of PE-SH,
the reaction of PE-I with the potassium salt of xanthic acid
KSC(dS)-OEt was investigated. Polyethylenes bearing a dithio-
carbonate end group (3a) were obtained in quasi-quantitative
yields as shown in entries 11 and 12 in Table 4. After a reduction
step with LiAlH4, clean PE-SH were obtained as attested by 1H
NMR analyses (Figure 4) and MALDI-TOF mass spectrometry
(Figure 5) in which only the expected PE-SH population was
observed making this strategy a really powerful method for PE-
thiol-functionalization.
Acknowledgment. The authors thank Dr. Fernande Boisson
ꢀ
ꢀ
(Service de RMN du Reseaux des Polymeristes Lyonnais, CNRS
UMR 5223) and Dr. Hank Debruyn (The University of Sydney)
for fruitful discussions. The financial support from the French
National Agency for Research (ANR 08-JCJC-Tricky-0115-
01) and competitiveness clusters Plastipolis and Axelera is
acknowledged.
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