Synthesis of linear α-olefins via polyhomologation

Article abstract of DOI:10.1021/ma050152k

Linear α-olefins (LAOs) of controlled molecular weight and low PDI were synthesized in high yield by the polyhomologation reaction of dimethylsulfoxonium methylide (1) with triallylborane (2). Following polymerization, propionic acid or trimethylamine N-oxide dihydrate (TAO) was used to afford α-vinyl-ω-methyl or α-vinyl-ω-hydroxy end-functionalized polymethylene, respectively. Controllable molecular weights up to 13 kDa were obtained by varying the initial monomer/catalyst ratio, and the resultant polymers had very narrow polydispersities (PDI < 1.08). Although polymer yields were generally high, quantities of oxidized side products were observed in α-vinyl-ω-methyl samples; however, these oxidized side products could be removed for samples of low molecular weights (C_n ≤ 36).

Full text of DOI:10.1021/ma050152k

7286
Macromolecules 2005, 38, 7286-7291
Synthesis of Linear R-Olefins via Polyhomologation
Carl E. Wagner, Andrew A. Rodriguez, and Kenneth J. Shea*
Department of Chemistry, University of California, Irvine, California 92697
Received January 24, 2005; Revised Manuscript Received June 23, 2005
ABSTRACT: Linear R-olefins (LAOs) of controlled molecular weight and low PDI were synthesized in
high yield by the polyhomologation reaction of dimethylsulfoxonium methylide (1) with triallylborane
(2). Following polymerization, propionic acid or trimethylamine N-oxide dihydrate (TAO) was used to
afford R-vinyl-ω-methyl or R-vinyl-ω-hydroxy end-functionalized polymethylene, respectively. Controllable
molecular weights up to 13 kDa were obtained by varying the initial monomer/catalyst ratio, and the
resultant polymers had very narrow polydispersities (PDI < 1.08). Although polymer yields were generally
high, quantities of oxidized side products were observed in R-vinyl-ω-methyl samples; however, these
oxidized side products could be removed for samples of low molecular weights (Cn e 36).
Introduction
by treatment of the oligomeric and polymeric triorga-
noboranes (3) with propionic acid could provide condi-
tions for the protolysis of the carbon-boron bonds of 3.
As an alternative to propionic acid, we also investigated
milder methods for the protolysis of polymeric triorga-
noboranes 3 using 2-hydroxypyridine8 (Scheme 1).
Linear R-olefins (LAO) in the range of C6-C20 repre-
sent an important starting material for many com-
mercial polymers, including detergents, lubricants, and
comonomers for ethylene polymerizations to form LDPE
and VLDPE.1 A variety of transition metal catalysts (Ni,
Zr, Cr) have been used to synthesize LAOs through the
oligomerization of ethylene. An alternative synthesis of
LAOs with controlled molecular weight distributions
remains in high demand. Studies in our laboratory have
concentrated on developing a new approach to synthe-
sizing LAOs using a living polymerization of dimethyl-
sulfoxonium methylide (1) by trialkylboranes. In this
reaction, the alkyl groups of the trialkylboranes undergo
repetitive 1,2-migrations with ylide monomers to build
the carbon backbone of the developing polymethylene
polymers one carbon at a time. This process of repetitive
1,2-migration, or homologation has been termed poly-
homologation.2 The polyhomologation reaction has dem-
onstrated its versatility in the construction of novel
materials ranging from star polymers3 to copolymers of
polymethylene and poly(dimethylsiloxane)s4 to the com-
mercially important commodity copolymer poly(ethylene-
b-styrene).5 By developing and combining new monomer
sources, the polyhomologation reaction has also yielded
a route to a material with the same chemical composi-
tion as an ethylene-propylene copolymer.6
For synthesis of controlled molecular weight distribu-
tions of linear R-olefins, we have focused on utilizing
the polyhomologation reaction with a suitable trialkyl-
borane initiator and installing a terminal vinyl group
either following or prior to polyhomologation. Prior
efforts in pursuit of the former concentrated on the
polyhomologation of triethylborane with dimethylsul-
foxonium methylide (1)7 followed by heating (250 °C)
under high vacuum (0.05 mmHg) in an attempt to effect
retro-hydroboration to install a terminal vinyl group.
These efforts were not successful. While the retro-
hydroboration is expected to be thermodynamically
favored at high temperature, the process, even at these
elevated temperatures, is kinetically prohibitive.
Results and Discussion
Synthesis and Properties of Triallylborane. Tri-
allylborane (2) was prepared as described by the method
of Zakharkin and Stanko.9 Allylaluminum sesquibro-
mide, formed by the reaction of allylbromide with solid
aluminum granules, was reacted with boron trifluoride
etherate to give triallylborane in 60% yield after two
distillations (Scheme 2).
In contrast to most alkylboranes, triallylborane dis-
plays high reactivity toward water, alcohols, and other
nucleophiles in reactions that often result in cleavage
of the C-B bond.10 Since ylide 1 has been shown to react
with electron-deficient olefins,11 its mode of reaction
with triallylborane was in question. The application of
triallylborane for the synthesis of functionalized, linear
R-olefins by the polyhomologation reaction would rep-
resent a novel use for this versatile reagent.
Polyhomologation of Triallylborane. In the syn-
thesis of LAOs, triallylborane (2) serves as an initiator
while dimethylsulfoxonium methylide (1) is the meth-
ylene monomer source. One equivalent of DMSO is
expelled for each equivalent of ylide 1 consumed.
Random insertion into all three carbon-boron bonds
results in the formation of a polymeric organoborane
(3). The living nature of the polyhomologation reaction
allows control of the average length of each growing
chain of 3.12 Providing each branch grows randomly,
subsequent oxidation of 3 provides R-vinyl-ω-hydroxy-
polymethylene (5) (Scheme 3).
Toluene solutions of ylide 1 were preheated to 80 °C
and treated with an aliquot of a toluene solution of 2.
The ylide was rapidly consumed (5 min). TAO13 was
added directly to the solution to affect oxidation under
reflux conditions. TAO was used in preference to basic
hydrogen peroxide to affect oxidation because the latter
results in a small amount (1-2%) of polymethylene with
twice the average molecular weight of a given sample
due to radical coupling.14 Removal of solvents followed
by filtering and washing of the polymeric solids with
An alternative approach using triallylborane (2) as
initiator was explored. Following polyhomologation of
triallyborane (2), reductive cleavage of the B-C bond
* Corresponding author. E-mail: kjshea@uci.edu.
10.1021/ma050152k CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/30/2005

Macromolecules, Vol. 38, No. 17, 2005
Linear R-Olefins 7287
Scheme 1
Scheme 2
Scheme 3
Table 1. GPC and 1H NMR Data of
r-Vinyl-ω-hydroxypolymethylene 5 (Scheme 3)
Table 2. GPC and 1H NMR Data of Vinylpolymethylene
(4) Produced by Scheme 2
trial DPa (calcd) Mnb (g/mol) DPb DPc 1H NMR PDI
trial DPa (calcd) Mnb (g/mol) DPb DPc 1H NMR PDI
1
2
3
4
5
6
20
40
38
55
1
2
64
1038
2426
70
65
1.04
1.05
498
712
1396
6690
13202
31
47
1.12
1.05
1.03
1.05
1.06
127
169
183
54
72
a The DP refers to n (Scheme 2). b Data refer to GPC in o-xylenes
at 100 °C calibrated with linear polyethylene standards. c Data
refer to 1H NMR end-group analysis (i.e., the ratio of the terminal
hydroxymethylene proton to the broad polymethylene peak of 4).
108
400
800
96
133
473
937
a The DP refers to n (Scheme 6). b Data refer to GPC in o-xylenes
at 100 °C calibrated with linear polyethylene standards. c Data
refer to 1H NMR end-group analysis (i.e., the ratio of the terminal
hydroxymethylene proton to the broad polymethylene peak of 5).
respectively. In all cases, the DP by GPC and 1H NMR
end-group analysis correlated closely to the calculated
values. The data are summarized in Table 1.
The GPC traces for all samples in Table 1 were
monomodal. Both the relative and absolute correlations
between the DP calculated for trials 3 and 4 (54 and
108) and the DP determined by GPC (47 and 96)
demonstrate molecular weight control for the synthesis
of 5. Further, the low PDI for all trials is indicative of
a living polymerization.
Reactions of 3 with two proton sources, propionic
acid15 and 2-hydroxypyridine,8 were explored (Scheme
1). For protolysis by propionic acid, reactions were run
with initial ratios of ylide:triallylborane of 192 and 381.
It was anticipated that this reaction would yield R-vinyl-
ω-methyl-polymethylene (4) with n ) 64 and 127,
respectively. GPC data and 1H NMR end-group analysis
for these experiments are presented in Table 2.
For trials 1 and 2 in Table 2, after ylide consumption,
the toluene and DMSO were removed in vacuo, and the
product was dissolved in a 1:1 o-xylene:propionic acid
(20 equiv) solution and refluxed under nitrogen for 3
days.
methanol, water, and hexanes gave near-quantitative
crude yields of polymer 5. Samples were further purified
by reprecipitation in toluene/acetonitrile to afford an
opaque white polymer in 88-94% yield. The structural
assignment of the polymer 5 is as follows.
The 1H NMR spectra of 5 (C6D6, 77 °C) have features
consistent with R-vinyl-ω-hydroxypolymethylene (5).
Diagnostics include resonances at 5.78 ppm (ddt, J )
17.0, 10.4, 6.6 Hz, 1H), corresponding to the vinylic
methine proton of 5. Also, resonances at 5.02 ppm (dd,
J ) 17.1, 1.8 Hz, 1H) and at 4.95 ppm (dd, J ) 10.2, 1
Hz, 1H) correspond to the terminal vinylmethylene
protons of 5. A triplet at 3.39 ppm (t, J ) 6.5 Hz, 2H)
and a quartet at 2.01 ppm (q, J ) 6.9, 2H) correspond
to the hydroxymethylene and methylene protons of the
allylic carbon of 5, respectively. A broad signal at 1.35
ppm corresponds to the polymethylene protons of 5. The
doublet at 0.93 ppm arises from the incorporation of
1-2% ethylidene groups in the polymethylene backbone
arising from dimethylsulfoxonium ethylide, formed as
a minor product in the synthesis of ylide 113 (Figure 1).
The degree of polymerization (DP) was established by
For protolysis by 2-hydoxypyridine, reactions were
run with initial ratios of ylide:triallylborane of 138 and
276. Following protolysis of the carbon-boron bond of
3, these reactions were expected to yield R-vinyl-ω-
methylpolymethylene (4) with n ) 46 and 92, respec-
1
GPC and H NMR end-group analysis (Table 1). Reac-
tions were run with initial ratios of ylide:triallylborane
of 60, 120, 162, 324, 1200, and 2400. If all three allyl
groups of triallylborane undergo homologation, these
reactions should produce R-vinyl-ω-hydroxypolymeth-
ylene (5) with n ) 20, 40, 54, 108, 400, and 800,
1
tively. GPC data and H NMR end-group analysis for
these experiments are presented in Table 3.
For trials 1 and 2 in Table 3, after ylide consumption,
2-hydroxypyridine (16 equiv) was added to the reaction

7288 Wagner et al.
Macromolecules, Vol. 38, No. 17, 2005
Figure 1. Representative 1H NMR spectrum of the polymer 5 (C6D6, 77 °C). The spectrum shown corresponds to trial 1 in Table
1. The broad singlet at 0.5 ppm has been assigned to the hydroxyl proton of the end group.
Table 3. GPC and 1H NMR Data of Vinylpolymethylene
oxygen-containing functional groups in R-vinyl-ω-func-
tional-olefin comonomers.16
(4) Produced by Scheme 3
trial DPa (calcd) Mnb (g/mol) DPb DPc 1H NMR PDI
Oligomeric r-Vinyl-ω-methylpolymethylene in
the C9-C36 Range. To evaluate the effectiveness of the
protolysis of the carbon boron bonds of oligomer (3), a
reaction with a ratio of ylide:triallylborane of 36 was
conducted and followed by treatment with propionic acid
(Scheme 2). After the ylide was consumed, the toluene
and DMSO were removed in vacuo, and the crude
oligomeric 3 was refluxed in neat propionic acid (360
equiv, 3 days). The short-chain oligomeric R-vinyl-ω-
methylpolymethylene (4) was extracted with hexanes
and ether, and the organic layers were washed with
saturated potassium carbonate, dried over sodium
sulfate, and removed in vacuo to give an 86% yield of 4
after purification by chromatography (SiO2, hexanes).
This reaction should yield oligomeric linear R-vinyl-ω-
methylpolymethylene centered approximately at C15.
The GC chromatograph of this reaction is shown in
Figure 2.
1
2
46
92
665
1282
43
87
61
113
1.08
1.03
a The DP refers to n (Scheme 3). b Data refer to GPC in o-xylenes
at 100 °C calibrated with linear polyethylene standards. c Data
refer to 1H NMR end-group analysis (i.e., the ratio of the terminal
hydroxymethylene proton to the broad polymethylene peak of 4).
solution, and the reaction was refluxed for 3 days.
Solvents were removed in vacuo, and the recovered
polymeric product 4 for each trial was redissolved in
toluene and precipitated with acetonitrile to afford 80-
86% yield of 4 after filtering and washing with water,
methanol, and hexanes. The 1H NMR spectra of the
polymeric samples of 4 produced by all reactions in
Table 2 and Table 3 indicate that approximately 20-
30% of the polymer chains have some terminal oxygen
functionality. Chemical shifts of approximately 3.5 ppm
Each peak in the spectrum has a mass separation of
14.0 m/z. The peak at the retention time of 4.57 min
has a mass (EI) of 140.2 m/z and corresponds to
1-decene. The PDI of the distribution shown in Figure
3 is 1.09. The calculated DP from the Mn of the
distribution corresponds to the C20 linear R-olefin.
Importantly, there was no evidence for any oxygen-
terminated linear R-olefins in the GC spectrum or the
1H NMR spectra of oligomer 4 (Figure 3). The absence
of oxygen-terminated linear R-olefins in this oligomeric
sample of 4 may be attributed to the isolation of the
oligomers by extraction and their purification by chro-
1
in the H NMR of these oligomers are consistent with
terminal hydroxyl (or peroxy) groups. An earlier study
of the reaction products of tris(polymethylene)borane
with oxygen suggests the oxygenated biproducts are
comprised of ω-hydroxy- and peroxy-terminated poly-
methylene.14 The oxygen-terminated chains may arise
from oxidation of the polymeric alkylborane (3) by
adventitious oxygen over the 3 days required for the
reaction. Comparison of the theoretical DP with the DP
determined by GPC indicates the absolute molecular
weights can be controlled within a narrow range. While
perfectly oxygen-free polymeric samples of R-vinyl-ω-
methylpolymethylene (4) are frequently desired to serve
as comonomers with ethylene with oxygen-sensitive
polyethylene metallocene catalysts for the synthesis of
novel comb polymers, there have been reports of ethyl-
ene polymerization Ni(COD)2 catalysts that tolerate
1
matography. The H NMR spectra of 4 have features
consistent with the proposed assignment of an R-vinyl-
ω-methylpolymethylene structure.
While the center of the distribution is slightly higher
than that calculated on the basis of the stoichiometry,
1
the GC chromatograph and the H NMR spectra of 4

Macromolecules, Vol. 38, No. 17, 2005
Linear R-Olefins 7289
Figure 2. GCMS chromatograph of oligomeric linear R-vinyl-ω-methyl-polymethylene (4) produced by the polyhomologation
reaction. The distribution ranges from C9 to C36. The peak at the retention time of 22.17 min has a mass (EI) of 364.4 m/z and
corresponds to the C26 linear R-vinyl-ω-methylpolymethylene (4). The baseline distribution of peaks offset from the main distribution
correspond by mass to isomeric, R-olefin chains that incorporate 1-2% ethylidene groups.13
dardization of toluene solutions of dimethylsulfoxonium meth-
demonstrate that triallylborane can be used to access
linear R-vinyl-ω-methylpolymethylene (4) in the C9-C36
range by polyhomologation in high yield.
ylide (1) have been described elsewhere.14
Triallylborane (2). The method of Zakharkin and co-
workers was followed to synthesize 2.9 To a flame-dried 250
mL round-bottom flask sealed with rubber septum and equipped
with a magnetic stir bar, reflux condenser, and thermometer
was transferred solid aluminum granules (12.03 g, 446 mmol)
and HgCl2(s) (88 mg, 0.32 mmol). Dry, N2-purged diethyl ether
(60 mL) was added, and the solution was stirred and heated
to 35 °C. Allyl bromide (52.1 mL, 602 mmol) was added
dropwise under a nitrogen atmosphere. Care must be taken
to add the allyl bromide slowly, since the reaction with
aluminum is exothermic. The reaction mixture was stirred at
reflux in an oil bath set at 67 °C for 3 h. The reaction mixture
was then transferred via syringe into a septum-sealed flame-
dried 250 mL round-bottom flask containing BF3‚Et2O (20.0
mL, 142 mmol) under nitrogen. This solution was refluxed at
70 °C for 3 h. The ether was removed by short-path distillation
under nitrogen. The residue was then distilled under vacuum
(20 mmHg) with a short-path distillation head at a head
temperature of 65 °C to give crude triallylborane as a clear,
colorless liquid (17.00 g, 78%). 11B NMR (237 MHz, CDCl3) of
this distillate showed a major peak, δ 80.7 (br s) 83%, and
minor peaks at δ 50.6 (br s) 11%, and δ 32.0 (br s) 6%. The
peak at δ 80.7 (br s) has been assigned to triallylborane. On
the basis of chemical shift, the peak at δ 50.6 has been
tentatively assigned to a borinic ester of some type, most likely
the product of triallylborane’s reaction with oxygen. A second
vacuum distillation (20 mmHg) yielded pure triallylborane as
a clear, colorless liquid (13.07 g, 60%). 1H NMR (500 MHz,
CDCl3) δ: 2.2 (br s, 6H), 4.9 (br s, 6H), 5.89-5.98 (p, J ) 10.6,
3H). 13C NMR (125.8 MHz, CDCl3) δ: 134.8, 114.7 (br s), 34.4
(br s). 11B NMR (237 MHz, CDCl3) δ: 80.6 (br s). IR (neat):
3076, 2999, 2973, 2914, 1807, 1635, 1420, 1370, 1270, 1170,
Conclusions
Triallylborane (2) was employed in the polyhomolo-
gation reaction to synthesize R-vinyl-ω-functionalized-
polymethylene of controlled molecular weight and low
PDIs. Following polyhomologation of triallylborane with
oxidation by TAO provides access to polymeric (C40-
C800) samples of R-vinyl-ω-hydroxypolymethylene (5) of
controlled molecular weight and low PDI. Following the
polyhomologation of triallylborane with reduction by
propionic acid or 2-hydroxypyridine yields impure poly-
meric (C40-C180) R-vinyl-ω-methylpolymethylene (4)
contaminated with 20-30% of an oxygen-terminated
impurity. However, oligomeric (C9-C36) R-vinyl-ω-meth-
ylpolymethylene (4) can be isolated in high yield by the
polyhomologation of triallylborane followed by reduction
with refluxing propionic acid and purification by chro-
matography.
Experimental Section
General Information. All 1H NMR spectra were acquired
at 400 or 500 MHz. Chemical shifts (δ) are listed in ppm
against internal references. Coupling constants (J) are re-
ported in hertz. All 13C NMR spectra were acquired at 125.8
MHz. Chemical shifts (δ) are listed in ppm against solvent
carbon peaks as an internal reference. All 11B NMR spectra
were acquired at 237 MHz. All chemical shifts (δ) are listed
in ppm against boron trifluoride etherate as an external
reference. All GPC was conducted on a high-temperature gel
permeation chromatograph using xylenes as the mobile phase.
Pure hydrocarbons and narrow molecular weight polyethylene
were used as calibration standards. The synthesis and stan-
993, 900 cm-1
.
r-Vinyl-ω-methylpolymethylene (4). General Method.
Toluene solutions of ylide 1 were preheated to 80 °C and
treated with an aliquot of a toluene solution of triallylborane

7290 Wagner et al.
Macromolecules, Vol. 38, No. 17, 2005
Figure 3. 1H NMR spectrum of the oligomer 4 (C6D6, 67 °C). The integration of the terminal methyl group (Hf) is high due to the
incorporation of 1-2% ethylidene groups.13
(2). The ylide was rapidly consumed (5 min), solvents were
removed in vacuo, and a 1:1 o-xylenes:propionic acid (20 equiv)
was added and the reaction was refluxed for 3 days. Removal
of solvents followed by filtering and washing of the polymeric
solids with methanol, water, and hexanes gave near-quantita-
tive crude yields of polymeric 4. Purification by reprecipitation
in toluene/acetonitrile afforded a purified white polymer 4 in
yields of 80-90% (spectrocopic data for trial 1 of Table 2). 1H
NMR (500 MHz, toluene-d6, 78 °C) δ: 5.78 (ddt, J ) 17.0, 10.3,
6.7 Hz, 1H), 5.00 (dd, J ) 17.2, 1.7 Hz, 1H), 4.94 (dd, J ) 9.6,
1.0 Hz, 1H), 3.36 (m, 0.3 H), 1.98 (q, J ) 6.9, 2H), 1.35 (m,
130H), 0.91 (m, 3H). 13C NMR (125.8 MHz, toluene-d6, 78 °C)
4 was extracted with hexanes and ether, and the organic layers
were washed with saturated potassium carbonate, dried over
sodium sulfate, and removed in vacuo to give an 86% yield of
oligomeric 4 after purification by chromatography (SiO2,
1
hexanes). H NMR (500 MHz, C6D6, 77 °C) δ: 5.80 (ddt, J )
17.0, 10.3, 6.6 Hz, 1H), 5.02 (dd, J ) 17.1, 1.7 Hz, 1H), 4.97
(d, J ) 10.2 Hz, 1H), 2.01 (q, J ) 7.2, 2H), 1.35 (m, 66H), 0.91
(t, J ) 6.9, 5H). 13C NMR (125.8 MHz, C6D6, 77 °C) δ: 139.6,
114.7, 34.5, 32.7, 30.5, 30.4, 30.3, 30.2, 30.1, 29.8, 29.7, 23.4,
14.5. IR: 2918, 2850, 1464, 992, 910, 730, 720 cm-1
.
δ: 30.1. IR: 3447 (br), 2918, 1473, 1463, 730, 719 cm-1
.
Acknowledgment. We thank the Chemistry Divi-
sion of the National Science Foundation for financial
support of this work (Grant CHE-9617475). C.W. thanks
Allergan Inc. for a graduate fellowship.
r-Vinyl-ω-hydroxypolymethylene (5). General Method.
Toluene solutions of ylide 1 were preheated to 80 °C and
treated with an aliquot of a toluene solution of triallylborane
(2). The ylide was rapidly consumed (5 min), and trimethyl-
amine-N-oxide dihydrate (3.1 equiv) was added to affect
oxidation under reflux conditions. Removal of solvents followed
by filtering and washing of the polymeric solids with methanol,
water, and hexanes gave near-quantitative crude yields of
polymeric 5. Purification by reprecipitation in toluene/aceto-
nitrile afforded a purified white polymer 5 in yields of 80-
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1H), 3.36 (m, J ) 6.5 Hz, 2H), 2.00 (q, J ) 6.9, 2H), 1.35 (m,
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MA050152K