Short-Chain Branched Polar-Functionalized Linear Polyethylene via "tandem Catalysis"

Article abstract of DOI:10.1021/acs.macromol.6b00581

Cationic Pd^II complex 1 chelated by an N-fixed phosphine sultam has been synthesized and structurally characterized. Exposure of 1 to ethylene resulted in the formation of short-chain olefins (1-butene: 2-butene: 1-hexene: 1-octene = 86:7:6:1) with a high catalytic activity of 10⁵ mol_E mol_Pd^-1 h^-1. By combination of 1 and one of the well-known phosphinesulfonato Pd^II catalyst precursors 2-5, linear polyethylenes containing methyl, ethyl, and n-butyl branches of up to 100 per 1000 C were generated from the polymerization of ethylene alone in a "tandem catalysis" one-pot approach. In further exploitation of this concept, linear polyethylenes with both various short-chain branches and a choice of different polar functional groups incorporated into the main chain were obtained for the first time from the copolymerization of ethylene and polar vinyl monomers (methyl acrylate, N-isopropylacrylamide, methyl vinyl sulfone, acrylonitrile, ethyl vinyl ether, vinyl acetate, and allyl bromide). All these apolar and polar branches are incorporated into the linear polyethylene backbones to varying degrees, while the type of initiating and terminating chain ends of the resulting polyethylenes depends significantly on the nature of polar vinyl monomer.

Full text of DOI:10.1021/acs.macromol.6b00581

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Article
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Short-Chain Branched Polar-Functionalized Linear Polyethylene via
“Tandem Catalysis”
Zhongbao Jian and Stefan Mecking*
†Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitatsstrasse 10, 78457 Konstanz,
̈
Germany
S
* Supporting Information
ABSTRACT: Cationic PdII complex 1 chelated by an N-fixed
phosphine sultam has been synthesized and structurally
characterized. Exposure of 1 to ethylene resulted in the
formation of short-chain olefins (1-butene: 2-butene: 1-
hexene: 1-octene = 86:7:6:1) with a high catalytic activity of
105 molE molPd h−1. By combination of 1 and one of the
−1
well-known phosphinesulfonato PdII catalyst precursors 2−5,
linear polyethylenes containing methyl, ethyl, and n-butyl
branches of up to 100 per 1000 C were generated from the
polymerization of ethylene alone in a “tandem catalysis” one-
pot approach. In further exploitation of this concept, linear
polyethylenes with both various short-chain branches and a
choice of different polar functional groups incorporated into the main chain were obtained for the first time from the
copolymerization of ethylene and polar vinyl monomers (methyl acrylate, N-isopropylacrylamide, methyl vinyl sulfone,
acrylonitrile, ethyl vinyl ether, vinyl acetate, and allyl bromide). All these apolar and polar branches are incorporated into the
linear polyethylene backbones to varying degrees, while the type of initiating and terminating chain ends of the resulting
polyethylenes depends significantly on the nature of polar vinyl monomer.
Over the past two decades, less oxophilic late-transition-
metal catalysts, especially palladium(II) catalysts, have been
developed for the insertion polymerization of ethylene with a
range of polar vinyl monomers to generate functionalized
polyethylenes.5 The seminal cationic α-diimine PdII catalysts
form highly branched amorphous, liquid ethylene-acrylate
copolymers with ester groups at the end of branches as a
result of extensive “chain-walking” (Chart 1A).6 Corresponding
dinuclear α-diimine PdII catalysts were recently reported to
produce branched copolymers containing ester groups both at
the end of branches and also on the main chain (Chart 1B).7
Neutral phosphinesulfonato PdII catalysts represent another
prototypical motif, yielding linear functionalized polyethylenes
with only very low degrees of methyl branching and
incorporation of polar vinyl monomer into both the main
chain and the chain ends (Chart 1C).8 These neutral PdII
catalysts are compatible with a broad scope of polar vinyl
monomers including even such difficult candidates as
acrylonitrile, vinyl acetate, vinyl ether or acrylic acid.9 These
properties render the neutral PdII catalyst attractive candidates
for a “tandem catalysis” concept to both short chain branched
and polar functionalized polyethylenes.
INTRODUCTION
■
Polyethylene (PE) is the highest volume of all synthetic plastics
produced, and it is used ubiquitously in a variety of different
applications. These different requirements can be met by
adjusting the mechanical properties and processing character-
istics of polyethylenes via their microstructures. For instance, an
introduction of short-chain branches (SCB) into the backbone
of linear polyethylene alters the crystallinity and in turn the
density, rigidity, hardness, and permeability.1 As an extension of
these well-established principles, an incorporation of polar
functional groups into the backbone of inherently nonpolar
polyethylene even in small amounts can also significantly
enhance such properties as adhesion and compatibility with
other materials.2 Thus, polyethylene with a combination of
both short-chain branches and polar moieties in the backbone
appears attractive.
Using Ziegler-type or metallocene-type catalysts, short-chain
branched polyethylenes are produced industrially by copoly-
merization of ethylene with nonpolar α-olefins such as 1-
butene, 1-hexene, and 1-octene.3 An alternative route to
generate such materials by polymerization of ethylene alone has
been termed “tandem catalysis”, employing a dimerization (or
oligomerization) catalyst that generates the comonomers in situ
in combination with a copolymerization catalyst.4 The high
oxophilicity of the traditional early transition-metal catalysts
employed, however, appears prohibitive for additional incor-
poration of polar vinyl monomers.
Received: March 22, 2016
Revised: May 2, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b00581
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Chart 1. Functional Polyethylenes with Different
Microstructures
RESULTS AND DISCUSSION
Synthesis of Ligand and PdII Complex. Although the less
■
sterically encumbered O-atom of the sulfonamide moiety of II
(Chart 2) chelates more easily to the PdII center in the catalyst
precursor,11b the N-atom of the sulfonamide moiety may also
compete for coordination to the PdII center of the active species
as suggested by theoretical studies.11a To exclude this additional
parameter, we fixed the N-atom in the form of a cationic
phosphine sultam PdII complex (Chart 2, III). N-phenyl-
benzenesulfonamide was prepared, which underwent ortho
n
metalation, as well as N-metalation, with 2 equiv of BuLi to
form dilithiosulfonamide that further reacted with benzophe-
none to generate the carbinol (Scheme 1). This carbinol
underwent thermal cyclodehydration at 230 °C to yield the
desired sultam.13 The resulting sultam was again metalated, and
then reacted with (dianisyl)chloro phosphine to afford the
desired phosphine sultam. Similar to reported procedures for
acyclic phosphine sulfonamides,11a treatment of the phosphine
sultam with 1 equiv of [(COD)PdMeCl] at room temperature
for 30 min, followed by reaction with AgSbF6 in the presence of
dmso at room temperature for another 10 min selectively
generated the cationic phosphine sultam PdII complex 1 in a
high yield (Scheme 1).
A comprehensive mechanistic understanding has revealed
that an important structural feature of PdII catalysts containing
phosphine sulfonates (Chart 2, structure I), which renders
1
Indicative H NMR resonances at δ = 0.62 and 2.70 ppm
arise (cf. Supporting Information, Figure S4) from the protons
of the Pd−CH3 group and a coordinated dmso molecule
(compared to δ = 2.55 ppm for free dmso), respectively. The
solid-state structure of 1 was determined by X-ray diffraction
analysis (Figure 1). The phosphine sultam chelates to the PdII
center in a κ2-P,O bidentate fashion, resulting in a distorted
square planar coordination geometry. The dmso molecule
binds to the PdII center in a κ1-O mode (Pd1−O5: 2.129(3) Å).
As anticipated, the N-atom is remote to the PdII center and
blocked from coordination. The Pd1−O4 bond (2.277(2) Å)
in 1 is longer than in the analogous neutral dianisyl
phosphinesulfonato PdII complex (2.165(3) Å)14 and a cationic
phosphine−diethyl phosphonate PdII complex (2.164(2) Å),12a
and is much longer than that in the phosphine−phosphine
monoxide PdII complex (2.087(4) Å).12b The Pd1−P1 bond
(2.217(9) Å) is slightly shorter than in the aforementioned
three PdII complexes (2.235(6) Å, 2.232(1) Å and 2.232(2) Å,
respectively).12,14 That is, complex 1 has a slightly stronger
Pd−P bond but a significantly weaker Pd−O bond compared to
other known neutral and cationic PdII complexes that are
precursors to polymerization catalysts.
Chart 2. PdII Catalyst Precursors with Different Phosphine
Ligands
them capable of insertion chain growth, is the presence of one
hard sulfonate oxygen (O) and one soft phosphine (P) σ-donor
ligand.10 On the basis of this P/O framework, very recently
cationic PdII catalysts chelated by phosphine sulfonamides
(Chart 2, structure II) have been reported,11,12 which catalyze
the dimerization of ethylene to predominantly generate 1-
butene.
We now report on the generation of both short chain
branched and polar functionalized polyethylene (Chart 1D)
from ethylene and a polar vinyl monomer as the sole feedstocks
via a “tandem catalysis” approach employing a combination of a
neutral and a cationic PdII catalyst (Chart 2, structures I and
III).
Ethylene Oligomerization. Under pressure reactor
conditions, exposure of 1 to ethylene at 80 °C for 30 min
resulted in a high catalytic activity of 105 molE molPd h−1 as
−1
Scheme 1. Synthesis of Phosphine Sultam and PdII Complex 1
B
DOI: 10.1021/acs.macromol.6b00581
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two metal sites toward ethylene must be well-matched, and the
catalysts must not interfere with each other chemically.4f In
view of these issues, the combination of catalyst precursors 1
and 2 (Chart 3) appears reasonable. Because of their overall
Chart 3. Phosphinesulfonato PdII Catalyst Precursors 2−5
Applied in Tandem Catalysis
similar structure, no specific unfavorable interactions are
anticipated between active species formed from 1 and 2. The
catalytic activity and lifetime of 1 toward ethylene compare to
those of 2 as monitored by mass flow (Table 1, entries 1 and
2).9p Additionally, 2 is well-known to afford linear polyethylene,
and to copolymerize ethylene and 1-hexene.9p,15
By a one-pot polymerization procedure with ethylene as the
only monomer starting with catalyst precursor 1 and
subsequently adding 2, 3, or 4, respectively (cf. Supporting
Information and the Experimental Section), linear poly-
ethylenes containing methyl (Me), ethyl (Et), and n-butyl
(Bu) branches were produced. Under otherwise identical
conditions, with an increasing amount of 1, the Et and Bu
branches increase as anticipated, resulting in a decrease of the
melting point (Tm). However, polymer molecular weights
decrease as well (Table 1, entries 5−8). By combination of 1
with a precursor to a more active polymerization catalyst 3,9p
the degree of branching in the resulting linear polyethylene
decrease as expected (Table 1, entries 6 vs 9). Conversely, by
Figure 1. ORTEP plot of 1 drawn with 50% probability ellipsoids.
Hydrogen atoms and the SbF6 counterion are omitted for clarity.
Selected bond lengths [Å]: Pd1−O4 2.277(2), Pd1−P1 2.217(9),
Pd1−C1 2.022(4), Pd1−O5 2.129(3).
monitored by mass flow (Table 1, entry 1). This is
approximately twice as high compared to cationic phosphine
sulfonamide PdII catalysts reported previously,11 thus the
fixation of the N-atom in 1 obviously enhances catalytic
1
activity. Both H NMR spectroscopy and GC-MS analyses of
the resulting products revealed the formation of 1-butene as the
major product, along with 2-butene, and small amounts of 1-
hexene and 1-octene (typical ratio: 86:7:6:1). The formation of
1-olefin products predominately is attractive for employing this
catalyst in tandem catalysis of ethylene to generate branched
polyethylene.
Short-Chain Branched Linear Polyethylene. For the
approach of tandem catalysis to be viable, the reactivity of the
a
Table 1. Short-Chain Branched Linear Polyethylene Produced by One-Pot Tandem Catalysis
step 1
oligo
step 2
pE [bar]
(1/2)
t [min]
(1/2)
yield
[g]
b
g
c
c
c
d
d
e
entry
1
(co)poly
act.
Me brs /1000C Et brs /1000C Bu brs /1000C Mn [103] Mw/Mn Tm [°C]
f
1 (3.5)
−
−
−
10/−
−/5
−/5
−/10
10/5
10/5
10/5
10/5
10/5
10/5
10/5
5
30/−
−/30
−/30
−/30
30/60
30/60
30/60
30/60
30/40
30/60
30/60
60
4.6
263
280
370
57
−
3
−
−
−
−
−
−
1
−
12.6
130
122
2.5
1.5
0.9
0.5
4.1
7.3
0.8
0.4
0.6
0.8
−
−
2.2
−
134
136
142
96
h
2
2 (3.5)
4.9
3.7
i
3
j
3 (2)
<1
∼1
1
−
−
1.6
2.2
2.2
2.2
2.0
2.0
2.5
2.0
4.6
2.1
1.8
1.9
4
−
4 (3.5)
2 (20)
2 (20)
2 (20)
2 (20)
3 (20)
4 (20)
4 (20)
2 (3.5)
2 (8)
1.0
5
1 (5)
1 (10)
1 (20)
1 (40)
1 (10)
1 (10)
1 (40)
1 (40)
1 (40)
1 (40)
−
13.4
11.5
7.4
67
33
42
55
62
2
6
58
3
2
88
7
37
4
3
26
8
4.7
24
3
5
14
9
19.0
3.5
143
18
1
−
3
128
87
10
11
1
68
93
18
19
17
−
1.1
6
2
5
75
k
12
1.5
43
1
1
107
103
107
−
k
13
5
60
3.4
43
2
1
k
14
l
2 (20)
2 (20)
2 (20)
5
60
6.6
33
2
1
m
15
−
5
60
0.25
3.2
1.3
48
−
4
−
−
−
2.5
n
16
−
20
32
2.1
96
a
b
Reaction conditions: toluene 100 mL; 80 °C; catalyst precursors 1, 2, 3, or 4 (μmol); 1000 rpm, unless stated otherwise. Overall activity (104 g
c
d
molPd−1 h−1) for polymer formation, determined from the mass of polymer formed. Determined by quantitative 13C NMR in C2D2Cl4 at 130 °C. g
e
f
mol−1: determined by GPC in 1,2,4-trichlorobenzene at 160 °C vs linear polyethylene. Determined by DSC. Yield of the oligomerization step,
g
h
i
−1
determined from the mass of ethylene consumed. Oligomerization activity in 104 g molPd h−1. Reference 9p: 80 °C. Reference 9p: 70 °C.
j
k
l
m
Reference 16: 80 °C. 1 and 2 were added together. 2; toluene 50 mL; 95 °C; neat 1-butene (4 bar). Activity for 1-butene homopolymerization.
n
Ethylene (5 bar) and neat 1-butene (2 bar) were added together.
C
DOI: 10.1021/acs.macromol.6b00581
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combination of 1 with a precursor 4 to a less active
polymerization catalyst,16 the degree of branching in the
resulting linear polyethylenes reaches ca.100 (Me, 2; Et, 93; Bu,
5) per 1000 C (Table 1, entries 6 and 8 vs 10 and 11).
Likewise, by a one-pot single step polymerization procedure
exposing a mixture of 1 and 2 to ethylene as the only monomer
(cf. Supporting Information and the Experimental Section),
linear polyethylenes containing short-chain branches were also
obtained (Table 1, entries 12−14).
The microstructures of the resulting polymers were revealed
by 1D- and 2D- 1H and 13C NMR spectroscopy. In view of the
very low number of branches (Me, 1; Et, 2) in the polymer
from entry 9 of Table 1, the signals of isolated ethyl branches
EBE (predominantly from 1-butene incorporation) and Me
branching (from chain walking) in the in-chain structures
(Chart 4) are readily confirmed (cf. Supporting Information,
octene into the main chain cannot be observed.20 The presence
of a diad (BBE) of incorporated 1-butene is in line with the
observation that the polymerization catalyst employed is
capable of 1-butene homopolymerization in the absence of
ethylene (Table 1, entry 15, Figure S20), to generate an
oligomer with an average degree of polymerization of ca. 7. In
addition, four kinds of terminating chain ends are also detected,
arising from chain transfer by β−H elimination after ethylene
or 1-butene insertion, respectively. Note that, after standing at
130 °C for a long time (6 h) in C2D2Cl4, trisubstituted olefin
end groups in the polymer are unexpectedly observed, which
originate from the isomerization of terminal olefins (Figure
S19).
Linear Polyethylene Containing Short-Chain Branches
and Polar Functional Groups. Because of the tolerant nature
of late transition metals toward polar functional groups, the
tandem catalysis by 1 and 2 offers itself in particular for
additional incorporation of polar vinyl monomers. In view of
the different regiochemistry (2,1-/1,2-) of insertion of polar
vinyl monomers with different electronic properties, electron-
poor as well as electron-rich, respectively, vinyl monomers were
studied.
Chart 4. Microstructures of Linear Polyethylene with Short-
Chain Branches
Electron-Poor Polar Vinyl Monomers. As the most
prominent electron-poor polar vinyl monomer, acrylate was
investigated. Under pressure reactor conditions, by a one-pot
single step polymerization procedure (cf. Experimental
Section), exposure of both 1 and 2 to ethylene and methyl
acrylate (MA) resulted in the formation of a branched ester-
functionalized linear polyethylene (Table 2, entry 1), in which
the degree of Et branching is ca. 13/1000 C and the density of
ester groups is ca. 41/1000 C. Likewise, by a one-pot
polymerization procedure with ethylene and MA as the only
feedstocks starting with catalyst precursor 1 and subsequently
adding 2 (cf. Experimental Section), a branched ester-
functionalized linear polyethylene with incorporations of 22/
1000C Et branches and of 41/1000 C ester groups was formed
(Table 2, entry 2). As expected, the incorporations of short
chain branches and polar groups significantly decrease the
melting point of polyethylene. Taking into account spectro-
scopic data of ethylene/1-butene copolymers18 and ethylene/
MA copolymers reported previously,9p quantitative 13C NMR
spectroscopic analysis of the resulting polymers is consistent
with 1-butene and MA incorporation into the main chain as
well as at the initiating and terminating chain ends (cf.
Supporting Information, Figure S21). Note that a very small
amount of consecutive 1-butene insertions (BBE) was also
detected.
Applying this one-pot polymerization procedure to the
copolymerization of ethylene and electron-poor N-isopropyla-
crylamide (NIPAA) also afforded a branched amide-function-
alized linear polyethylene (Table 2, entry 3).21 Note that
catalytic activity, the incorporation of branches, and the NIPAA
incorporation are lower compared to acrylate copolymerization,
indicating that amide functions retard the polymerization
reaction more strongly than ester function.9g Likewise, the
electron-poor methyl vinyl sulfone (MVS) could also be
incorporated in the tandem catalytic copolymerization with
ethylene to generate a linear polyethylene with short-chain
branches and sulfone functions,22 although the catalytic activity
and the incorporations were again lower than in acrylate
copolymerization (Table 2, entry 4). It is worth noting that the
tandem catalysts 1/2 were also applicable to such difficult
candidate as electron-poor acrylonitrile (AN) to yield a
Figure S12).17,18 In addition, the signals of EHE were
undoubtedly identified by comparison to an ethylene/1-hexene
copolymer sample prepared independently by copolymerization
of ethylene and added 1-hexene using iron catalyst (cf.
Supporting Information, Figure S7).19 To obtain a sample
devoid of additional branches from 1-hexene and 1-octene, the
copolymerization of ethylene and added 1-butene was also
performed by catalyst precursor 2 alone, affording an ethylene/
1-butene copolymer with a degree of Me and Et branching of 4
and 32 per 1000C, respectively (Table 1, entry 16). In the 13C
NMR spectrum of this ethylene/1-butene copolymer, the
signals of EBE, BBE, BEB, and Me branching are observed (cf.
Supporting Information, Figure S18).18 On the basis of the
aforementioned well-identified signals, microstructures of a
representative polymer (Me, 3; Et, 62; Bu, 5) obtained from
tandem catalysis were fully assigned (Table 1, entry 8). As
shown in Chart 4 and Figure 2, there are two types of saturated
initiating chain ends, namely n-alkyl and 2-ethyl- n-alkyl groups.
The former is possibly formed by insertion of ethylene into a
Pd−Me or Pd−H bond or 1,2-insertion of 1-butene into a Pd−
H bond, the latter 2-ethyl motif likely arises from 2,1-insertion
of 1-butene into a Pd−H bond. In the main chain structure,
EBE, BBE, BEB, EHE, and Me branching are observed. The
signal of a hexyl branch from the insertion of in situ-formed 1-
D
DOI: 10.1021/acs.macromol.6b00581
Macromolecules XXXX, XXX, XXX−XXX