GPC and ESI-MS analysis of labeled poly(1-hexene): Rapid determination of initiated site counts during catalytic alkene polymerization reactions

Article abstract of DOI:10.1021/ja105775r

Accurate active-site counts are necessary for the establishment of olefin polymerization kinetics, yet current techniques are often tedious and limited in sensitivity. Herein, we describe the development of a novel method for determining the fraction of initiated catalyst using standard gel-permeation chromatography (GPC). The first insertion of monomer into the chromophore-bearing zirconocene 1 generates a single equivalent of a labeled polymer chain. The polymer-bound label can be quantified as a function of polymer molecular weight using a GPC with a UV detector; simultaneous RI detection allows both the extent of monomer conversion and the molecular-weight distribution to also be established. We estimate that monomer to catalyst ratios of as high as 10000:1 can be measured using this technique.

Full text of DOI:10.1021/ja105775r

Published on Web 09/28/2010
GPC and ESI-MS Analysis of Labeled Poly(1-Hexene): Rapid Determination of
Initiated Site Counts during Catalytic Alkene Polymerization Reactions
Beth M. Moscato, Bolin Zhu, and Clark R. Landis*
Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison Wisconsin 53706
Received June 30, 2010; E-mail: landis@chem.wisc.edu
d8/0.2 mL of chlorobenzene-d5), clean activation is observed, generating
Abstract: Accurate active-site counts are necessary for the establish-
the monomeric uninitiated catalyst 3a (Scheme 1). This species has been
ment of olefin polymerization kinetics, yet current techniques are often
tedious and limited in sensitivity. Herein, we describe the development
of a novel method for determining the fraction of initiated catalyst using
1
extensively characterized by H and 19F NMR. Spectral data indicate
displacement of the [MeB(C6F5)3]- counterion by coordination of the
anilinyl ring to the zirconium center.
standard gel-permeation chromatography (GPC). The first insertion of
As anticipated, 3a catalyzes the polymerization of 1-hexene
monomer into the chromophore-bearing zirconocene 1 generates a
(Scheme 2). At -33 °C, catalyst initiation can be readily monitored
single equivalent of a labeled polymer chain. The polymer-bound label
by NMR observation of the decrease in 3a, which quantitatiVely gen-
can be quantified as a function of polymer molecular weight using a
erates a single equivalent of polymer-bound chromophore (4) upon
GPC with a UV detector; simultaneous RI detection allows both the
initiation. 1-Hexene consumption also can be monitored as a function
extent of monomer conversion and the molecular-weight distribution
of time. Assuming that all of the initiated catalyst propagates, this
to also be established. We estimate that monomer to catalyst ratios of
permits apparent rates of catalyst initiation and propagation to be
as high as 10 000:1 can be measured using this technique.
established (kiapp ) 0.00017 mol-1 L-1 s-1; kpapp ) 0.399 mol-1 L-1
s-1; both rate laws are first-order in zirconocene complexes and
1-hexene).8
The development of high-throughput organometallic synthesis and
screening techniques has accelerated the discovery of highly active
homogeneous olefin polymerization catalysts.1 Modern metallocene and
Scheme 2. Polymerization Initiated by 3a
postmetallocene catalysts are exceedingly active and generate highly
tailored polyolefins.2 The mechanisms of catalytic polymerization can be
complex;3 one important factor that limits mechanistic studies is practical
determination of catalyst speciation under realistic conditions. Herein, we
describe a novel approach to rapid catalyst-site counting that utilizes an
ionizable chromophore to label polymer chains and is amenable to high
throughput conditions using standard gel-permeation chromatography
(GPC) and mass spectrometry techniques.
Accurate active site counts are a vital but often neglected element of
polyolefin polymerization kinetic analysis.4 Most techniques require the
Chromophore-labeled polymer samples (CPol) generated from poly-
reaction of propagating, polymeryl-bearing catalytic centers with an NMR-
active5 or radioactive6 label, followed by tedious quantification. Alternative
approaches, such as in situ NMR spectroscopy, can also be used but
currently are limited to relatively slow processes at millimolar catalyst
concentrations.7 We reasoned that selective labeling of polyolefins with a
UV-chromophore would allow the concentration of an initiated catalyst
to be directly quantified as part of standard polymer analysis by GPC
with a UV detector.
To test this approach, we designed a labeled catalyst system, rac-
(SBI)Zr(Chrom)Me (1). 1 employs an industrially relevant ansa-metal-
locene backbone and a chromophore-bearing substituent, N,N-dimethyl-
anilinyldimethylsilyl-methylene. 1 can be readily synthesized in four steps
from commercially available starting materials and is stable indefinitely
under glovebox conditions.
merization reactions quenched with MeOD can be analyzed by ESI-MS,
GPC, and NMR. Simple polyolefins are not easily ionized, precluding
many mass spectral analyses.9 However, the first chain grown from 3a
contains an ionizable aniline group (Scheme 2). Although true polymer
mass distributions cannot be obtained from ESI-MS of these polymers,9d
high mass resolution enables chain-end analysis through measurement of
the ratio of deuterated to unsaturated chain ends, which differ by 3 amu
(Figure 1); this ratio reflects the ratio, at the time of quench, of actively
propagating catalyst polymeryls to chains liberated by ꢀ-hydride elimina-
tion. However, label decomposition under electrospray conditions and
cumulative instrument damage induced by large amounts of un-ionized
polyhexene discourage routine ESI-MS analyses.
Scheme 1. Activation of 1a
Figure 1. ESI-MS analysis of CPol sample. Conditions: [1] ) 9.8 mM;
[2] ) 11.2 mM; [1-Hexene] ) 0.47 M; 0.6 mL of tol-d8/0.2 mL of
chlorobenzene-d5, -40 °C, 3.5 h. Inset shows isotopic resolution.
Upon addition of excess B(C6F5)3 (2; 9 µmol in 0.2 mL of toluene-d8)
to a solution of 1 in toluene/chlorobenzene (8 µmol of 1, 0.5 mL of toluene-
9
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10.1021/ja105775r  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Results from Analysis of CPol Samples by NMR and GPCa
NMR Data
% Conv
GPC Data
c
Mw
Entry
[Zr]b (mM)
[B(C6F5)3] (mM)
[Hexene] (M)
Time (min)
% Init
% Init
% Conv
PDId
1
2
3
4
5
6
7
5
6
6
5
5
1.5
3.0
9
9
9
9
9
2.5
4.5
0.8
0.8
0.8
0.8
0.4
0.8
0.8
5
30
60
180
60
60
e
14
18
22
13
24
20
4f
71
95
100
89
52
1.9
13
17
21
12
1.8
69
104
112
90
17 900
32 600
31 200
34 400
34 500
38 000
40 500
2.0
2.2
2.3
2.3
2.2
2.3
2.2
21
18
49
73
60
77
a Reaction conditions: -33 °C in toluene-d8/chlorobenzene-d5 (0.7 mL/0.2 mL), with 5.45 mM diphenylmethane present as an internal standard.
Analysis procedures are described in the Supporting Information. b Absolute catalyst concentrations were established by NMR prior to initiation.
c Weight-average molecular weight. d Polydispersity index (Mw/Mn). e Peak could not be detected. f Reported percent conversion is uncertain due to low
value.
Alternatively, GPC analysis of quenched CPol samples yields
substantial, quantitative information. Unlike simple polyolefin
samples, two-detector GPC analysis of CPol samples yields two
distinct traces: an RI trace, which shows the mass concentration of
bulk polymer, and a UV-vis trace, which reflects the molar
concentration of polymers derived from 4 as a function of molecular
weight (Figure 2). Because each initiated catalyst must produce a
chromophore-labeled polymer, this technique uniquely enables
simultaneous monitoring of how much catalyst has undergone
initiation, the mass distribution of the first chain grown at a catalyst,
and mass distribution of the bulk polymer using standard equipment.
sensitivity can be readily improved using this technique by either
increasing the concentration of polymer or developing better
chromophores.
The significance of this approach is twofold. First, initiated
catalyst concentrations and molecular weight distributions as a
function of time are among the two crucial components necessary
for detailed kinetic studies (the other components, such as end group
analysis, can be readily determined using standard NMR techniques
or mass spectrometry).10 At short reaction times the UV-detected
mass distributions are particularly sensitive to initiation kinetics.
Second, the method is practical: it uses common instrumentation,
is suitable for very active catalysts, and can be automated for high
throughput.
Although experiments with 1 establish that careful choice of
catalyst precursors generates polymers suitable for ESI-MS detec-
tion and quantitative GPC analysis, this chromophore is not ideal.
First, the label, which bears a N,N-dimethylanilinyl group, is
susceptible toward reaction with cationic metallocenes. Additionally,
4 decomposes in the presence of acid and is slightly unstable even
in the presence of base. Tentative evidence suggests this may be
due to cleavage of the Si-phenyl bond. Even with added NEt3,
reaction workup must be relatively rapid and sample analysis must
occur within a week or inaccurate concentrations of 4 will be
measured. Furthermore, 4 is expected to have different initiation
rates (but not propagation or chain transfer rates) than other common
initiating alkyl groups such as methyl and benzyl. In the presence
of additives such as MAO, aluminum alkyls, or diethyl zinc that
can undergo transmetalation reactions with polymerization catalysts,
the chromophore-labeled polymer may not reflect the total initiated
catalyst.11
Figure 2. GPC analysis of CPol sample. Conditions are described in Table
1, entry 3. Red line: UV trace (left axis); blue line: RI trace (right axis).
To test the quantitative accuracy of this labeling technique, results
from GPC analysis were directly compared to those from NMR
analysis of quench-labeled polymer samples (Table 1). At the
specified time of quench, samples intended for GPC analysis were
quenched with a mixture of methanol-d4 and NEt3 and then
transferred to a precooled NMR probe, where the concentration of
4 and the remaining hexene concentration were established via
quantitative 1H NMR. Following workup, samples were diluted into
10 mL of THF and analyzed on a Viscotek GPC equipped with
refractive index and UV-vis detectors. As shown in Table 1, results
from NMR and GPC analysis agree well, uniquely demonstrating
determination of the concentration of initiated sites of an alkene-
polymerization catalyst directly from GPC analysis of bulk poly-
olefin samples.
In conclusion, we have shown that GPC analysis of polyolefins
synthesized from a labeled catalyst can provide highly accurate
initiated site counts and bulk polymer mass distributions. This
technique is rapid and versatile, can detect as little as 0.1 nmol of
chromophore, and uses common instrumentation. In order to
facilitate routine application of these methods we are developing
more chemically inert chromophores.
The sensitivity of the GPC technique was compared with that
2
of other site counting methods. Previously, we estimated that H
Acknowledgment. We thank Professor Mahesh Mahanthappa
for GPC access, Dr. Kevin Kreisel and Dr. Maren Buck for help
with GPC analysis, Professor Lloyd M. Smith for ESI-MS access,
Dr. Brian Frey for help with ESI-MS, and the DOE and Dow
Chemical for funding.
labeling, through use of a labeled catalyst or through quench-
labeling with MeOD, was sensitive to samples with more than one
active site per 3000 monomer insertions, at best.5a At minimum,
samples shown above contain a 4/monomer ratio of 1:1200. Dilution
experiments demonstrate that as little as 0.1 nmol of 4 can be
detected and roughly quantified by GPC; this corresponds to a
4/monomer ratio of ca. 10 000:1 for GPC analysis at a polymer
concentration of 1 mg/mL. Unlike common NMR techniques,
Supporting Information Available: Synthesis of 1; characterization
of 1, 3a, and 4; conditions and preliminary apparent kinetic information
for 3a-catalyzed 1-hexene polymerization, procedure for ESI-MS and
9
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C O M M U N I C A T I O N S
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GPC analysis of CPol, and supplemental NMR and GPC spectra and
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
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