Awakening a dormant catalyst: Salicylaldimine systems for ethene/tert-butylstyrene copolymerization

Article abstract of DOI:10.1039/b818829g

A group of readily available zirconium catalysts incapable of ethene-co-styrene polymerization are remarkably active and selective for the production of the new polymer ethene-co-tert-butylstyrene via a single site mechanism.

Full text of DOI:10.1039/b818829g

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www.rsc.org/dalton | Dalton Transactions
Awakening a dormant catalyst: salicylaldimine systems for
ethene/tert-butylstyrene copolymerization†
Giles W. Theaker,a Colin Mortonb and Peter Scott*a
Received 23rd October 2008, Accepted 3rd November 2008
First published as an Advance Article on the web 10th November 2008
DOI: 10.1039/b818829g
A group of readily available zirconium catalysts incapable of
ethene-co-styrene polymerization are remarkably active and
selective for the production of the new polymer ethene-co-
tert-butylstyrene via a single site mechanism.
In this context we noted Bercaw’s study into stoichiometric
1,2-migratory insertion of a range of a-methylstyrenes into a
metal-carbon bond.23 Rates followed the order of the Hammett
parameter for the para substituent (sp) i.e. OMe > Me > H > F >
CF3. We set out to see whether this s effect could be used to (i) in-
crease the rate of 1,2-insertion in our target copolymerization, thus
increasing comonomer incorporation and overall rate, and/or (ii)
reduce the rate of production of the dormant state, the rationale
being that a s-donating para substituent would oppose build-up
of negative charge at the benzylic position during 2,1-insertion.
We report here a dramatic substituent effect of this kind;
salicylaldiminate catalysts which do not copolymerize ethene and
styrene are very effective for ethene/4-tert-butylstyrene (TBS—a
monomer previously used in copolymerizations with styrene24–26
and extensively with CO27–31), giving a new class of essentially
monodisperse polymers at high rates and catalytic stability.
Salicylaldimine ligands Ln (n = 1–3) were chosen to provide
a range of electron donating abilities in the subsequent com-
Ethylene-styrene (ES) copolymer resins have many potential
applications,1 but commercial realisation of a chemical process
for their production has been hampered by the absence of robust
catalysts that are compatible with both monomers. For example,
Ziegler–Natta catalysts for polythene are generally sluggish and
unselective in ES copolymerization.2–7 A good deal of experimental
and theoretical effort has been expended on elucidating the cause
of this behaviour, and the consistent picture that emerges is that
the benzylic species I (Fig. 1) formed by 2,1-insertion of the
styrene into the growing polymer chain is resistant to insertion of
a further monomer; the arene fragment coordinates strongly to the
electrophilic metal centre8 creating a motif that is familiar from the
solid state molecular structures of zirconium benzyls.9 In contrast,
the product of 1,2-insertion II inserts further monomers more
readily. Nevertheless, titanium constrained geometry catalysts
(CGCs) have been quite successful in this catalysis, giving high
activity and comonomer reactivity.10 Klosin, Timmers and co-
workers at Dow have recently described stable aryl-substituted
CGCs which promote styrene insertion, perhaps via interaction of
the aryls with incoming comonomer.11 Most recently Marks has
shown that dinuclear CGC catalysts exhibit significantly greater
activities than their monomeric counterparts as a result of a related
bimetallic cooperative effect.12
plexes Ln ZrCl2 (see Fig. 2, ESI†) since this is known to affect
2
thermal stability in the homopolymerization of ethene32 and might
also have an effect on the relative rates of the two insertion modes
(Fig. 1).
Fig. 2 Precatalysts Ln ZrCl2 used in this study.
2
Table 1 contains the data for a series of runs for the homopoly-
merization of ethene and copolymerizations of ethene with styrene
or tert-butylstyrene at 40 ◦C. The catalysts performed as expected
in the ethene homopolymerizations runs (1–3), giving steady gas
uptake over ca. 10 min (Fig. 3) before stirring became difficult
and the reaction was terminated. Table 1 also shows the various
molecular weights as determined by GPC, including the carbon
backbone numbers Cn calculated from Mn and Mw. As expected,33
the polyethenes produced by these catalysts are of low molecular
weight compared with those from metallocene or constrained
geometry catalysis,10,34 and had multi-modal Mw distributions
with broad high molecular weight (low retention time) shoulders
(Fig. 4). The observed modality has been attributed to the presence
of a number of isomeric forms of the catalyst species.35–37
Fig. 1 Insertion of styrenic monomers into a growing polymer chain.
For the salicylaldiminate (a.k.a. FI) systems developed by
Fujita13–15 and Coates16–18 no such ES copolymerisation has been
reported19 despite their many other successes.20,21 The lack of
reactivity is most probably as result of the prevalence of the
2,1-insertion mechanism for a-olefins in these catalysts22 giving
unreactive benzyls of type I.
aDepartment of Chemistry, University of Warwick, Gibbet Hill Road,
Coventry, UK CV4 7AL. E-mail: peter.scott@warwick.ac.uk; Tel: +44 24
7652 3238; Fax: +44 24 7657 2710
bInfineum UK Ltd, Milton Hill, Abingdon, UK OX13 6BB
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b818829g
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Table 1 Polymerisation of ethene, ethene-co-styrene and ethene-co-TBS by catalysts 1–3/methyl aluminoxane (MAO)
Cn
Runa
Cat.
Comon.
Mon./M Yield/g
Mass prod.b
Mol prod.c
Comon. (mol%)
Tm/◦C
Mn
Mw
9835
12950
9295
n/d
n/d
n/d
5215
6470
5085
PDi
Mn
Mw
1
2
3
4
5
6
7
8
9
1
2
3
1
2
3
1
2
3
—
—
—
Styrene
Styrene
Styrene
TBS
TBS
TBS
—
3.1
4.3
5.8
1.4
0.4
1.7
6.4
6.3
7.7
2.60
3.82
3.05
0.51
0.21
0.38
2.81
4.20
3.88
92.9
136.2
108.8
12.5
7.6
—
—
—
n/a
n/a
n/a
4.9
7.4
11.3
123
123
123
—
—
—
122
114
92
2035
2345
2420
n/d
n/d
n/d
2800
2275
2580
4.8
5.5
3.8
—
—
—
1.86
2.84
1.97
145 702
167 924
173 663
n/d n/d
n/d n/d
n/d n/d
162 302
120 342
120 237
—
—
0.70
0.70
0.70
0.70
0.70
0.70
9.6
81.6
111.0
90.5
a Reaction conditions: Al/Zr = 1000; 1.2 bar ethene; 40 ◦C; overall volume = 100 ml. gpoly molcat h-1 bar-1 (/106). c molmon molcat h-1 bar-1 (/103),
b
-1
-1
where molmon = mol of monomer converted.
refractive index (RI) detected GPC. Run 4 (Fig. 4) is typical with
a higher molecular weight aromatic polymer (PS) indicated by
postive RI response and the principally aliphatic polymer (PE) at
lower molecular weight.
Replacement of styrene with TBS led to a dramatic change in
behaviour (runs 7–9). The catalyst stability was largely unchanged
vs. the ethene homopolymerizations, and indeed as a result of the
higher solubility of the polymers, the copolymerizations could be
conducted for extended periods without technical difficulty. Also,
substantial comonomer incorporation (5–11 mol%) was observed
even at this relatively low concentration of TBS; under similar
conditions, Cp2ZrCl2 and (Me2SiCp¢NtBu)TiCl2 incorporate ca. 2
and5% styrene respectively.38 Whilea detailedanalysis ofreactivity
ratios is in progress, it is encouraging that the overall rate is only
slightly depressed cf. the homopolymerisation. For example from
the molar productivity figures [molpoly molcat h-1 bar-1 (/10-3)] it
-1
can be seen that the average rates for copolymer syntheses are only
Fig. 3 Ethene uptake for polymerizations in Table 1.
13–18% lower than for the ethene homopolymerisations. This is a
remarkable observation, given the performance of these catalysts
in runs 4–6 and previously observed effects that the addition of
styrenic monomers leads to an order of magnitude reduction in
this parameter.
Unlike the homopolymers of runs 1–3, ethene-co-TBS poly-
mers produced in runs 7–9 were fully extractable into THF
(no detectable PE contaminant) and gave essentially unimodal
molecular weight distributions with a minor lower weight shoulder
(see run 7, Fig. 4). PDI’s were as low as 1.86 (run 9). This
observation is consistent with one of the possible isomeric forms
of the catalyst being predominant in the copolymerization. Also,
it has been noted that the addition of styrenic monomer to
metallocene catalysed ethene polymerizations, as well as greatly
lowering the productivity, lowers the molecular weight of the
polymer produced by as much as half.10 For the current system, no
such reduction in Mn or Cn was apparent on moving from ethene
homopolymerization to ethene-co-TBS.
Fig. 4 Refractive index detected GPC for selected polymers.
The 13C NMR spectra of the TBS/ethene copolymers (see
ESI†) are remarkable in that they show unusually sharp and
solitary peaks, suggesting high dispersity of the comonomer.
This is corroborated by the lack of peaks indicating sequential
TBS units, or TBS-E-TBS triads (this also indicates the absence
of poly-TBS). Ethyl and vinylic end groups were also readily
Upon addition of purified styrene (0.7 M), but otherwise under
the same conditions, the catalysts performed very poorly (runs
4–6). In some cases, ethene uptake was recorded for a short
period before the system was seen to deactivate (Fig. 3). This
behaviour was reproducible, and the system gave small quantities
of a “blend” of principally poly(styrene) (PS) and poly(ethene)
(PE) as shown by NMR spectroscopy, solvent extraction, and
1
assigned from H and 13C NMR and corresponding correlation
=
spectra; the absence of –C(Ar) CH2 end groups (which would
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result from b-elimination following 1,2-insertion of the styrene,
Fig. 1) corresponds with our observation that addition of the
styrene does not lead to a reduction in the chain length of the
polymer.
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This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 6883–6885 | 6885
©