Analysis of the products of the anionic oligomerisation of a phosphaalkene using MALDI-TOF mass spectrometry

Article abstract of DOI:10.1039/b406160h

The first use of MALDI-TOF MS in the study of the products of RLi (R = Me, Bu) initiated oligomerisation of P=C bonds is reported. These studies may be considered as models for analogous polymerisation reactions. The detected linear products with R-P and

Full text of DOI:10.1039/b406160h

Analysis of the products of the anionic oligomerisation of a
phosphaalkene using MALDI-TOF mass spectrometry†
Bronwyn H. Gillon and Derek P. Gates*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia,
Canada V6T 1Z1. E-mail: dgates@chem.ubc.ca
Received (in West Lafayette, IN, USA) 26th April 2004, Accepted 1st June 2004
First published as an Advance Article on the web 1st July 2004
The first use of MALDI-TOF MS in the study of the products of
RLi (R = Me, Bu) initiated oligomerisation of PNC bonds is
reported. These studies may be considered as models for
analogous polymerisation reactions. The detected linear prod-
ucts with R–P and C–H end-groups are consistent with a chain
growth mechanism similar to that known for olefins. Inter-
estingly, the results suggest that backbiting may compete with
chain growth.
revealing endgroups which, thus far, have been undetectable by
NMR spectroscopy. To date, we have not been able to successfully
analyse polymeric samples of 2 by MALDI-TOF MS. Therefore,
we attempted to synthesise small oligomers that might be easier to
ionise using MALDI techniques.
Small oligomeric poly(methylenephosphine)s were prepared
conveniently from the ambient temperature reaction of 1 (3 equiv.)
with in situ generated Mes(Me)P–C(Ph)
Li (d 31P = 245 ppm) in
2
diethyl ether (Scheme 1). Our previous work on the polymerisation
The addition polymerisation of olefins is one of the most important
methods of polymer synthesis and forms the basis for the
production of many commodity organic materials. There has been
considerable interest in the synthesis and reactions of phosphaalk-
enes, and the remarkable parallels between PNC and CNC bonds in
of 1 with MeLi (5%) in a minimum of solvent required a
temperature of 150 °C. Significantly, the present results suggest
2
that in solution high temperatures may not be necessary. After
oxidation and workup, analysis by 31P NMR spectroscopy revealed
that, analogous to oxidised 2, only a broad signal (d = 45 ppm) was
1
1
molecular chemistry. We have been interested in expanding the
observed. Interestingly, end-groups were not identified in the H or
31
so-called “phosphorus–carbon analogy” to polymer science, and
have recently reported the high temperature polymerisation of
phosphaalkene 1 to give poly(methylenephosphine) 2.2 The
incorporation of inorganic elements into the polymer backbone is
an active area of research and often leads to materials with novel
properties.3 Previously, addition polymerisation was often dis-
missed as unsuitable for inorganic systems. Now that 1 has been
polymerised, one question that must be addressed is whether the
initiation and propagation steps are analogous to those known for
olefins. If so, by analogy with olefins, living polymers and novel
block or end-functionalised polymers may be accessible. Knowl-
edge of the structure of 2, including end-groups, will provide
insight into the mechanism of their formation. Here, we report the
ambient temperature oligomerisation of 1 using anionic initiators
and the detection of oligomers with up to 11 repeat units using
MALDI-TOF MS. This work demonstrates that chain growth
proceeds in solution without heating and the observation of end-
groups confirms that the mechanism of anionic polymerisation for
PNC bonds is analogous to that for olefins. To our knowledge,
oligomeric phosphaalkenes larger than diphosphetaines have not
previously been characterised by mass spectrometry.5,6
P NMR spectra and are possibly obscured by broad signals of the
aryl and alkyl side-groups.
A sample for MALDI-TOF analysis was prepared with the
matrix (2,5-dihydroxybenzoic acid) using the layer method. A
typical MALDI-TOF mass spectrum is shown in Fig. 1. Re-
markably, a series of ions spaced by the mass of the monomer unit
7
,4
(332 Da) were observed that were assigned to linear oligomers 3
(n = 3–11). The masses of the oligomers were within experimental
n
accuracy (±0.1%) of the calculated masses for protonated species
+
(3
n
+H) . For example, inset A in Fig. 1 clearly shows an isotope
+
pattern with lowest mass 1676 Da which is 1 less than (3
5
+H)
; calcd. 1677 g mol ). The isotope pattern is
consistent with that expected with the most intense ion due to the
2
1
110 5 5
(C111H P O
Scheme 1 Reagents and conditions: i, MeLi (1 equiv), Et
C, 30 min; ii, 1 (3 equiv), 25 °C, 16 h; iii, H O (1 drop); iv, H
CH Cl , 25 °C, 30 min, the higher oligomers were isolated by precipitation
from a CH Cl solution with hexanes, 54%.
2
O, 280 °C to 25
°
2
2 2
O
(excess),
2
2
2
2
The emergence of soft ionisation (i.e. MALDI) mass spectro-
metric techniques to investigate high molecular weight (MW)
macromolecules is revolutionising the field of synthetic polymers,7
however their use is still relatively uncommon in the analysis of
8
inorganic systems. Importantly, MALDI-TOF MS gives absolute
masses rather than relative (i.e. GPC) and usually only molecular
ions are observed without fragmentation. We have been interested
in analysing poly(methylenephosphine) 2 by MALDI-TOF MS.
One objective is to obtain accurate MW’s for 2 since laser light-
scattering has shown that the MW is underestimated by GPC (vs.
2
polystyrene standards). Moreover, MALDI-TOF MS may provide
mechanistic insight into this new polymerisation reaction by
†
Electronic supplementary information (ESI) available: experimental
procedures and an additional figure. See http://www.rsc.org/suppdata/cc/
b4/b406160h/
Fig. 1 MALDI-TOF MS of oligomer mixture. * tentative assignment.
1
868
C h e m . C o m m u n . , 2 0 0 4 , 1 8 6 8 – 1 8 6 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

presence of 1 3 13C (observed at 1677). Remarkably, oligomers of
confirm whether backbiting is indeed involved and, if so, develop
methods to minimise it. These results may provide a rationale for
the relatively low MWs (ca. 10 g mol by GPC) and isolated
yields (ca. 30–40%) thus far obtained for polymer 2 and related
polymers12 prepared using anionic initiators.
+
21
up to 11 repeat units [(311+H) , 3669 Da; calcd., 3669 g mol
]
4
21
were observed where the most intense ion was due to the presence
13
of 2 3 C in the oligomer. The observation of ions consistent with
the presence of CH and H end-groups in 3 suggests that linear
3
n
products are obtained and that the mechanism of chain growth is
analogous to that for olefins. Confirmation of this postulate was
obtained by varying the initiating species. Specifically, we have
initiated oligomerisation of 1 with BuLi (0.5 equiv.) and quenched
We are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada for financial support of this
work. We also thank Dr. Yun Ling and Marshall Lapawa for
assistance with the MALDI-TOF MS.
2
with H O. The MALDI-TOF mass spectra of the oligomers were
consistent with linear species containing Bu and H end-groups,
respectively. Oligomers with up to 6 repeat units were observed.
One puzzling feature of the MALDI-TOF mass spectra,
regardless of whether MeLi or BuLi is used, is the unexpected
presence of a second series of oligomers spaced by the monomer
unit (332 Da). This second series can clearly be seen in Fig. 1.
Remarkably, each peak in this series is situated exactly halfway (i.e.
Notes and references
1 For recent reviews dealing with the phosphaalkenes, see: F. Mathey,
Angew. Chem. Int. Ed., 2003, 42, 1578; K. B. Dillon, F. Mathey and J.
F. Nixon, Phosphorus: The Carbon Copy; Wiley: New York, 1998; M.
Yoshifuji, J. Chem. Soc., Dalton Trans., 1998, 3343.
2
C.-W. Tsang, M. Yam and D. P. Gates, J. Am. Chem. Soc., 2003, 125,
480.
1
±
n
166 Da) between the ions assigned to the linear oligomers (3 ).
3
For reviews of inorganic polymers see: I. Manners, Angew. Chem., Int.
Ed. Engl., 1996, 35, 1602; H. R. Allcock, Adv. Mater., 1994, 6, 106; J.
E. Mark, H. R. Allcock and R. West, Inorganic Polymers, Prentice Hall,
New Jersey, 1992; H. R. Allcock, Chemistry and Applications of
Polyphosphazenes, Wiley: Hoboken, New Jersey, 2003; D. P. Gates,
Annu. Rep. Prog. Chem., Sect. A, 2003, 99, 453.
This observation suggested multiply charged species. However,
after close inspection of the mass spectra this hypothesis was ruled
9
out since the ions were spaced by integer mass units. After further
consideration, we assigned these signals to linear oligomers with an
extra phosphorus group at the end of the chain (4
shows signals consistent with 4 with calculated mass of 1842 Da.
Structures with an extra CR and P–Me end-group are unlikely
since a CPh –CPh backbone linkage would be necessary to
n
). Fig. 1, inset B,
4
For recent work on phosphorus polymers, see: C. A. Jaska, A. J. Lough
and I. Manners, Inorg. Chem., 2004, 43, 1090; R. C. Smith and J. D.
Protasiewicz, J. Am. Chem. Soc., 2004, 126, 2268; V. A. Wright and D.
P. Gates, Angew. Chem. Int. Ed., 2002, 41, 2389; C. H. Walker, J. V. St.
John and P. Wisian-Neilson, J. Am. Chem. Soc., 2001, 123, 3846; H. R.
Allcock and R. Prange, Macromolecules, 2001, 34, 6858; H. Dorn, R. A.
Singh, J. A. Massey, J. M. Nelson, C. A. Jaska, A. J. Lough and I.
Manners, J. Am. Chem. Soc., 2000, 122, 6669; V. Maraval, R. Laurent,
B. Donnadieu, M. Mauzac, A.-M. Caminade and J.-P. Majoral, J. Am.
Chem. Soc., 2000, 122, 2499; V. Chunechom, T. E. Vidal, H. Adams
and M. L. Turner, Angew. Chem. Int. Ed., 1998, 37, 1928.
5
2
2
2
account for the observed masses.
The reason for this second series of oligomers, is not obvious. It
is possible that these species simply result from fragmentation of
n
3 , however, this is relatively uncommon in polymer MALDI-TOF
7
,10
mass spectrometry.
Although if the P–C chains are easily
could be accounted for. However,
fragmented during ionisation, 4
n
if fragmentation during ionisation is facile, we should see
oligomeric fragments in attempted MALDI analysis of polymer 2.
Under analogous MALDI conditions we have not observed any
5
6
For examples of diphosphetaines, see: A. Mack, U. Bergsträber, G. J.
Reib and M. Regitz, Eur. J. Org. Chem., 1999, 3, 587; M. Schmitz, S.
Leininger, U. Bergsträber and M. Regitz, Heteroat. Chem., 1998, 9,
3
4
mass spectra for the polymer 2 (GPC M = 5 3 10 – 1 3 10 g
n
4
53.
2
1
mol ). In addition, oxidised 2 is thermally stable up to 320 °C as
determined by TGA which seems to suggest a stable backbone not
likely to fragment easily. Therefore, although we cannot rule out
A fascinating oligomerisation chemistry exists for phosphaalkynes. See,
for example: T. Wettling, J. Schneider, O. Wagner, C. G. Kreiter and M.
Regitz, Angew. Chem., Int. Ed. Engl., 1989, 28, 1013; R. Bartsch, P. B.
Hitchcock and J. F. Nixon, J. Chem. Soc., Chem. Commun., 1989, 1046;
V. Caliman, P. B. Hitchcock, J. F. Nixon, M. Hofmann and P. v. R.
Schleyer, Angew. Chem., Int. Ed. Engl., 1994, 33, 2202; F. Tabellion, A.
Nachbauer, S. Leininger, C. Peters, F. Preuss and M. Regitz, Angew.
Chem. Int. Ed., 1998, 37, 1233; A. M. Arif, A. R. Barron, A. H. Cowley
and S. W. Hall, J. Chem. Soc., Chem. Commun., 1988, 171; D. A. Loy,
G. M. Jamison, M. D. McClain and T. M. Alam, J. Polym. Sci. (A),
n n
fragmentation of 3 as a source of 4 , it is unlikely. If these are not
fragments, these observations may have important implications on
the proposed mechanism of anionic polymerization of 1. This will
now briefly be elaborated upon.
1
999, 37, 129.
S. F. Macha and P. A. Limbach, Curr. Opin. Solid State Mater. Sci.,
002, 6, 213; M. J. Stump, R. C. Fleming, W.-H. Gong, A. J. Jaber, J.
J. Jones, C. W. Surber and C. L. Wilkins, Appl. Spectrosc. Rev., 2002,
7, 275; S. D. Hanton, Chem. Rev., 2001, 101 , 527.
7
8
2
3
For examples of MALDI-TOF of inorganic macromolecules, see: H. R.
Allcock, C. R. de Denus, R. Prange and W. R. Laredo, Macromolecules,
n
We speculate that species 4 (n = 3–10), if not arising from
fragmentation, could also result from backbiting during the
polymerisation reaction. The proposed mechanism would involve
nucleophilic attack of the growing carbanionic chain-end on either
2
001, 34, 2757; B. M. White, W. P. Watson, E. E. Barthelme and H. W.
Beckham, Macromolecules, 2002, 35, 5345; F. H. Köhler, A. Schell and
B. Weber, Chem. Eur. J., 2002, 8, 5219; J.-C. Blais, C.-O. Turrin, A.-M.
Caminade and J.-P. Majoral, Anal. Chem., 2000, 72, 5097; V. Marvaud,
D. Astruc, E. Leize, A. Van Dorsselaer, J. Guittard and J. C. Blais, New
J. Chem., 1997, 21, 1309.
a P or C site in its own backbone – attack on C would give 4
n
after
H
2
O/H workup. Backbiting reactions of the type proposed are
2 2
O
uncommon in olefin polymerisation. However, similar backbiting
processes are very common in the anionic ring-opening polymer-
9 If a complex was doubly charged, the difference between peaks would
be 0.5 Da.
3
,11
isation of cyclic trisiloxanes [(R
nism necessitates the formation of cyclic oligomers (5
inspection of Fig. 1 (inset A and B) show that ions are detected
exactly 16 Da below 3 and 4 , respectively. These signals could be
due to a loss of oxygen or may also be assigned to cyclic oligomers
and 6 . Moreover, cyclic species are observed in the analysis of
BuLi initiated oligomers, confirming that they are 5 and 6 rather
than linear minus oxygen. Cyclic species are only observed up to n
5 for the MeLi initiated reaction. Further studies are underway to
2
SiO)
3
].
A backbiting mecha-
1
1
0 H. J. Räder and W. Schrepp, Acta Polymer., 1998, 49, 272.
1 For MALDI-TOF detection of cyclics in ROP of siloxanes or dioxane-
ones, see: M. Barrère, C. Maitre, M. A. Dourges and P. Hémery,
Macromolecules, 2001, 34, 7276; J. Libiszowski, A. Kowalski, R.
Szymanski, A. Duda, J.-M. Raquez, P. Degée and P. Dubois,
Macromolecules, 2004, 37, 52.
n
, 6 ). Close
n
n
n
5
n
n
n
n
1
2
2 Anionic polymerisation of RPNCR has been extended to a variety of
monomers: M. Yam, C.-W. Tsang, K. Noonan, J. Kingsley, B. O.
Patrick and D. P. Gates, Manuscript in preparation.
=
C h e m . C o m m u n . , 2 0 0 4 , 1 8 6 8 – 1 8 6 9
1869