The addition polymerization of a P=C bond: A route to new phosphine polymers

Article abstract of DOI:10.1021/ja029120s

Addition polymerization, the most general method of preparation for organic polymers, has successfully been extended to P=C bonds. The polymerization of a phosphaalkene has been initiated by thermolysis or with alkyllithium reagents. The unprecedented pol

Full text of DOI:10.1021/ja029120s

Published on Web 01/17/2003
The Addition Polymerization of a PdC Bond: A Route to New Phosphine
Polymers
Chi-Wing Tsang, Mandy Yam, and Derek P. Gates*
Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer,
British Columbia, Canada, V6T 1Z1
Received October 28, 2002 ; E-mail: dgates@chem.ubc.ca
The preparation of macromolecules possessing inorganic ele-
ments in the backbone is an area of current interest due to the
prospect of finding materials with unique properties.1-3 However,
the design of suitable methods for their preparation has hindered
their widespread development. Perhaps the most general and
industrially important method of organic polymer synthesis is the
addition polymerization of olefins. This method has often been ruled
out as a feasible method of inorganic polymer synthesis because
suitable unsaturated precursors are not readily available.1b,d,4 In
recent years, the synthesis and study of compounds containing stable
(p-p)π bonds involving the heavier elements (n > 2) of groups
14 and 15 has attracted considerable attention.5 However, their use
as precursors to inorganic polymers is virtually unexplored.6,7
A
goal of our research program is to polymerize stable heavier
element-containing multiple bonds. As a starting point we have
chosen PdC bonds which are known to exhibit numerous parallels
to the chemistry of CdC bonds.8 Furthermore, as a consequence
of strong, relatively nonpolar P-C bonds in the main chain, we
anticipate such a polymer to be air-stable through simple chemical
modification.9 In this work, we report the first polymerization of a
phosphaalkene (1) to give poly(methylenephosphine) 2, a new
polymer with phosphorus(III) atoms in the main chain.
Figure 1. 13C NMR spectra of (a) Mes(Me)PCPh2H in CDCl3 and (b) 2 in
CDCl3. (*) indicates CDCl3.
revealed several sharp resonances and a broad signal (-10 ppm)
in the phosphine region. After precipitation (×4) of viscous THF
solutions of the residue with hexanes, poly(methylenephosphine)
1
2 was obtained as a colorless solid (yield ) 7%). The H and 13C
NMR spectra exhibited signals consistent with the proposed
structure of 2. The 13C NMR spectrum of 2 (Figure 1b) shows
resonances which are remarkably similar to those of the model
system Mes(Me)PCPh2H (Figure 1a), and their breadth is consistent
with the polymeric nature of 2. In particular, the backbone
quaternary P-C(Ph)2-P moiety was detected as a broad resonance
at 52 ppm (Figure 1b). Importantly, GPC (THF) analysis of the
new polymer 2 resulted in an estimated Mn of 11500 g/mol (vs
polystyrene) and a polydispersity index (PDI; Mw/Mn) of 1.25.
Poly(methylenephosphine) 2 is reasonably air- and moisture-
stable in the solid state; however, it slowly forms phosphine oxide
3a after 4 d in CH2Cl2 solution when exposed to air. The oxidized
The instability of phosphaalkenes bearing small substituents has
often been attributed to undesired “polymerization” reactions.8c,10
However, these reactions were likely uncontrolled, and the products
were never characterized. To study reactions with polymerization
initiators, isolable phosphaalkenes with minimal steric protection
are desired. We have reported that treating Mes*PdCH2 (Mes* )
2,4,6-tri-tert-butylphenyl)11 with cationic initiators (i.e., GaCl3,
AlCl3, HOTf) results in intramolecular C-H activation of the Mes*
group, effectively preventing chain growth.12 In our search for other
candidates for polymerization, phosphaalkene 1 (Mes ) 2,4,6-
trimethylphenyl), the first stable compound with a localized PdC
bond, attracted our attention.13,14 Interestingly, although 1 was
isolable, it has been reported that employing the 2-methylphenyl
substituent at P resulted in a “polymeric material” rather than
phosphaalkene.13 No spectroscopic data for this material were
provided. Thus, 1 appeared to be an ideal candidate for polymer-
ization because it is isolable but has minimal kinetic stabilization
and was unlikely to undergo intramolecular C-H activation.
Monomer 1 was prepared according to a literature procedure,15
and after vacuum distillation of the crude product (bp ) 150 °C;
0.1 mmHg), a gummy, pale-brown residue remained. In addition
to 1 (δ ) 233), the 31P NMR spectrum of the residue in CH2Cl2
31P
polymer 3a can be obtained quantitatively by NMR (δ ) 47),
and in moderate isolated yield (60%) by treating a CH2Cl2 solution
of 2 with excess H2O2 (30% in H2O). The molecular weight of air-
and moisture-stable 3a was estimated using GPC (Mn ) 8800; PDI
) 1.27) and was similar to that for 2. Similarly, poly(methylene-
phosphine) 2 reacts with elemental sulfur in CH2Cl2 to yield
31P
phosphine sulfide 3b (84%; δ ) 52) for which the molecular
weight was estimated using GPC (Mn ) 11900; PDI ) 1.24). Due
to possible inaccuracy of estimated molecular weights versus
polystyrene, the polymer 3b was analyzed using light-scattering
GPC and viscometry. Significantly, the molecular weight deter-
mined for 3b (Mw ) 35000) using light scattering was much higher
than that estimated using polystyrene standards. The thermal
stability of the polymers was evaluated using TGA, and they were
stable to weight loss until 265 °C (2), 320 °C (3a), and 220 °C
(3b), whereupon they each lost 85-95% of their mass.
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J. AM. CHEM. SOC. 2003, 125, 1480-1481
10.1021/ja029120s CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
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We shifted our attention to possible anionic initiators. When 1
and MeLi (0.05 equiv) in a minimum of THF (ca. 0.2 mL) were
heated in a sealed tube at 150 °C for 24 h the solution became
increasingly viscous.16 The polymer mixture was quenched with
MeOH and precipitated with hexanes; the isolated polymer (2, yield
1
) 30%) showed 31P, 13C, and H NMR spectra identical to those
obtained from distillation. The molecular weight of poly(methyl-
enephosphine) 2 was estimated (vs polystyrene) using GPC (Mn )
6600, PDI ) 1.55). The polymerization of 1 in the presence of
MeLi is reproducible, and the estimated molecular weights for 2
were between 5000 and 10000 g/mol. Monomer 1 can also be
polymerized using BuLi (0.05 equiv) [2 (25%): Mn ) 5400, PDI
) 1.15].
The mechanism of the polymerization of 1 using anionic species
is of interest. The addition of anionic reagents (i.e., MeLi) across
PdC and PtC bonds has been reported. For example, the linear
dimer MePdC(Mes*)PdC(Mes*)Li has been prepared by treating
the phosphaalkyne (Mes*CtP; 2 equiv) with MeLi (1 equiv).17
Moreover, Mes(Me)P(O)CHPh2 has been isolated after quenching
the reaction of 1 and MeLi with MeOH. The authors concluded
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Significantly, the formation of 4 from 1 and MeLi represents the
first step in the putative anionic polymerization of 1. We have
repeated the reaction of 1 with MeLi (1 equiv) and observed 4
31
(δ P ) -45) quantitatively which, after treatment with H2O, gave
31
Mes(Me)PCPh2H (δ P ) -24). Furthermore, monomer 1 polymer-
izes in the presence of 4 (0.05 equiv; 24 h; 150 °C) to yield 2
(39%; Mn ) 6200; PDI ) 1.29). Additional evidence for the
involvement of 4 in the polymerization of 1 using RLi is provided
by the fact that pure 1 does not polymerize at 150 °C in the absence
of initiator. These results are consistent with an anionic mechanism;
however, further studies must be conducted to confirm this
hypothesis.19
In summary, we have reported the first polymerization of a Pd
C bond to yield a stable new class of functional phosphorus
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Acknowledgment. The support of the Natural Sciences and
Engineering Research Council (NSERC) of Canada is gratefully
acknowledged. We also thank Dr. Scott Clendenning (U. Toronto)
for the GPC-LLS analysis of 3b.
(16) Thus far, at lower temperatures (e120 °C) only very small quantities of
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(19) A [2 + 2]cycloaddition/ring-opening polymerization mechanism is also
Supporting Information Available: Full experimental procedures,
spectroscopic data, and additional figures (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
possible.
JA029120S
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