Experimental and Theoretical Study of the Living Polymerization of
N-Silylphosphoranimines. Synthesis of New Block
Copolyphosphazenes
Silvia Sua ́ rez Sua ́ rez,
†
David Presa Soto,
†
Gabino A. Carriedo,*
,†
Alejandro Presa Soto,*
,†
and Anne Staubitz*
,‡
†
Department of Organic and Inorganic Chemistry, IUQOEM, University of Oviedo, Julia ́ n Clavería, 33006 Oviedo, Spain
‡
Otto-Diels-Institute for Organic Chemistry, University of Kiel, Otto-Hahn-Platz 3/4, 24118 Kiel, Germany
* S Supporting Information
ABSTRACT: The sequential living polymerization of N-silylphosphoranimines for the
synthesis of polyphosphazene-b-polyphosphazene diblock copolymers (PP-b-PP) has been
studied both experimentally and theoretically. For the experiments, BrMe
2
PN-SiMe
3
,
[Cl
3
PNPCl
3
][X] (X = PCl
6
-
, Cl
-
), Cl
3
PN-SiMe
3
, ClMe
2
PN-SiMe
3
, and
[Me
3
PNPMe
2
Cl]
+
were used as representative model reagents. Density functional
theory (DFT) calculations in the gas phase adjusted for solvent effects on ClMe
2
PN-
SiMe
3
, [Cl
3
PNPCl
3
]
+
, Cl
3
PN-SiMe
3
, and ClMe
2
PN-SiMe
3
confirmed the
experimental observations. The results have shown the necessity of starting with mono-
end-capped initiators to avoid the formation of triblock chains. It was also demonstrated
that the nature of the nucleophilic N-silylphosphoranimines and the electrophilic cationic
end groups of the living polyphosphazenes strongly affects the polymerization reaction,
imposing limits to its synthetic potential. Thus, good electron donor N-silylphosphor-
animines, i.e. XR
2
PN-SiMe
3
, react better with electron-deficient cationic end groups
such as N-PCl
3
+
, probably by molecular orbital (MO) control. The results led to the
designed synthesis of well-defined PP-b-PP block copolymers with narrow molecular weight distributions of formula [N
P(Ph)(Me)]
n
-b-[NP(OCH
2
CF
3
)
2
]
m
and [NP(Ph)(Me)]
n
-b-[NP(O
2
C
12
H
8
)]
m
, which are excellent candidates for
micellation studies.
■
INTRODUCTION
The presence of elements other than carbon in polymeric
chains (inorganic polymers) can generate interesting and useful
properties such as flexibility at low temperatures, thermal,
radiative, and oxidative stability, flame retardancy, new electrical
and optical features, and novel chemical reactivity.
1a,b
Since the
early studies of F. Gordon A. Stone in this area,
1c-e
the
synthesis of those materials remains an important challenge.
Within the large variety of monomeric precursors containing
main-group elements in the structure, the phosphoranimines
R
3
PNR′, discovered in 1919 by Staudinger and Meyer,
2
have
contributed to significant advances in this field. The chemistry
of these molecules has been the subject of intense study.
3
Phosphoranimines undergo a variety of reactions such as 1,2-
additions,
4
cyclodimerizations,
5
aza-Wittig,
6
and catalytic meta-
thesis
7
and have been used as ligands in main-group
8
and
transition-metal chemistry.
9
Moreover, they are important
monomeric precursors for a large variety of polyphosphazenes,
[NPX
2
]
n
, a class of inorganic polymers consisting of
alternating phosphorus and nitrogen atoms in the backbone.
The first known example, [N PCl
2
]
n
(poly-
(dichlorophosphazene)), was prepared in the late 1890s as a
cross-linked, hydrolytically unstable elastomer.
10
Since then,
various synthetic methods to obtain non-cross-linked [N
PCl
2
]
n
(1) have been achieved, on the basis of the ring-opening
polymerization (ROP) of hexachlorocyclotriphosphazene [N
PCl
2
]
3
(2), either at ca. 200-250 °C (Chart 1, eq 1)
11
or at
room temperature in the presence of trialkylsilyl carboranes as
initiators.
12
Non-cross-linked [NPCl
2
]
n
(1) proved to be an
essential intermediate to synthesize numerous polyphospha-
zene derivatives [NPX
2
]
n
, by macromolecular substitution of
the chlorine atoms by oxygen or nitrogen donor nucleophiles
(X).
13
However, some polyphosphazenes, mainly those bearing
alkyl or aryl substituents, are inaccessible by these strategies,
because treatment of [NPCl
2
]
n
(1) with aryl or alkyl
Grignard or lithium reagents leads to decomposition of the
polymers. Another route to access this pivotal polymer is the
use of phosphoranimines. Thus, heating (N-phosphoryl)-
trichlorophosphoranimine Cl
3
PNP(O)Cl
2
affords [N
PCl
2
]
n
(1) at ca. 250 °C by condensation polymerization
which proceeds with elimination of P(O)Cl
3
(Chart 1, eq 2).
14
Furthermore, the introduction of phosphoranimines as
monomers allowed the successful preparation of poly(alkyl/
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: October 20, 2011
Published: January 24, 2012
Article
pubs.acs.org/Organometallics
© 2012 American Chemical Society 2571 dx.doi.org/10.1021/om201012g | Organometallics 2012, 31, 2571-2581