Controlled synthesis of polyphosphazenes with chain-capping agents

Article abstract of DOI:10.3390/molecules26020322

N-alkyl phosphoranimines were synthesized via the Staudinger reaction of four different alkyl azides with tris(2,2,2-trifluoroethyl) phosphite. N-adamantyl, N-benzyl, N-t-butyl, and N-trityl phosphoranimines were thoroughly characterized and evaluated as

Full text of DOI:10.3390/molecules26020322

molecules
Article
Controlled Synthesis of Polyphosphazenes with
Chain-Capping Agents
†
Robert A. Montague and Krzysztof Matyjaszewski *
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA;
rmontague.ky@roadrunner.com
*
†
Correspondence: matyjaszewski@cmu.edu or km3b@andrew.cmu.edu
Current address: 2217 Forest Avenue, Ashland, KY 41101, USA.
Abstract: N-alkyl phosphoranimines were synthesized via the Staudinger reaction of four different
alkyl azides with tris(2,2,2-trifluoroethyl) phosphite. N-adamantyl, N-benzyl, N-t-butyl, and N-trityl
phosphoranimines were thoroughly characterized and evaluated as chain-capping compounds in the
anionic polymerization of P-tris(2,2,2-trifluoroethoxy)-N-trimethylsilyl phosphoranimine monomer.
All four compounds reacted with the active chain ends in a bulk polymerization, and the alkyl
1
end groups were identified by H-NMR spectroscopy. These compounds effectively controlled the
molecular weight of the resulting polyphosphazenes. The chain transfer constants for the monomer
and N-benzyl phosphoranimine were determined using Mayo equation.
Keywords: N-alkyl phosphoranimines; polyphosphazenes; phosphoramidate esters; fluoride;
trifluoroethoxide; N-methylimidazole initiators; anionic; chain-capping agents
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ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Montague, R.A.;
1. Introduction
Matyjaszewski, K. Controlled
Synthesis of Polyphosphazenes with
Chain-Capping Agents. Molecules
The most important examples of inorganic polymers [
1–3] are the polysiloxanes—(R Si-
2
O)
n
[4–8], polysilanes—(R Si) 11], and the polyphosphazenes—(R P=N)n− [12–14].
n
[9–
2
2
They have been the subject of significant research due to desirable properties, such as
biocompatibility, high thermal resistance, oxidative stability, UV resistance, interesting
photoelectronic behavior, and high flexibility that are often difficult or impossible to achieve
in carbon-based organic polymers. Much of the current research on polyphosphazenes is
focused on applications in the life sciences as drug delivery vehicles and other biological
applications [15–18].
2
021, 26, 322. https://doi.org/
1
0.3390/molecules26020322
Academic Editors:
Sławomir Rubinsztajn, Marek Cypryk
and Wlodzimierz Stanczyk
Received: 20 December 2020
Accepted: 7 January 2021
Published: 10 January 2021
The field of polyphosphazene synthesis has expanded significantly with the advent
of more efficient and faster synthetic routes such as the ambient temperature preparation
of poly(dichlorophosphazene) by the PCl
P-trichlorophosphoranimine, [13 19] the polymerization of N-trimethylsilyl-P-tris(2,2,2-
-initiated polymerization of N-trimethylsilyl-
5
,
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional clai-
ms in published maps and institutio-
nal affiliations.
trifluoroethoxy)phosphoranimine by antimony pentachloride initiator, [20] and poly
(organophosphazenes) from partially halogenated phosphoranimines [21]. These reac-
tions are examples of cationic polymerization of phosphoranimines. The cationic route has
also led to polyphosphazenes with functional and terminal groups at the chain ends [22
–27]
as well as many other derivative copolymers with more complex architecture [28–32].
The first catalyzed/initiated polymerization of a phosphoranimine monomer by fluo-
ride ion was previously reported (Scheme 1a) and then extensively investigated [33]. The suc-
cessful conversion of these monomers to polyphosphazenes initiated/catalyzed by fluoride
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license (https://
creativecommons.org/licenses/by/
anion, trifluoroethoxide anion, and N-methylimidazole was described [34–36]. Polyphosp-
hazene random and block copolymers with mixed alkoxy/alkoxyether substituents were
subsequently prepared by the fluoride/anionic method and characterized [37]. Other ex-
amples include polyphosphazenes with alkyl or aryl substituents [38], polyphosphazenes
with electronegative nitropropoxy groups bound to the phosphorus atom of the N-silylated
phosphoranimine [39], polymerization of N-silyl-P-diethyl phosphoranimine with fluoride
4
.0/).
Molecules 2021, 26, 322. https://doi.org/10.3390/molecules26020322
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 322
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and phenoxide initiators [40], and P-tris(trifluoroethoxy)-N-trimethylsilyl phosphoran-
imine polymerized with water in the presence of N-methylimidazole initiator in diglyme
solution polymerization [41].
Scheme 1. (a) Anionic chain growth condensation polymerization. (b) Chain growth vs. step growth macrocondensation.
The use of chain capping agents is a technique to prepare well-defined polyphos-
phazenes produced by anionic polymerization of phosphoranimines, since active chain
segments are subject to late-stage condensation reactions that can increase molecular weight
with diminished control of chain length. In Scheme 1b, active anionic chain ends of the
forming polymer can react with a monomer molecule via chain growth condensation (with
elimination of trimethylsilyl trifluoroethoxide), or it may react with a partially polymerized
chain segment to advance molecular weight via a macrocondensation. The preparation
of four N-alkyl phosphoranimine compounds and their efficiency as chain end-capping
agents in polyphosphazene synthesis was briefly mentioned before [42].
In this paper, we report more detailed experimental data obtained using these com-
pounds, and present evidence of their incorporation as unique chain end groups during
the formation of polyphosphazenes by the anionic route.
2. Results and Discussion
2.1. Synthesis and Characterization of N-Alkyl Phosphoranimines
The synthetic route to the four N-alkyl phosphoranimines prepared in this study was
via the Staudinger reaction [43] is shown in Scheme 2, in which tris(2,2,2-trifluoroethyl)
phosphite reacted with an alkyl azide to form the P-tris(2,2,2-trifluoroethoxy)-N-alkyl
phosphoranimine with the evolution of nitrogen gas.

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Scheme 2. Staudinger synthesis of P-tris(trifluoroethoxy)-N-alkylphosphoranimines.
The N-alkyl compounds formed consisted of the N-adamantyl, N-benzyl, N-t-butyl,
and N-trityl phosphoranimines. Yields, boiling or melting points, densities, and refractive
indices are shown in Table 1.
Table 1. Physical data for N-alkyl phosphoranimines.
Refractive0
Compound
% Yield
B.P.(M.P.)
Density
Index, nD2
◦
◦
1
(Adamantyl)
46.3
98.9
34.4
56.5
93 C/2.5 torr, (0–2 C)
1.606 g/mL
1.664 g/mL
1.568 g/mL
1.4076
1.4022
1.3445
◦
2
3
(Benzyl)
(t-Butyl)
95 C/2.5 torr
◦
93 C/52 torr
◦
4
(Trityl)
(80–85 C)
The new compounds were characterized by NMR, fast atom bombardment, and GC-
mass spectrometry, with the resulting data presented in Table 2. The data obtained from
the NMR spectra are consistent with the proposed structures of the four compounds, as are
the mass spectrometry measurements for molecular weight of the parent ions.
Table 2. NMR and mass spectrometry data for N-alkyl phosphoranimines.
31P-NMR,
ppm
Mass
Spectrometry
Compound.
1H-NMR ppm
Theor. MW
4
.28 (p): 6H 3J(POCH)
=
7.6 Hz
1
(Adamantyl)
−27.3 (m)
1.73(d): 6H
.65(br. S.):6H
477 **
433 ˆ
477
1
2
.05(br. s.):3H
3
4
.20 (p): 6H J(
=
=
POCH)
7
.2 Hz
3
2
3
(Benzyl)
−13.19 (m)
4.32 (d): 2H J (
433
PNCH)
2
2 Hz
7
.30 (m): 5H
.28 (p): 6H J (
3
4
=
=
POCH)
(t-Butyl)
−28.95 (m)
−29.01 (p)
8.2 Hz
399 *
399
585
1
.23 (s): 9H
3
4
.05 (p): 6H J (
POCH)
4
(Trityl)
7.8 Hz
.31(m): 15H
585 **
7
*
Molecular ion; ˆ Partially decomposes on silica GC column; ** Molecular ion, FAB mass spec.
Elemental analyses of these compounds for found and theoretical percentages are
displayed in Table 3. The determined percentages of elements from the analyses are in
good agreement with the assigned structures.

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Table 3. Elemental analyses of N-alkyl phosphoranimines.
Compound
1
% C
% F
% H
% N
%O
%P
Theory/Found
40.28
40.58
35.83
35.49
4.45
4.54
2.94
3.22
10.06
-
6.47
6.33
Theory
Found
(
Adamantyl)
3
3
6.06
5.73
39.47
38.12
3.03
2.98
3.24
3.16
11.8
-
7.15
6.77
Theory
Found
2
3
(Benzyl)
3
2
0.10
9.36
42.83
41.88
3.80
3.65
3.51
3.27
12.03
-
7.76
7.75
Theory
Found
(t-Butyl)
5
4
1.32
8.14
29.22
23.89
3.63
3.60
2.39
2.25
8.21
-
5.29
*
Theory
Found
4
(Trityl)
Notes: Fluorine interferes with oxygen determination. * Sample size insufficient for P-analysis.
FTIR spectra further confirmed the structures with the appearance of the broad absorp-
−
1
tion bands of the P=N moieties between 1270–1310 cm , and the characteristic absorptions
of aliphatic or aromatic protons as expected for the particular structure. The data collected
are shown in Table 4, and an example IR spectrum of the N-adamantyl phosphoranimine is
shown in Figure 1.
Table 4. FTIR band assignments of N-alkyl phosphoranimines.
Absorbance Wavenumbers
Functional Group
Assignment
Compound
−
1
cm
2
870–2950
CH aliph.
P-O-C
P=N
1
1
435
270
1
(Adamantyl)
1
180, 960
090
60
P-O
C-O
CF3
1
6
3
020–3120
305
150, 970
090
00–750
70
CH arom.
P=N
P-O
C-O
CH arom.
CF3
1
1
2
3
(Benzyl)
1
7
6
2
1
1
970
420
310
CH aliph.
P-O-C
P=N
(t-Butyl)
1
170, 970
085
75
P-O
C-O
CF3
1
6
3
1
000–3110
420–1440
CH arom.
P-O-C
P=N
1
300
1180
090
4
(Trityl)
P-O
C-O
1
7
6
20–750
60–670
CH arom.
CF3

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Figure 1. FT-IR spectrum of P-tris(trifluoroethoxy)-N-adamantyl phosphoranimine.
.2. Reactivity of N-Alkyl Phosphoranimines
The phosphoranimines are air-sensitive and hydrolyze easily to the corresponding
2
phosphoramidite esters, (CF CH O) P(=O)-NHR, as confirmed by mass spectrometry
3
2
2
and NMR. For example, the GC-mass spectrum of the N-adamantyl phosphoranimine
in (wet) diethyl ether showed two major peaks with fragments that are consistent for
the phosphoranimine (molecular ion 477), and the corresponding N-adamantyl phospho-
ramidate ester (molecular ion 395). A third peak in trace amount is in good agreement
with the 1-azidoadamantane starting reagent, and all three species contain the adamantyl
fragment (135).
The proton-coupled 31P-NMR spectrum of the GC-MS sample showed the phosphora-
nimine signal at
−
27.4 ppm, and the new species at +7.0 ppm. The new signal is a sextet
3
with a coupling constant of 7.3 Hz, as expected for J
coupling.
POCH
1
The H-NMR spectrum of this sample showed the ethyl ether proton signals at
1.18 ppm (t) and 3.45 ppm (q); two distinct sets of the different adamantyl protons from 1.58
to 2.10 ppm; amidate proton at 2.80 ppm (d); and trifluoroethoxy protons at 4.27 ppm (p).
2
The amidate doublet has a coupling constant of J
= 9 Hz. This data supports the
PNH
structure assignment for the hydrolysate.
The phosphoramidate ester can arise from nucleophilic attack by water (or OH ) on
the electrophilic phosphorus atom of the phosphoranimine, as shown in Scheme 3.
−

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Scheme 3. Reaction of P-tris(trifluoroethoxy)-N-adamantylphosphoranimine with H O.
2
Similar behavior was reported in the study of an N-silylated phosphoranimine [44].
This could be a general reaction of phosphoranimines bearing trifluoroethoxy (or other
leaving) groups bound to the phosphorus center.
A sample of the N-benzyl phosphoranimine was exposed to air and was converted to a
colorless crystalline solid with a
δp = +8.3 ppm, which is consistent with the corresponding
N-benzyl phosphoramidate ester, (CF CH O) P = (O)-NHCH Ph [45].
3
2
2
2
In addition to the phosphoramidate ester major product, a trace signal at
was also observed. This high field resonance arises from the hexacoordinate species
−
152.5 ppm
−
(
CF CH O) P , in which the phosphorus center carries a formal negative charge. This
3
2
6
31
compound was previously prepared and its P-NMR chemical shift was reported as
154.6 ppm [46]. Electronegative groups stabilize the phosphorus hexacoordinated
state [47].
This species has also been observed as a trace component of polymerizing mixtures
−
of the P- tris(2,2,2-trifluoroethoxy)-N-trimethylsilyl phosphoranimine monomer, and in
unreacted fractions of tris(2,2,2-trifluoroethyl) phosphite that were distilled from reaction
mixtures. It is probably formed by a thermal rearrangement of phosphorus compounds
bearing trifluoroethoxy ligands.
A pentacoordinated phosphorus compound was observed in the 31P-NMR spectrum of
a sample of the clear colorless N-benzyl phosphoranimine that had crystallized after several
weeks in a desiccator. Its chemical shift of
compounds as previously reported [45], and sufficiently electronegative ligands tend to
stabilize such species [45]. The penta-(2,2,2-trifluoroethoxy) phosphorane, P(OCH CF ) ,
−
72 ppm is typical of five-coordinate phosphorus
2
3 5
31
was previously prepared and its P-NMR chemical shift reported as
−
76.6 ppm [46].
Another example of a penta-coordinated phosphorus compound with an N-benzyl ligand
is (CF ) (F)P[N(CH ) (CH Ph)] with a δp = −68.8 ppm [48].
3
3
3
2
The mass spectrum of the crystallized sample of the N-benzyl phosphoranimine
showed a high mass peak of 351. The pentacoordinate phosphorus compounds shown in
Scheme 4 are potential isomers with MW = 353 and are reasonable structural assignments
for the species observed. They may exist in equilibrium with their phosphonium salts of
+
−
the general formula, R P R [45].
4
Scheme 4. Pentacoordinated phosphorus structures.

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2.3. Polymerization Studies
Bulk Polymerization with Addition of N-alkyl Phosphoranimines
Experiments were conducted in which small samples of these compounds were added
to polymerizing mixtures of the tris(2,2,2-trifluoroethoxy) phosphoranimine monomer
and corresponding polyphosphazene. The initial experiment probed the reactivity of
the N-alkyl phosphoranimine in bulk polymerization. P-tris(2,2,2-trifluoroethoxy)-N-
trimethylsilyl phosphoranimine monomer, 1.5 mmol (CF CH O) P = N-Si(CH ) , was
3
2
3
3 3
treated with 1 mol% tetrabutylammonium fluoride (TBAF) in a dry NMR tube and heated
◦
at 150 C for 15 min. Then, 0.6 mmol of N-benzyl phosphoranimine was injected by syringe,
mixed, and heated for another 15 min.
The reaction was cooled to room temperature, and 31P-NMR spectra confirmed poly-
mer formation, along with a few percent oligomers. There were two very small doublets
(
+8.5,
indicate formation of a phosphazene dimer as a minor side product, probably formed by a
coupling reaction of monomer and the N-benzyl compound. A small signal at 72 ppm
−
2.5 ppm, JPNP = 73 Hz) in the spectrum of the reaction mixture (less than 2%) which
−
indicated the presence of ca. 2% of the pentacoordinate phosphorane discussed above.
The sample was then heated for an additional 15 min and final spectra recorded. A
control reaction without N-benzyl phosphoranimine was run concurrently. The polymer
samples were dissolved in diglyme and precipitated from excess cold chloroform, then
re-dissolved in diglyme and precipitated again from 90/10 CHCl MeOH with thorough
3/
washing of the white polymer solids.
1
The H-NMR spectra of the isolated polymer in d -acetone solution showed the
6
polymer signal at 4.55 ppm, n-butyl protons from the TBAF initiator, and new signals at
7.35 ppm indicating the aromatic protons of the benzyl group (Figure 2).
1
Figure 2. H-NMR spectrum of N-benzyl-capped poly(bis(trifluoroethoxy)phosphazene).
The 31P-NMR spectrum of this sample showed the polymer signal at
−
7.05 ppm, but
no signals for the N-benzyl phosphoranimine, dimer, or phosphorane. The aromatic proton
signals were thus ascribed to reaction of the N-benzyl compound with the polymer chain.
Integration of the polymer and benzyl proton signal ratio correlated with incorporation of
one N-benzyl phosphoranimine residue per chain, indicative of a functionalized chain-end.
Similar results were obtained using the other N-alkyl phosphoranimines. After heating

Molecules 2021, 26, 322
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◦
mmol of monomer and 1 mol% TBAF initiator for 15 min at 150 C, 3 mmol of the N-alkyl
6
compound (neat or in solution) was added by syringe and heating continued for a total
time of 1 h.
1
The polymer samples were precipitated twice as described before, and the H-NMR
spectra showed signals for polymer, the n-butyl groups of the TBAF, and the corresponding
alkyl group. The presence of the trityl-capped chains is particularly distinctive by the
multiple aromatic proton signals of the formed polymer product, as seen in Figure 3. The
proton signals for the adamantyl and t-butyl chain-end groups are partially obscured by
the TBAF initiator residue, but they can be seen and identified. The 3 P-NMR spectra of the
experimental polymer samples showed no signals for unreacted N-alkyl phosphoranimines.
The control reaction of monomer and TBAF showed proton signals for polymer and n-
butyl groups, and GPC confirmed higher molecular weight. The yield of the polymer was
significantly higher, indicating a more complete monomer conversion.
1
1
Figure 3. H-NMR spectrum of N-trityl-capped poly(bis(trifluoroethoxy)phosphazene).
The molecular weights of the polymer samples obtained show the effect of the ad-
dition of the N-alkyl compounds. As seen in Table 5, the molecular weight and yield of
polymer were significantly reduced by the addition of the N-alkyl compound compared to
the control.
Table 5. Molecular weights, dispersity, yields of N-capped polyphosphazenes (bulk polymerization
◦
at 150 C 1% TBAF, 3 mmol N-alkyl phosphoranimine addition, heated for 1 h).
Polymer End Group
Mn
Mw/Mn
% Grav. Yield
Adamantyl
Benzyl
t-Butyl
21,960
19,867
21,297
17,362
29,012
1.40
1.34
1.49
1.51
1.70
22
25
34
33
71
Trityl
Control (no N-alkyl)
In related experiments, bulk polymerizations with monomer-equivalent additions
◦
of N-benzyl phosphoranimine (NBP) at specific intervals during 150 C polymerization
using 1 mol% TBAF initiator showed that the polymer molecular weight was terminated

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upon addition compared to controls (*) with no N-benzyl compound (Table 6). Neither
conversion nor molecular weight increased after addition of the NBP.
Table 6. Effect of addition time of P-Tris(2,2,2-trifluoroethoxy)-N-trimethylsilyl phosphoranimine on
◦
conversion and molecular weight (bulk at 150 C, 1% TBAF).
Minutes
before Add.
%
NBP Sample
1
Time, Min.
Mn
Mw/Mn
Conversion
2
–
5
–
–
20
2
20
5
7123
9757
11,472
15,646
18,188
1.08
1.29
1.45
1.40
1.72
9
2
*
35
29
54
81
3
4
5
*
*
20
*
controls, no N-benzyl phosphoranimine.
The effect of various amounts of N-benzyl phosphoranimine was examined by prepar-
ing samples containing monomer, 1 mol% TBAF initiator, and N-benzyl compound in
◦
the ratios shown in Table 7. After heating at 150 C for 45 min, the molecular weights of
3
1
the polymers obtained were measured by GPC and conversion determined by P-NMR
spectra integration. These data show that at least 2% N-benzyl phosphoranimine was
needed to effectively limit polymer molecular weight and conversion of monomer, with
20% inhibiting polymerization.
Table 7. Effect of various monomer: N-benzyl phosphoranimine ratios on conversion and molecu-
lar weight.
Monomer: N-Benzyl
Mn
Mw/Mn
% Conversion
mol. Ratio
5
:1
Polymer not detected
–
–
7
52
86
86
2
5
0:1
0:1
4849
1.02
1.50
1.73
1.84
10,183
14,952
15,195
1
00:1
Control
The polymer molecular weight, dispersity, and yield are decreased by the N-alkyl
phosphoranimine addition. However, the data also show that perfect stoichiometric
agreement between the amount of the capping agent and degree of polymerization (DP)
was not always observed. For example, the 20:1 and 50:1 samples have DPs of about 20
and 50, respectively, but the other ratios exhibit some variability. The lower than expected
DP of the 100:1 sample may arise from other transfer reactions in the system.
Using the data from Table 7, a graph was constructed, as shown in Figure 4. The
following Mayo equation was used to determine the chain transfer to the capping agent
constant, Ctr,X, and the constant for transfer to monomer, Ctr,M
:
1/DPn = Ctr,M + Ctr,X [X]/[M]
From the graph, the slope of the line is the chain transfer constant to capping agent and
the y-intercept is the chain transfer to monomer constant. From the equation, the values for
−
1
−3
Ctr,X and Ctr,M are 8.54
×
10 and 7.2
×
10 respectively. There was a very small signal
,
1
from trimethylsilyl protons in the H-NMR spectrum of precipitated polymer at 0.12 ppm
(
Figure 2), which indicated that some chains could be terminated by trimethylsilyl groups,
albeit in trace amount.

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Figure 4. Mayo plot of 1/DP as function of [X]/[M] ratio.
A final experiment was conducted in which a 1:1.7 mol mixture of monomer and N-
◦
adamantyl phosphoranimine with 1% (mol) TBAF was heated for several hours at 150 C,
well beyond the normal time required for complete conversion of monomer to polymer in
31
the presence of the fluoride. Analysis of the reaction mixture by P-NMR showed that only
3% conversion of the monomer occurred, producing polymer and oligomers, with 87% of
1
the spectrum signals arising from unreacted monomer and the N-adamantyl compound.
Under these forcing conditions, the polymerization was retarded to a major extent, but not
completely inhibited by the N-adamantyl phosphoranimine. The amount of polymer in the
reaction mixture was less than 7% and could not be isolated for inspection of end groups.
31
Significantly, no phosphorus doublets were observed in the P-NMR spectrum, thus,
the monomer and N-adamantyl compound did not react to form a phosphazene dimer.
This result and that of the end-capping experiments indicate that the N-adamantyl phos-
phoranimine reacts preferentially with active chain ends.
The active species, particularly in the early chain-growth stage of the polymerization,
should be an anionic polymer chain-end [49]. The phosphorus atom of the N-alkyl phos-
phoranimine has similar reactivity as in the N-silylated monomer (Ctr,X ~1) but forms an
unreactive group. The stable alkyl group of the new phosphoranimine leads to termination
of the chain, whereas the labile silyl group of the monomer results in continued propagation
of the chain, as shown in Scheme 5.
Scheme 5. Chain propagation vs. chain termination.

Molecules 2021, 26, 322
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Chain termination with these compounds is a degradative transfer reaction in which
an active chain is terminated and a new active species, trifluoroethoxide anion, is produced.
The number of active species in the system is unchanged, and the anion can initiate a new
chain, as reported in an earlier study [49,50].
3. Conclusions
Four N-alkyl phosphoranimines were synthesized and found to be effective chain
capping agents for polyphosphazenes prepared by anionic initiation of a phosphoranimine
monomer. The NMR and GPC data support their incorporation into the chain as end groups.
The use of chain-capping/terminating compounds in the anion-initiated polymerization of
a phosphoranimine monomer has been demonstrated and may provide a useful technique
for further functionalization of polyphosphazenes. For example, it may be possible to add
alkenyl groups to short phosphazene chain ends as a route to prepare inorganic/organic
graft copolymers, as well as to prepare block copolymers with controlled molecular weight.
4. Materials and Methods
4.1. Materials
Tris(2,2,2 trifluoroethyl) phosphite and trimethylsilyl azide were obtained from Aldrich
Chemical Co., Milwaukee, WI, USA and purified by distillation at reduced pressure from 4
Angstrom molecular sieves to molecular sieves. Tetra-n-butyl ammonium fluoride TBAF
in THF, 1.0M and TBAF-silica gel were obtained from Aldrich and were used as received.
Benzyl azide was obtained from Johnson-Mathey Co., London, England and purified by
recrystallization from dry benzene-hexane solution. 1-Azidoadamantane and sodium azide
from Aldrich were used as received. Trityl bromide and t-butyl alcohol (Fisher Scientific-
Fisher Scientific Co., LLC Hampton, NH, USA), and BF .ET O from Aldrich were used
3
2
as received.
4.2. Monomer Synthesis
A clean 100 mL three-neck round bottom flask with magnetic stir-bar, water jacketed
condenser, glass or Teflon stopcock, and Claisen tube with 250 mL pressure-equalizing
◦
addition funnel was oven-dried at 150 C overnight, assembled hot with natural rubber
septa and gas inlet and outlet, and cooled under a dry nitrogen purge. All ground-glass
joints were sealed with a light application of stopcock grease and wrapped with Teflon
tape. The flask was covered with aluminum foil to prevent photolysis of the azide. In
accordance with a literature procedure [51], distilled tris(2,2,2-trifluoroethyl) phosphite
(
0.4 mol) was combined with equimolar distilled trimethylsilyl azide and refluxed for 24 h
◦
at 120 C, followed by two successive additions of equimolar azide under agitation at 24 h
intervals, for a final mole ratio of 1:3 phosphite:azide. During each azide addition, the
reaction flask was cooled to 0 C, then the temperature was slowly raised to 120 C. At the
end of the 72-h period, an orange/amber liquid was observed in the flask. Upon distillation
◦
◦
at reduced pressure, a first fraction of unreacted clear, colorless azide was obtained at
◦
4
5
0–50 C/85 mm, and a second fraction of clear, colorless liquid monomer distilled at B.P.
◦
0–60 C/0.5 mm. Yield of P-tris (2,2,2 trifluoroethoxy)-N-trimethylsilyl phosphoranimine
monomer: 95% based on the phosphite.
4.3. Synthesis of N-Alkyl Phosphoranimines
The adamantyl azide (21 mmol) and an equimolar amount of the tris(2,2,2-trifluoroethyl)
phosphite were combined in a three-neck round bottom 100 mL flask with condenser over
an electrically heated and thermostated oil bath. The oil temperature was increased gradu-
◦
◦
ally from 50 C to 140 C over a 1.75 h period, in order to moderate the pressure increase in
the system as the nitrogen outgassed. The N-adamantyl phosphoranimine distilled under
◦
reduced pressure (93 C/2.5 torr) as a clear, colorless liquid which crystallized as colorless
◦
needles with a melting point near 0 C as shown in Table 1.

Molecules 2021, 26, 322
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In a similar fashion, 136 mmol benzyl azide and equimolar amount of phosphite were
◦
combined with stirring at 40–80 C for 22 h. CAUTION: very vigorous reaction with rapid
outgassing and pressure build-up in apparatus! N-benzyl phosphoranimine distilled as a
◦
clear colorless liquid at 95 C/2.5 torr. The t-butyl azide was prepared by the reaction of
1
00 mmol t-butyl alcohol and 120 mmol trimethylsilylazide in the presence of 120 mmol
BF .Et O, following a published procedure [52]. Due to difficulty in isolating the azide
3
2
by distillation, 100 mmol phosphite was added to this reaction at room temperature (RT)
◦
for the Staudinger coupling in situ, at 90 C for 18 h. N-t-butyl phosphoranimine distilled
from the reaction as a clear, colorless liquid. Trityl azide was prepared by the reaction of
sodium azide suspended in MeCN with trityl bromide in benzene at RT over several days,
in a manner similar to published methods [53
combined with an equimolar amount of phosphite at 60–140 C over a 2-h period. N-trityl
,
54]. The crystallized azide (13.5 mmol) was
◦
phosphoranimine crystallized as an amber, fibrous solid in high purity.
4.4. Bulk Polymerization of Monomer and N-Alkyl Phosphoranimines
The bulk polymerizations of the monomer phosphoranimine with and without the N-
alkyl compounds were conducted in NMR tubes as indicated in the Results and Discussion
section. Monomer in the specified amount was charged via syringe and the specified
amount of TBAF initiator solution was added by microliter syringe. Heat was supplied
by a mineral oil bath over an RCT Tekmar hotplate unit for the specified time period. The
N-alkyl phosphoranimine was added neat if liquid, or as a solution by syringe as indicated.
After the reaction was cooled to RT, the work-up entailed dissolving the solid polymer
mass in 2–5 mL THF, and adding the solution to excess cold chloroform to precipitate
the polymer. The polymer mass was re-dissolved in a 90/10 blend of chloroform and
methanol and re-precipitated. The re-precipitated polymer was allowed to stand under the
◦
mother liquor at
−
20 C overnight to maximize precipitation, and was collected on a clean,
tared glass frit with thorough washing to remove any unreacted materials. The collected
white polymer solid was dried in a vacuum desiccator overnight before weighing. Yield
was calculated on the basis of monomer weight minus the condensate by-product as the
theoretical yield.
4.5. Characterization of N-Alkyl Phosphoranimines and Polymers
NMR spectra were recorded on an IBM NR/300 MHz FT NMR spectrometer. Trimethyl
3
1
phosphite in C D was used as an external standard for P-NMR spectra, with a
δ
P of
6
6
31
1
1
41.0 ppm (85% phosphoric acid H PO = 0 ppm). The P-NMR and H-NMR spectra were
obtained in CDCl solution or in d acetone as internal standard ( H-NMR = 7.24 ppm).
3
4
6
1
3
Abbreviations for NMR signals: s = singlet; m = multiplet, d = doublet, t = triplet, q =
quartet, p = pentet, br. = broad. Integration symbols, such as “6H”, signify six protons of a
particular type as shown in the table.
Mass spectra were obtained using a Hewlett Packard 5890 gas chromatograph with a
silica column and equipped with a 5970 series mass spectrometer. Run time was 40 min
◦
◦
with 2 min solvent delay; T (initial) = 100 C for 10 min, heating rate of 10 C/minute,
◦
and T (final) = 250 C for 5 min. FTIR spectra were obtained on a Nicolet 5DXB FT-IR
spectrometer. Samples were analyzed in KBr pellets, or thin translucent films between
sodium chloride plates as appropriate, using polystyrene film standard. Melting points
were measured with a digital melting point apparatus from Electrothermal Eng. Ltd. at a
◦
heating rate of 1 C/minute. Refractive indices were measured on the Bausch and Lomb
◦
Abbe’
−
3L refractometer at 20 C. Elemental analyses were provided by Midwest Microlab
of Indianapolis, IN. Gel Permeation Chromatography (GPC) was performed on polymer
samples by first dissolving the solid polymer (0.2–0.5 g) in 1 mL THF-HPLC grade, and
filtering through 0.5 micron Teflon filter, 20 microliters of this solution was injected into
the carrier solvent stream (THF with 0.1% tetra-n-butyl ammonium bromide, a literature
procedure for polyphosphazenes [55]) at a flow rate of 1.0 mL/minute. Ultrastyragel
columns (10,000; 1000; 100 Angstroms) and a Waters 410 differential refractometer were

Molecules 2021, 26, 322
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◦
used at 35 C internal temperature. Data acquisition and calculations were performed with
a Nelson 900 analytical interface and Samsung 286 personal computer. Calibration was
based on polystyrene standards of low to high molecular weights.
Author Contributions: Conceptualization: R.A.M. and K.M. Methodology: R.A.M. and K.M. Valida-
tion: R.A.M. and K.M. Investigation planning: R.A.M. and K.M. Data acquisition: R.A.M. Formal
analysis: R.A.M. and K.M. Writing—original draft: R.A.M. Writing—review and editing: K.M. Project
administration: K.M. Funding acquisition: K.M., R.A.M. Both authors have read and agreed to the
published version of the manuscript. All authors have read and agreed to the published version of
the manuscript.
Funding: K.M. acknowledges support from PPG Industries, Inc., Eastman Kodak, Xerox Corp.,
Hoechst-Celanese. R.A.M. acknowledges support from PPG Industries, Inc.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in this article.
Acknowledgments: This paper is dedicated to Professor Julian Chojnowski on the occasion of
his 85th birthday. R.A.M. and K.M. acknowledge and thank Frank Burkus for his assistance with
the experiments in the preliminary research study [56], and Kasi Somajajula of the University of
Pittsburgh for the FAB-Mass Spectrometry measurements.
Conflicts of Interest: The authors declare no conflict of interest. The sponsors had no role in the
design, execution, interpretation, or writing of the study.
Sample Availability: Samples of the compounds are not available from the authors.
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