Fluorescent polyolefins by free radical post-reactor modification with functional nitroxides

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.06.021

The 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) has been synthesized and successfully grafted in the melt onto a random poly(ethylene-co-1-ottene) copolymer. Functionalized polyolefins have been prepared by coupling reaction between NfO-TEMPO free radicals and macroradicals which have been formed by H-abstraction induced by the presence of a peroxide. In order to deepen insight into the functionalization mechanism, the reaction has been investigated by Electron Paramagnetic Resonance (EPR). EPR spectra collected during the reaction run, have evidenced the decrease of TEMPO signal as a consequence of temperature increasing. This decrease has been attributed to the formation of a covalent bond between macroradicals and nitroxide free radical. The resulting functionalized polyolefins, PO-g-(NfO-TEMPO), have been characterized by FT-IR and ¹H NMR which has allowed to evaluate their functionalization degree, whereas UV-Vis and fluorescence spectroscopy have been used to investigate their optical properties. The comparison with low molecular weight model compounds, has allowed to state that our methodology can be conveniently adopted to prepare fluorescent polyolefins where the optical properties of the chromophore has been completely transferred to polymer backbone both in solution and in the condensed phase.

Full text of DOI:10.1016/j.reactfunctpolym.2012.06.021

Reactive & Functional Polymers 72 (2012) 695–702
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Fluorescent polyolefins by free radical post-reactor modification with functional
nitroxides
⇑
Francesca Cicogna , Serena Coiai, Calogero Pinzino, Francesco Ciardelli, Elisa Passaglia
CNR-ICCOM UOS Pisa, Area della Ricerca CNR, via G. Moruzzi 1, 56124 Pisa, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) has been synthesized and suc-
cessfully grafted in the melt onto a random poly(ethylene-co-1-ottene) copolymer. Functionalized poly-
olefins have been prepared by coupling reaction between NfO-TEMPO free radicals and macroradicals
which have been formed by H-abstraction induced by the presence of a peroxide. In order to deepen
insight into the functionalization mechanism, the reaction has been investigated by Electron Paramag-
netic Resonance (EPR). EPR spectra collected during the reaction run, have evidenced the decrease of
TEMPO signal as a consequence of temperature increasing. This decrease has been attributed to the for-
mation of a covalent bond between macroradicals and nitroxide free radical. The resulting functionalized
polyolefins, PO-g-(NfO-TEMPO), have been characterized by FT-IR and 1H NMR which has allowed to
evaluate their functionalization degree, whereas UV–Vis and fluorescence spectroscopy have been used
to investigate their optical properties. The comparison with low molecular weight model compounds,
has allowed to state that our methodology can be conveniently adopted to prepare fluorescent polyole-
fins where the optical properties of the chromophore has been completely transferred to polymer back-
bone both in solution and in the condensed phase.
Received 18 April 2012
Received in revised form 6 June 2012
Accepted 30 June 2012
Available online 6 July 2012
Keywords:
Functionalization of polymers
UV–Vis spectroscopy
Fluorescence
EPR
Pro-fluorescent nitroxide
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
production. In this contest, the radical coupling between a macro-
radical, formed onto the polyolefin backbone by H-abstraction, and
Significant advances have been made recently in the develop-
ment of functional polymers that have been able to undergo
light-induced shape changes [1–7]. On the other hand, the modifi-
cation of emission spectra of luminescent molecules in relation to
environment variation has been conveniently applied to obtain
information about conformation of polymers, compatibility of
polymer blends, deformation and alignment of polymer chains.
These materials have been prepared by mixing or linking these
functional molecules to the polymer matrices.
The ability to modulate the emission of luminescent molecules
through changes of their supramolecular structure has stimulated
the development of several potential technological applications of
dyes. Notably, high-performance optical devices both in absorption
and in emission have been designed and fabricated [8–10].
Besides the physical mixing of dyes with polymer matrices,
chromophores can be also covalently inserted onto the polymer
backbone by employing various techniques. If polymerization is
applied, the insertion takes place during the process, while, partic-
ularly in the case of polyolefin-based materials, the free radical
post-reactor modification allows to modify the material after its
a functional nitroxide has been a very interesting tool to graft spe-
cific functionalities onto polymer chains. 2,2,6,6-Tetramethylpi-
peridine-1-oxyl (TEMPO) derivatives have been successfully used
in post-reactor modification processes to graft polar species such
as –OH or ester groups onto polymers [11]. For example, 4-hydro-
xy-2,2,6,6-tetramethylpiperidine-1-oxyl (HO-TEMPO) and 4-ben-
zoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (BzO-TEMPO) have
been used to functionalize poly(ethylene-co-1-octene) with good
efficiency and minor extent of side reactions, such as degradation
and cross-linking, which generally affect the free radical function-
alization process of polyolefins in the melt [12].
The versatility of this functionalization method can be extended
to the preparation of functional polyolefins if ad hoc modified nitr-
oxides are used in the post-reactor modification process.
Molecules bearing nitroxides covalently linked to fluorophore
(profluorescent nitroxides) have been synthesized and used, in
the literature, as probe of polypropylene degradation [13–15].
The main feature of profluorescent nitroxides is the partial or total
reduction of the fluorescence emission due to a quenching of the
singlet excited state of the chromophore by the nitroxyl free radi-
cal. The chromophore fluorescence has been completely restored
when the nitroxide has been scavenged by an alkyl radical to pro-
duce an alkoxy adduct [16,17].
⇑
Corresponding author. Tel.: +39 050 3153393; fax: +39 050 3152555.
E-mail address: francesca.cicogna@pi.iccom.cnr.it (F. Cicogna).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.06.021

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F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
In this work, a pro-fluorescent nitroxide, the 4-(1-naphthoate)-
2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO), has been syn-
thesized and successfully grafted in the melt onto a random
Gas (Nitrogen, CUR): 10; Gas Sources 1 & 2 (air; GS1 & GS2): 25;
GS2 Temperature (TEM): 300 °C; Ionspray Voltage (IS): 5.5 kV;
Declustering Potential (DP): 20 V; Entrance Potential (EP): 10 V;
Scan Range (Positive ions): 150–800 Th in 0.5 s.
poly(ethylene/a-olefin) copolymer. The coupling reaction between
macroradicals, formed by H-abstraction induced by peroxide, and
the nitroxide has produced the functionalized polyolefins (PO-g-
(NfO-TEMPO)). To insight into the mechanism, the reaction has
been studied by Electron Paramagnetic Resonance (EPR) by record-
ing EPR spectra during the reaction run. The structure of PO-g-
(NfO-TEMPO) has been investigated by FT-IR and 1H NMR whereas
by UV–Vis and fluorescent spectroscopy their light-response prop-
erties have been highlighted.
2.2.5. Thermo gravimetric analyses
Thermo gravimetric analyses of NfO-TEMPO has been carried
out by using a Mettler Toledo Stare System TGA/SDTA 851 Module.
The sample (about 10 mg) has been placed in alumina sample pan
and run has been carried out at a standard rate of 10 °C/min from
25 to 700 °C in the presence of nitrogen. Tonset = 240 °C.
2.2.6. Differential scanning calorimetry
A Perkin-Elmer DSC-7 differential scanning calorimeter thermal
analyzer equipped with a CCA7 cooling device has been used for
2. Experimental
DSC analysis of the random ethylene/a-olfin copolymer. Thermal
2.1. Materials and reagents
scans have been carried out on 10–15 mg sample under nitrogen
atmosphere. Previously the instrument has been calibrated by
using mercury (m.p. À38.4 °C) and indium (m.p. 156.6 °C,
A random copolymer ethylene/a-olefin having Mn = 48000 D;
Mw = 102000 D, polydispersity = 2.15 and MFR = 4.7 g/10 min has
been used as polymer matrix.
Di(tert-butylperoxy-iso-propyl)benzene (mixture of isomers)
(Perkadox 14S-FL, Akzo Nobel), 4-hydroxy-2,2,6,6-tetramethylpi-
peridine-1-oxyl (HO-TEMPO, Fluka), 1-naphthoic acid (NfCOOH,
Aldrich), 1-naphthoyl chloride (Aldrich), triethylamine (Aldrich),
D
H = 28.5 J/g) as references. The sample has been heated from
À30 to 120 °C then cooled to À30 °C and heated again to 120 °C
at a cooling/heating rate of 10 °C/min.
2.2.7. UV–visible spectra
.
Absorption spectra of solutions and polymer films have been re-
corded with a Perkin-Elmer Lambda 650 spectrophotometer. Poly-
mer solutions have been prepared in toluene by dissolving the
polymer at 80 °C, then solution has been cooled to room tempera-
ture. The concentration of the polymer in the solution has been
about 5 Â 10À2 M.
H2O2 35 wt.% (Aldrich), dimethyl sulphoxide (Aldrich), FeSO4 7H2O
(Aldrich), 1,3,5-trimethylbenzene (mesytilene, Aldrich), 1,1,2,2-
tetrachloroethane-d2 (Armar Chemicals, 99.6 atom% D, C2D2Cl4)
and silica gel 60 (Merck, 230–400 mesh) have been used as
received.
2.2. Instruments
2.2.8. Steady-state fluorescence spectra
Emission and excitation spectra of solutions and polymer films
have been acquired under isotropic excitation with a Perkin Elmer
luminescence spectrometer LS55 controlled by FL Winlab software
and equipped with the front-surface accessory. Origin 7.5, software
by Microcal OriginÒ, has been used for the analysis of the absorp-
tion and emission data.
2.2.1. FT-IR
FT-IR spectra have been recorded by a Fourier Transform Spec-
trometer Perkin Elmer FT-IR 1760-X at room temperature. Spectra
of microcrystalline samples have been obtained by mixing the
sample with potassium bromide powder. Spectra of polymers have
been obtained from films prepared by compression molding at
100 °C under a pressure of about 6 tons by using the Carter press
3851-0.
2.2.9. Melt functionalization procedure
Polyolefin functionalized samples have been prepared by using
a
discontinuous mechanical mixer Brabender Plastograph
2.2.2. 1H NMR
OHG47055 with a chamber of 30 cc (Table 1). Torque and temper-
ature data have been acquired by the Brabender Mixing software
WinMix ver. 1.0.
1H NMR quantitative analyses have been performed with a
spectrometer Varian Unity VXR 300 working at 300 MHz and spec-
tra have been recorded at 70 °C. About 25 mg of each polymer sam-
ple has been dissolved in 1 mL of tetrachloroethane-d2 (C2D2Cl4) at
80–90 °C. A known amount of a solution of 1,3,5-trimethylbenzene
in C2D2Cl4 (about 0.09 M) has been added to the mixture. About
0.5 mL of the resulting solution has been placed into an NMR tube
with an internal diameter of 0.5 cm and analysed. The 90° pulse
The functionalization of polyolefin has been carried out at
170 °C by reacting in the Brabender at 50 rpm, for 20 min, 15 g of
the masterbatch prepared as described later and 5 g of pristine
polymer. Master-batches, were obtained by melt mixing, at 100 °C
and 50 rpm in the Brabender, 20 g of pristine polyolefin, 4-(1-naph-
thoate)-2,2,6,6-tetramethylpiperidine-1-oxyl (NfO-TEMPO) and
Perkadox 14. The total mixing time at 100 °C was 7 min. Particu-
larly two master-batches have been prepared, the first contained
0.46 g (1.40 mmol) of NfO-TEMPO and 0.16 g (0.47 mmol) of
Perkadox 14, and has been used for the preparation of PO-g-(NfO-
TEMPO)_1. The second master-batch contained 0.92 g (2.80 mmol)
of NfO-TEMPO and 0.32 g, (0.95 mmol) of Perkadox 14, and has
been used to prepare PO-g-(NfO-TEMPO)_2. In both cases NfO-
TEMPO has been added 1.5 min after the polymer melting whereas
Perkadox 14 has been added 2 min later. The functionalized crude
samples have been extracted with boiling acetone for 16 h. The
amount of the acetone extracted fraction has been between 0.4
and 2 wt.% (Table 1). This extraction procedure have completely re-
moved all low molecular weight products, indeed, repeated disso-
lution and precipitations of the functionalized polymers have given
the same analytical results. The residues to the acetone extraction
amplitude has been 22 ls and the repetition time has been 60 s.
The number of transients of each spectrum has been about
1000–1200. For the qualitative analysis of functional TEMPO-Me,
about 10 mg of the sample have been dissolved in 0.5 ml of CDCl3
and the spectrum has been collected at room temperature.
2.2.3. Elementary analyses
Elementary analyses were executed at the microanalysis labo-
ratory of the Faculty of Pharmacy, University of Pisa.
2.2.4. The mass spectra
The mass spectra have been acquired on an AB Sciex API 4000
triple quadrupole mass spectrometer equipped with a turbo-V ion-
spray source, coupled to a Perkin Elmer Series 200 Micro HPLC
Pump. The experimental conditions have been as follows: Curtain

F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
697
Table 1
Amount of reagents in the feed, acetone extracted fraction, functionalization degree (FD) and conversion of NfO-TEMPO for the samples PO-g-(NfO-TEMPO)_1 and PO-g-(NfO-
TEMPO)_2.
Sample
NfO-TEMPO (%mola)
Peroxide (%mola)
Acetone ext.fract. (%)
FD (mol%)
Conversionb (%)
D
Torquec
PO-g-(NfO-TEMPO)_1
PO-g-(NfO-TEMPO)_2
0.28
0.56
0.09
0.18
1.2
2.5
0.13
0.18
46
32
0.25
0.53
a
b
c
The mole percentage was calculated on the basis of the amount of polymer in the feed (as moles of monomeric units).
Conversion was calculated as the percentage of grafted nitroxide with respect to its amount in the feed.
Torque value after polymer melting–End torque value.
have been dried to constant weight and analysed. All functional-
ized polymers have been completely soluble in hot toluene.
chromatography using silica gel as stationary phase and dichloro-
methane as eluant. The NfO-TEMPO-Me has been recovered from
the first band which eluted as a micro-crystalline uncolored solid
(yield 47%). IR (KBr):
1509, 1469, 1438, 1374, 1361, 1308, 1278, 1240, 1193, 1134,
1074, 1048, 1031, 1011, 957, 813, 719, 509 cmÀ1 1H NMR
(300 MHz, CDCl3): d = 8.87 (d, 1H, HNf, J = 8.0 Hz), 8.12 (d, 1H,
HNf, J = 8.1 Hz), 8.00 (d, 1H, HNf, J = 8.0 Hz), 7.88 (d, 1H, HNf,
J = 7.9 Hz), 7.51 (m, 3H, HNf), 5.35 (m, 1H, CH-OOCNf), 3.62 (s,
3H, NO-CH3), 2,00 (m, 2H, axial CH2 of TEMPO), 1.75 (m, 2H, equa-
torial CH2 of TEMPO), 1.23 (s, 12H, CH3 of TEMPO); elemental anal-
ysis calc (%) for C21H27NO3 (341.45): C 73.87, H 7.97, N 4.10; found:
C 73.81, H 7.85, N 4.01.
m = 3044, 2976, 2922, 1713, 1594, 1574,
2.2.10. X band EPR spectra
X band EPR spectra have been obtained by a Varian E112 spec-
trometer controlling the temperature by an OXFORD EPR 900 cryo-
stat. The EPR spectrometer has been interfaced to an IPC 610/
P566C industrial grade Advantech computer by means of a date-
acquisition system consisting of an acquisition board capable of
acquiring up to 500 000 12-bit samples per second including 32-
bit add to memory, thus giving on-line signal averaging [18], and
a software package specially designed for EPR experiments [19].
The spectra have been run by placing a small amount of the mas-
ter-batch, prepared as previously reported, into quartz tubes
(internal diameter 2 mm) and by rising gradually the temperature.
The sample has been collected and purified by dissolution in hot
toluene and precipitation in acetone. The purified sample has been
analysed by FT-IR whereas the acetone phase has been analysed by
GC–MS.
;
3. Results and discussion
3.1. Preparation and characterization of functionalized polyolefins
Functionalized polyolefin samples have been obtained by free-
radical grafting post-reactor modification. Reactions have been
carried out in the melt at 170 °C in a discontinuous mechanical
mixer and have been initiated by di(tert-butylperoxy-iso-pro-
pyl)benzene: different feed conditions have been used to obtain
two functionalized polyolefins having different functionalization
degrees (Table 1, Experimental).
The almost constant torque value recorded during runs, and the
very low difference between the initial and final torque, have sug-
gested a very good control of side reactions such as crosslinking or
degradation of the polymer matrix which have been usually de-
tected in the post-reactor modification of polyolefins with unsatu-
rated monomers (Table 1, Experimental).
2.2.11. Synthesis of 4-(1-naphthoate)-2,2,6,6-tetramethylpiperidine-
1-oxyl (NfO-TEMPO)
The synthesis of NfO-TEMPO has been carried out modifying a
reported procedure [20]. To a solution of 4-hydroxy-2,2,6,6-tetra-
methylpiperidine-1-oxyl (914 mg, 5.3 mmol) in 20 mL of dichloro-
methane, 0.80 mL of 1-naphthoylchloride (2.65 mmol) and
0.42 mL of triethylamine (2.65 mmol) have been added. The result-
ing mixture has been stirred under inert atmosphere for 24 h. The
reaction has been hydrolyzed with water (50 mL), the organic
phase has been extracted twice with water (2 Â 50 mL). The
dichloromethane phase has been dried over anhydrous sodium sul-
fate then solvent has been removed under reduced pressure. The
resulting orange solid has been purified by column chromatogra-
phy by using silica gel as stationary phase and dichloromethane
as eluant. The NfO-TEMPO has been recovered from the second or-
ange band which eluded as a micro-crystalline orange solid (yield
52%). m.p. 99–100 °C (lit. 100–102 °C) [20].
Crude samples have been extracted with boiling acetone for
16 h in order to remove all low molecular weight compounds (re-
agents and by-products), then analyzed by FT-IR and 1H NMR with
the aim of evaluating the occurrence of grafting and of determining
the functionalization degree (FD).
IR (KBr):
m = 3052, 2974, 2937, 2863, 1713, 1593, 1577, 1510,
Comparison of the FT-IR spectra of purified samples whit those
of pristine polymer (Fig. 1), have been evidenced some bands
clearly due to the grafted NfO-TEMPO. Particularly, the band at
1717 cmÀ1 can be associated to the carbonyl stretching of the ester
group while the band at 781 cmÀ1 has been due to the C–H out of
plane bending of naphthalene. Other ester absorptions have been
present in the spectral region between 1300 and 1130 cmÀ1 while
aromatic bands have been hidden by the polymer absorptions.
Normalization of all the spectra with respect to the 720 cmÀ1 band
of polyethylene (CH2 rocking), selected as reference band, has al-
lowed to highlight that the functionalization degree of PO-g-
(NfO-TEMPO)_2 has been higher than that of PO-g-(NfO-TEMPO)_1
in agreement with the larger amount of both nitroxide and perox-
ide used in the preparation of sample 2.
1462, 1377, 1364, 1311, 1278, 1216, 1197, 1180, 1134, 1075,
1030, 1013, 872, 815, 559, 509 cmÀ1; MS(CI): m/z: 349 [M + 23],
327 [M + 1], 346 [M]; elemental analysis calc (%) for C20H24NO3
(326.42): C 73.59, H 7.41, N 4.29; found: C 73.49, H 7.20, N 4.11.
2.2.12. Synthesis of 1-metoxy-4-(1-naphthoate)-2,2,6,6-
tetramethylpiperidine (NfO-TEMPO-Me)
To a stirred mixture of NfO-TEMPO (500 mg, 1.53 mmol), FeS-
O4 7H2O (673 mg, 2.29 mmol) and dimethyl sulphoxide (1.27 mL)
at room temperature, have been added, drop wise, a mixture of
H2O2 35 wt.% (0.39 mL, 4.6 mmol) and dimethyl sulphoxide
(5.1 mL). At the end of addition, the mixture has been stirred at
room temperature for 45 min. Then the reaction has been hydro-
lyzed with water (about 100 mL) and extracted with dichlorometh-
ane (3 Â 50 mL). The organic phase has been dried over anhydrous
sodium sulfate for 12 h, then the solvent has been removed under
reduced pressure. The resulting oil has been purified by column
.
The quantitative determination of functionalization degree (FD)
of PO-g-(NfO-TEMPO)_1 and _2 has been carried out by 1H NMR
analysis by dissolving a known amount of functionalized polymer
and mesytilene, in tetrachloroethane-d2 [12]. 1H NMR spectrum

698
F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
efficiency to about 50% [21]. Indeed, methyl radicals, produced by
720
b-scission reaction of primary radicals, have been reported to be
less reactive than primary radicals in H-abstraction reaction
[22,23], furthermore they can quickly react with TEMPO deriva-
tives thus subtracting functionalizing substrate from the reaction
medium. Moreover, it cannot be excluded that a fraction of func-
tionalizing reagents can be lost by evaporation out of the reaction
mixture. Finally, the quite probable immiscibility of the nitroxide
with the molten polymer can cause the functionalization reaction
to occur only at the polymer/nitroxide interface. All these adverse
events can limit the functionalization reaction efficiency.
Polymer
PO-
PO-
g
-(NfO-TEMPO)_1
-(NfO-TEMPO)_2
g
1717
781
3.2. EPR investigation of the grafting mechanism and of the thermal
stability of the functionalized sample
1800
1600
1400
1200
1000
800
600
400
Wavenumber (cm-1)
To deepen insight into the mechanism of the grafting, a mixture
composed of the polymer, peroxide and NfO-TEMPO, in the same
molar concentrations used for the functionalization reactions, has
been heated inside the cavity of the EPR instrument and EPR spec-
tra have been recorded at regular intervals of temperature. The
proposed mechanism of nitroxide grafting onto polyolefins has
foreseen the formation of a macroalkoxyamine by coupling reac-
tion between nitroxyl free radicals and macroradicals generated
by H-abstraction from polymer backbone. The reaction has been
started at the homolytic dissociation temperature of the peroxide
(up to 150 °C) (Scheme 1). Primary radicals can abstract hydrogen
atoms from the macromolecular backbone giving rise to macrorad-
icals or can undergo to b-scission producing methyl radicals which
have been reported to be less active in the hydrogen abstraction
reaction than primary radicals [12].
The initial EPR spectrum collected at room temperature has
been characterized by an asymmetric three line signal (Fig. 3a)
due to the nitroxide moiety whose rotation has been hidden by
the rigid polymer matrix.
By rising the temperature, EPR spectral pattern of TEMPO radi-
cal has evolved towards those of a radical species which has been
free to rotate (Fig. 3b). This behavior is in agreement with previ-
ously reported results about the use of functionalized TEMPO free
radicals as spin probes in the investigation of interaction of low-
molecular organic nitroxides with polymer matrix [24].
Fig. 1. FT-IR spectra of pristine polymer (dash line), PO-g-(NfO-TEMPO)_1 (bold
line) and PO-g-(NfO-TEMPO)_2 (straight line).
of PO-g-(NfO-TEMPO)_2 (Fig. 2) has showed four doublets at 8.94,
8.18, 8.06 and 7.93 ppm, respectively, and a multiplet centered at
7.56 ppm all due to protons of the naphthalene ring (see molecule
structure in Fig. 2 for attributions). The spectrum has been also
characterized by a multiplet centered at 5.43 ppm which can be
associated to the proton of the TEMPO ring in alpha position with
respect to the ester group (Hh in Fig. 2). The comparison between
the area of the peak of aromatic mesytilene protons (at
6.83 ppm) with those of naphthalene protons allowed the evalua-
tion of FD (Table 1). Very similar 1H NMR spectrum has been ob-
tained for PO-g-(NfO-TEMPO)_1 although the signals attributable
to the NfO-TEMPO grafting have been less intense.
On the bases of FD data, nitroxide conversion has been about
40% with respect to its amount in the feed. As discussed later,
the nitroxide functionalization of a polyolefin has occurred by a
coupling reaction between a nitroxide and a macroradical pro-
duced by primary radicals without transfer reactions. Thus the
amount of primary radicals, formed by thermal homolytic bond
cleavage of peroxide, can control the nitroxide conversion. How-
ever, primary radicals, besides giving H-abstraction, can be
involved in side reactions that lowered the peroxide H-abstraction
From spectral parameters, the correlation time (
sr) can be
calculated. It corresponds, on the average, to the time it takes the
Fig. 2. 1H NMR spectrum of PO-g-(NfO-TEMPO)_2, C2D2Cl4, T = 70 °C, 300 MHz.

F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
699
80
Δ
2RO
P
ROOR
+ P
64
48
32
16
RO
side reactions
+ ROH
O
N
P
+
X
X
N O P
0
0
40
80
120
160
200
macroalkoxy amine
T (°C)
P= polyolefin
X=1-naphthoate
Fig. 4. Correlation time (sr) as a function of EPR cavity temperature during heating
(-j-j-j-) and cooling (-N-N-N-) experiments.
of the nitroxide has been detected. At the end of experiment sr has
Scheme 1. Simplified reactions mechanism of nitroxide grafting.
reached an ending value which has been lower than the starting
one. The not complete recovery of the starting correlation time is
not easy to explain, however, it can be hypothesized that a fast
cooling rate and/or the presence of grafted nitroxide as well as of
side reaction products can interfere with the crystallization pro-
cess, causing a different crystalline domain distribution which
influences the mobility of the polymer chains [26].
Besides changing the spectral pattern, the increase of the
temperature during the EPR experiment have been caused also a
decrease of the area of the TEMPO signal (Fig. 5) and at 200 °C
the amount of free NfO-TEMPO radical has been about one hun-
dredth with respect its initial quantity.
No variation of the amount of free radicals has been observed
during the cooling of the sample from 200 °C to room temperature
suggesting that the covalent bond between nitroxide and macro-
radical has been stable at least in this experimental condition.
One may exclude that the thermal decomposition of the nitrox-
ide can be the cause of the decrease of the TEMPO signal, because,
as highlighted by the thermo gravimetric analysis, its onset tem-
perature has been about 240 °C, then the only possible route for
the TEMPO signal decreasing has been its reaction with another
radical species leading the formation of a covalent bond. However,
to further clarify the reaction pathway, at the end of the experi-
ment, the sample has been recovered, purified by dissolution and
a
b
3210
3232
3254
3276
3298
3320
Gauss
Fig. 3. EPR spectra of the mixture polymer, peroxide and nitroxide recorded at
25 °C (a) and 200 °C (b).
molecule to rotate by one radian and it is related to the ‘‘mobility’’
of the radical species in the matrix. Generally radical species have
been localized in the amorphous polymer phase where the inser-
tion of ‘‘external’’ molecules has been easier than in the con-
strained crystalline architecture [24,25].
4.0
3.2
2.4
1.6
0.8
0.0
The behavior of correlation time as a function of reaction tem-
perature (Fig. 4) has showed that at about 60 °C sr drastically has
decreased, suggesting that at this temperature the nitroxide has
had a larger degree of freedom. This can be due to the fact that,
as highlighted by DSC analysis of the polymer, at about this tem-
perature the polymer crystalline phase has been melting. Although,
on the bases of DSC and PE melting enthalpy, the fraction of crys-
talline phase has been only 5–10%, it can play a great role in the
mobility of the polymer chains. At higher temperature, the mobil-
ity of the nitroxide substantially, has not changed, although at
about 160 °C a very small increase of the correlation time can be
observed. This is probably due to some side reactions caused by lo-
cal concentration of the peroxide in a non mixed polymer matrix,
that locally has changed the polymer rheological behavior. During
the cooling of the sample from 200 °C to room temperature, the
correlation time practically has not changed until 120 °C, then,
lowering the temperature, a progressive hindering of free mobility
0
40
80
120
160
200
T (°C)
Fig. 5. Radical number of NfO-TEMPO molecules as a function of the EPR cavity
temperature.

700
F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
precipitation (see experimental) and analyzed by FT-IR. It has evi-
denced that the polymer has been functionalised with NfO-TEMPO
and its FD has been comparable to that of PO-g-(NfO-TEMPO)_1
which has been prepared in brabender starting from the same re-
agents amount. The acetone used to precipitate the polymer from
its toluene solution has been analyzed by GC–MS. This analysis
has evidenced the formation of the methoxy amine derivative of
the nitroxide together with other sub-products deriving from pri-
mary radicals decompositions [12]. All these observations have
been in agreement with the hypothesized grafting mechanism
(Scheme 1), which envisaged that the reaction between nitroxyl
free radicals and macroradicals gave rise to covalent bonds causing
the progressive decreasing of the free TEMPO radicals in the ana-
lysed mixture. Moreover, since the kinetic of peroxide decomposi-
tion has been controlled by the Arrhenius equation and depend on
the temperature of the reaction medium, then by increasing the
temperature, a higher amount of primary radicals, and conse-
quently macroradicals, can be generated so speeding up the de-
crease of TEMPO signal.
Finally, during the experiment, the only detectable EPR signal
has been that of TEMPO, no other signals like those of primary rad-
icals, deriving from peroxide decomposition, or carbon centered
macroradicals, generated by H-abstraction, have been intercepted.
According to the literature, the very reactive primary radicals are
difficult to be detected at high temperature [27] whereas the car-
bon centered macroradical signals [28,29] can be hidden by the
large amount of nitroxide present in the mixture. Moreover, the
steady state concentration of macroradicals has been supposed to
be low due to their very fast reaction with nitroxide free radicals
to form the stable covalent bond of the alkoxyamine [12,30,31].
band at about 220 nm (e
= 2x103 MÀ1cmÀ1) has been hidden by
the toluene absorption [20,32].
As far as the fluorescence emission spectra of low molecular
weight TEMPO derivatives is concerned (Fig. 7), it can be observed
that NfO-TEMPO has showed a very low emission intensity
whereas the NfO-TEMPO-Me emission maximum position and
intensity has been very similar to that of 1-naphthoic acid.
This behavior has been in agreement with literature reports on
nitroxide derivatives bearing fluorescent chromophore, indeed
fluorescence emission of the chromophore has been generally low-
ered by the presence of the nitroxide because the free radical
quenched the excited singlet states of the chromophore by an
intramolecular mechanism [17,20,32].
The comparison between the fluorescence quantum yield of
NfO-TEMPO (
both roughly estimated on the basis of fluorescence quantum yield
of 1-naphthoic acid ( = 0.033) [33], has confirmed the experimen-
U = 0.001) with those of NfO-TEMPO-Me (U = 0.03),
U
tal evidence of the low fluorescence emission intensity of NfO-
TEMPO and the higher fluorescence emission of NfO-TEMPO-Me.
Although these values have been only roughly estimated, there
has been a good agreement with those reported in the literature
for similar compounds [20,32,33].
Accordingly, a theoretical study carried out by DFT calculation
has succeeded in reproducing the experimental absorption and
fluorescence spectra of NfO-TEMPO-Me in toluene solution and
has confirmed that the electronic transition displayed a strong
p–
pà character involving the HOMO and LUMO orbitals localized
only on the naphthoate system [34].
The UV–Vis absorption and fluorescence emission spectra of
NfO-TEMPO-Me have been collected also from n-heptane solution
to simulate some possible weak interactions of this compound
with a completely apolar medium like those of a polyolefin matrix.
The emission spectrum of NfO-TEMPO-Me solved in n-heptane
(Fig. 10) has been shifted toward blue and has showed a partially
resolved vibrational structure with emission maxima at 331, 343
and 358 nm, whereas the absorption maximum has not been af-
fected by the solvent nature [20,33]. Emission spectra are generally
more sensitive to molecular surrounding variations than absorp-
tion spectra and, for these type of chromophores, the vibrational
structure can be detected only in the vapor phase or in non-polar
solvents. Accordingly, the presence of vibrational pattern in the
emission spectrum of NfO-TEMPO-Me dissolved in n-heptane has
suggested that intermolecular interactions between chromophores
have been very weak or not occurring at all, while the absence of
3.3. UV–Vis and fluorescence emission characterization
The UV–Vis absorption spectra of NfO-TEMPO and NfO-TEMPO-
Me (Fig. 6), which has been used as a model for the grafting of NfO-
TEMPO to the polymer chain, have showed a band centered at
298 nm due to the
p–
pà transition of the aromatic moiety alike
that of 1-naphthoic acid (NfCOOH). This has suggested that no
interactions between the naphthoic group and TEMPO has oc-
curred in NfO-TEMPO or NfO-TEMPO-Me and that the absorption
energy has been unaffected by the presence of the nitroxide both
in paramagnetic and diamagnetic form.
The weak nitroxide contribute in the visible region (460–
480 nm, e
= 11 MÀ1cmÀ1) has been detectable only from a very con-
centrated solution of NfO-TEMPO (10À2 M), while the ultraviolet
8000
Toluene solution
Toluene solution
NfCOOH
Nf-TEMPO
Nf-TEMPO-Me
NfCOOH
6000
Nf-TEMPO
Nf-TEMPO-Me
4000
2000
0
275
300
325
350
375
400
450
475
500
425
300
325
350
375
400
Wavelength (nm)
Wavelength (nm)
Fig. 6. UV–Vis absorption spectra of 1-naphthoic acid, NfO-TEMPO and NfO-
TEMPO-Me dissolved in toluene (1.4 Â 10À5 M).
Fig. 7. Fluorescence emission spectra of 1-naphthoic acid, NfO-TEMPO and NfO-
TEMPO-Me; all samples were dissolved in toluene (1.4 Â 10À5 M); kexc = 290 nm.

F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
701
the vibrational structure of NfO-TEMPO-Me in toluene solution has
confirmed the presence of solvent-chromophore interactions. Fi-
nally, no other bands due, for example, to excimer formation have
been detected at longer wavelengths both in toluene and n-hep-
tane solutions.
The UV–Vis and fluorescence emission behavior of functional-
ized polymer samples have been studied both by recording spectra
from toluene solutions and from polymer films.
Toluene solution
Polymer
PO-
g
-(NfO-TEMPO)_1
-(NfO-TEMPO)_2
PO-
g
358
The UV–Vis spectra of PO-g-(NfO-TEMPO)_1 and _2 dissolved in
toluene (Fig. 8a) have showed an absorption band centered at
297 nm which can be clearly associated with the absorption of
naphthalene sub-unit, thus suggesting that, neither the grafted
TEMPO nor the polymer chains, have interfered with the absorp-
tion properties of the chromophore. Absorbance value of PO-g-
(NfO-TEMPO)_1 has been lower than that of sample 2 in agreement
with different FDs of the two samples and with the fact that solu-
tions containing the same amount of repeating unit of polymers
have been prepared and used to collect spectra.
Similar spectra have been recorded on polymer films (Fig. 8b)
evidencing that the absorption properties of the chromophore have
been transferred to the polymer matrix both in solution and in the
bulk. However, in the case of spectra collected from polymer films
none conclusions about absorbance can be drawn since the thick-
ness of the polymer films have not been determined.
300
325
350
375
400
425
450
475
500
Wavelength (nm)
Fig. 9. Fluorescence emission spectra of pristine polymer 5 Â 10À3 M, NfO-TEMPO
1.4 Â 10À5 M, PO-g-(NfO-TEMPO)_1 and PO-g-(NfO-TEMPO)_2 both 5 Â 10À3 M; all
samples were solved in toluene; excitation at 290 nm.
NfO-TEMPO-Me and of 1-naphthoic acid dissolved in toluene
(Fig. 7). Again the emission intensity differences can be simply re-
lated to the functionalization degree of PO-g-(NfO-TEMPO)_1 and
_2.
Fluorescence emission spectra of PO-g-(NfO-TEMPO)_1 and _2
(Fig. 9) obtained from toluene solutions, have showed an emission
band centered at 358 nm which has been very similar to that of
Fluorescence emission spectra of PO-g-(NfO-TEMPO)_1 and _2
collected from polymer films (Fig. 10) have showed a not well re-
solved vibrational structure with emission maxima at 350 and
360 nm. These two bands are both slightly different from that re-
corded from toluene solutions spectra of the functionalized poly-
olefins (358 nm, Fig. 9) and from those recorded from NfO-
TEMPO-Me solved in n-heptane (331, 343 and 358 nm, Fig. 10).
Notably, the chromophore-toluene interactions are similar,
regardless of fact that NfO-TEMPO has been linked to a methyl
group or to the polyolefin backbone; whereas in the case of func-
tionalized polymer films, the presence of only a partially resolved
vibrational structure in the emission spectra has suggested that
some weak interactions between chromophores or between chro-
mophore and polymer have been probably established. The differ-
ences in the spectral pattern of functionalized polymer films and
model compound solved in n-heptane are likely due to the pres-
ence of the rigid surrounding polymer matrix which can interfere
with the conformational requirement for the S1 ? S0 transition.
A
297
Toluene solution
Polymer
PO-g-(Nf-TEMPO)_1
PO-g-(Nf-TEMPO)_2
300
325
350
375
400
Wavelength (nm)
343
B
358
PO-g-(NfO-TEMPO)_1(film)
NfO-TEMPO-Me(n-heptane)
NfO-TEMPO-Me(toluene)
Polymer film
Polymer
PO-g-(NfO-TEMPO)_1
PO-g-(NfO-TEMPO)_2
290
300
325
350
375
400
425
450
475
500
250
275
300
325
350
375
400
Wavelength (nm)
Wavelength (nm)
Fig. 8. UV–Vis absorption spectra of PO-g-(NfO-TEMPO)_1 and PO-g-(NfO-
TEMPO)_2. A: toluene solutions 5 Â 10À2 M. B: polymer films of unknown
thickness.
Fig. 10. Fluorescence emission spectra of PO-g-(NfO-TEMPO)_1 recorded on
polymer film and emission spectra of NfO-TEMPO-Me toluene and n-heptane
solutions excitation at 290 nm.

702
F. Cicogna et al. / Reactive & Functional Polymers 72 (2012) 695–702
Indeed, as proved by DFT calculations, the lowest energy con-
former of the S1 state of NfO-TEMPO-Me (in toluene) has showed
a co-planarity of the carbonyl moiety with the naphthyl ring,
whereas the carbonyl group in the ground state has been tilted
out of the plane of the naphthalene [34]. Although these calcula-
tions have been carried out considering the contribute of toluene
as bulk solvent, a similar conformational analysis can be hypothe-
sized for the n-heptane solutions.
Finally, excitation spectra of NfO-TEMPO-Me and PO-g-(NfO-
TEMPO)_1 and _2, which have been collected from toluene solu-
tions and using an emission wavelength of 360 nm, closely have
matched the absorption spectra of the fluorophore, thus confirm-
ing again that both TEMPO and polymer backbone has not inter-
fered with the emission properties of the chromophore.
National Project ‘‘New Polymer Systems with Electric and Optical
Functionalities via Nano and Micro Adhesive Dispersion to Produce
Materials and Devices for Smart Applications’’ POLOPTEL 2011–
2014, La Fondazione CARIPISA conv. 167/09.
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Acknowledgments
We thanks Dr. A. Raffaelli (CNR-ICCOM-UOS Pisa) for the
acquisition of mass-spectra. This research is funded by the