Frontal Polymerization of Diurethane Diacrylates
ALBERTO MARIANI,1 STEFANO FIORI,2 SIMONE BIDALI,1 VALERIA ALZARI,3 GIULIO MALUCELLI4
1Dipartimento di Chimica, UdR INSTM, Universita` di Sassari, 07100 Sassari, Italy
2R&D Department, Condensia Quimica SA, C/ La Cierva 8, 08184 Palau de Plegamans (BCN), Spain
3Materials Engineering Centre, UdR INSTM, NIPLAB, Universita` di Perugia, 05100 Terni, Italy
4Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, and local INSTM Research Unit,
Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
Received 17 January 2008; accepted 13 February 2008
DOI: 10.1002/pola.22675
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: This work deals with the preparation of poly(urethane acrylates) by using
two different polymerization techniques. Namely, the classical batch procedure has
been compared with frontal polymerization (FP). A thorough study on the effect of
initiator type, concentration, and on the velocity of the front and its maximum tem-
perature has been carried out. Moreover, two different synthetic ways have been
studied: the one step poly(urethane acrylate) preparation starting directly from 1,6
diisocyanato hexane and 2-hydroxyethyl acrylate, and the two step procedure consist-
ing of the synthesis of the corresponding diurethane diacrylate and of its subsequent
polymerization. The first method has the advantage of being faster but some caution
is necessary due to the excessive heat that is generated if the reaction conditions are
not properly chosen. The second approach requires a further step but has the advant-
age of being more controlled. DSC analysis did not show any significant difference by
comparing the thermal properties of the materials obtained by the two techniques
(batch and FP). However, since FP runs are very easy and fast to be performed,
FP should be seriously taken into proper account when these materials have to be
prepared. VC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3344–3352, 2008
Keywords: addition polymerization; differential scanning calorimetry (DSC); diure-
thane diacrylates; frontal polymerization; initiators; poly(urethane acrylates); radical
polymerization
INTRODUCTION
Diurethane diacrylates (DDs) are a class of
monomers widely used in polymer preparations;
namely, their reaction with diols or diamines is
often preferred for the obtainment of polyur-
ethane and polyureas, respectively.1–4
Prompted by the general interest on these
products, we have experimented the use of Fron-
tal Polymerization (FP) as alternative approach
for the synthesis of poly(urethane acrylates).
FP is a technique exploiting the heat released
during the polymerization reaction to promote a
self-sustaining front which travels along the
reactor by converting monomer into polymer. In
principle, once reached a steady state, FP could
propagate indefinitely.
FP was initially investigated by Chechilo
et al.,5 and lately extensively studied by Pojman
Correspondence to: A. Mariani (E-mail: mariani@uniss.it)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 3344–3352 (2008)
V
C 2008 Wiley Periodicals, Inc.
3344

and coworkers who polymerized acrylic mono-
mers6–8 and epoxy resins.9 Mariani et al. ob-
tainedpoly(dicyclopentadiene),10 polyurethanes,11,12
interpenetrating polymer networks,13 and unsat-
urated polyester/styrene resins.14 White and
coworkers investigated the curing of epoxy-
based materials.15 Pojman and coworkers pre-
pared thermochromic composites16 and polymer-
dispersed liquid crystal (PDLC) materials,17
Morbidelli and coworkers obtained homogenous
polymer blends18 and copolymers,19 Washington
and Steinbock synthesized hydrogels.20 Mariani
et al. prepared PDLC films21 and applied FP
to the consolidation of porous materials.22 Poj-
man et al. demonstrated FP with thiol-ene
chemistry.23 McFarland et al. used FP with
microencapsulated initiators.24,25 Crivello stud-
ied the design and synthesis of glycidyl ethers
that undergo FP,26,27 and hybrid free radical/cat-
ionic FP.28 Recently, Mariani et al. prepared
polymer-based nanocomposites with montmoril-
lonite29 and polyhedral oligomeric silsesquiox-
anes,30 Hu et al. frontally copolymerized ure-
thane-acrylates in dimethyl sulfoxide.31 Chen et
al. studied the FP of hydroxyethyl acrylate
(HEA),32 N-methylolacrylamide,33 and the prep-
aration of its hybrids with methylacrylamide;34
moreover, they studied the obtainment of
epoxy resins/polyurethane hybrid networks,35
and of polyurethane-nanosilica hybrid nanocom-
posites.36
Moreover, there are several patents relating
to FP.37–43 In this article, we report the first
application of FP:
i. To the components of the reaction between
1,6-diisocyanato hexane (HDI) and HEA
(molar ratio [HDI]/[HEA] ¼ 0.5 mol/mol
[Scheme 1(a)].
ii. To the DD derived by the preliminary reac-
tion of the above compounds [Scheme
1(b)].
Moreover, the obtained samples were com-
pared with the analogous ones prepared by the
conventional batch technique (CP). In Scheme 1,
HDI, HEA, and DD molecular structures are
depicted.
EXPERIMENTAL
HDI, HEA, dibutyltin dilaurate (DBTDL),
pyrocatechol (PC), 2,20-azobis-isobutyronitrile
(AIBN), benzoyl peroxide (BPO), Aliquat1 336,
t-butylperoxy-2-ethylhexylcarbonate (Luperox1
Scheme 1. Materials, polymerization routes and techniques used in the present
work.
POLYMERIZATION OF DIURETHANE DIACRYLATES 3345
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TBCE), t-butyl peroxide (tBP), and ammonium
persulfate were purchased from Sigma-Aldrich
and used without further purification.
Aliquat persulfate (APS) was prepared as
described in the literature44 starting from Ali-
quat1 336.
Temperature profiles and the maximum tem-
perature reached by the front [Tmax, Fig. 1(b)]
were measured using a K-type thermocouple
placed into the above mixture at 2 cm (65 mm)
from the bottom of the tube. The thermocouple
was connected to a digital thermometer (Delta
Ohm 9416) used for temperature reading and
recording (sampling rate: 1 Hz). The position of
the front (easily visible through the glass wall
of test tubes) was also measured as a function of
time [Fig. 1(a)].
Reproducibility of Tmax data was 615 8C and
that of front velocity, Vf, was 60.2 cm/min.
Error bars in the following plots indicate these
variation ranges.
DSC measurements were performed using a
Mettler DSC 30 Instr. equipped with a low temper-
ature probe. On each sample, two consecutive
scans were carried out in nitrogen atmosphere in
the temperature range from
80 up to þ200 8C
with a heating rate of 10 8C/min. Tg values were
determined by the 2nd thermal scan.
Synthesis of Acrylic acid 2-[6-(2-Acryloyloxy-
ethoxycarbonylamino)-hexylcarbamoyloxy]-
ethyl ester (DD)
A round bottom flask was loaded with 30 g (0.18
mol) of HDI and 0.2 g (3.1 10
4 mol) of DBTDL
dissolved in 200 mL of anhydrous tetrahydrofu-
ran. A little amount of t-butylcatechol was
added as radical inhibitor. A solution of 41 g
(0.36 mol) of HEA dissolved in 20 mL of anhy-
drous tetrahydrofuran was added dropwise at
5 8C. After 30 min, temperature was raised to
50 8C and the reaction prolonged for 20 min.
When the reaction was accomplished, the solu-
tion was poured into petroleum ether. The
obtained white precipitate was collected by fil-
tration and dried under vacuum. Yield 90%,
Scheme 1(b).
1H NMR (DMSO-d6; dH in ppm): 6.47 (2H);
6.13 (2H); 5.88 (2H); 4.31 (4H); 4.29 (4H); 3.17
(4H); 1.48 (4H).
In situ Formation of DD (from HEA and HDI)
and its FP [Scheme 1(a)]
In a typical run, a nonadiabatic glass test tube
(inner diameter: 16 mm) was loaded with appro-
priate quantities of HDI, DBTDL and PC (see
Results and Discussion). The mixture was homo-
geneously mixed with an adequate amount of
HEA ([HDI]/[HEA] ¼ 0.5 mol/mol) and a radical
initiator (APS, BPO, or AIBN).
A K-type thermocouple was placed into the
above mixture at 2 cm (65 mm) from the bottom
of the tube, and the temperature monitored by a
digital thermocouple reader. The FP reaction
was triggered by means of a hot soldering iron
tip (T  300 8C), by heating the external wall of
the tube in correspondence of the upper solution
layer, until the formation of a traveling front.
After FP was accomplished, the test tube was
cooled to room temperature. Afterwards, the
crosslinked polymer was extracted in Soxhlet
with diethyl ether to remove the unreacted prod-
ucts, and dried under vacuum for 12 h at 50 8C.
Figure 1. In situ formation of DD and its FP: typical front position (a) and temper-
ature profile (b) as a function of time. Conditions: [DBTDL]/[HDI] ¼ 0.1 mol %,
[APS]/[HEA] ¼ 0.6 mol %, [PC]/[DBTDL] ¼ 6 mol/mol.
3346 MARIANI ET AL.
Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

FP of DD [Scheme 1(b)]
In a typical run, a nonadiabatic glass test tube
(inner diameter: 16 mm) was loaded with
suitable quantities of DD (kept melt at  90 8C)
and a radical initiator (Luperox TBCE or tBP).
The mixture was rapidly homogenized and
cooled to  70 8C (solid state mixture).
A K-type thermocouple was placed into the
above mixture at 2 cm (65 mm) from the bottom
of the tube and the temperature monitored by a
digital thermocouple. The FP reaction was
ignited as described earlier.
After FP was accomplished, the test tube was
cooled to room temperature. Afterwards, the
crosslinked polymer was extracted in Soxhlet
with diethyl ether to remove the unreacted
products, and dried under vacuum for 12 h at
50 8C.
Batch Samples
The same reaction mixtures as described earlier
were also allowed to polymerize by the CP in
nonstirred batch reactors at 100 8C for 1 h.
RESULTS AND DISCUSSION
As well as it happens in many frontal reactions,
all FP runs were characterized by constant Vf
[Fig. 1(a)]. In Figure 1(b), a typical FP tempera-
ture profile is depicted with the definition of
Tmax; this is the maximum temperature experi-
mented by the thermocouple junction and corre-
sponds to the front temperature.
In situ Formation of DD and its Frontal
Polymerization [Scheme 1(a)]
As known, free-radical polymerization of acryl-
ates and step-growth synthesis of polyurethanes
are very exothermic. Starting from this consid-
eration, we have investigated a system derived
from the in situ reaction of HDI and HEA
([HDI]/[HEA] ¼ 0.5 mol/mol).
Some FP blank runs were performed to ascer-
tain the actual polymerization mode (Table 1).
As reported in Table 1, it is evident that only
FP1 was obtained by pure FP (NOTE: with the
term ‘‘pure’’ FP we mean that no simultaneous
‘‘spontaneous polymerization,’’ SP, occurs. SP is
the process related to the spontaneous tendency
of the reaction mixture to polymerize at room
temperature, even in absence of any ignition).
Indeed, BR2 mixture led to the formation of the
blocked diisocyanate (as described in the experi-
mental section) followed by self-ignited FP.45
This term is used when a reaction mixture,
without any external localized stimulus, under-
goes local ignition (generally due to a local tem-
perature increase) and subsequent FP. SP dif-
fers from self-ignited FP in that, whereas the
first phenomenon happens simultaneously in
the whole reaction mixture, generally with a
gradual increase of temperature, self-ignited FP
occurs as a consequence of a sudden increment
of temperature in a restricted area.
It should be underlined that a certain amount
of PC was necessary to achieve pure FP, its role
being that of increasing the pot-life as a conse-
quence of partial DBTDL inhibition.
Starting from this preliminary study, we have
first oriented our research work to the obtain-
ment of compositions which undergo pure FP.
Although APS was chosen as the preferred
radical initiator, AIBN and BPO were used as
well. APS was mainly utilized because it does
not give rise to bubbles during the polymeriza-
tion reaction. DBTDL was used as the catalyst
for urethane formation and PC as the inhibitor
to increase the pot-life of the mixture,11,12 which
was stable for several hours (pot life  8 h
depending on the composition).
Table 1. In Situ Formation of DD and Its FP: Blank Runs
Sample
[APS]/[HEA]
(mol %)
[DBTDL]/[HDI]
(mol %)
[PC]/[DBTDL]
(mol/mol)
Polymerization
Mode
BR1 – – – No polymerization
BR2 – 0.17 6 FP (self-ignition)
BR3 0.60 – 6 No polymerization
BR4 – – 6 No polymerization
FP1 0.60 0.17 6 FP
[HDI]/[HEA] ¼ 0.5 mol/mol.
POLYMERIZATION OF DIURETHANE DIACRYLATES 3347
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In Figure 1, the position of the front as a
function of time, and a temperature profile for a
typical FP experiment are depicted ([DBTDL]/
[HDI] ¼ 0.1 mol %, [APS]/[HEA] ¼ 0.6 mol %,
[PC]/[DBTDL] ¼ 6 mol/mol). These plots are
useful for the calculation of Vf and Tmax.
In a series of experiments, the catalyst concen-
tration was varied (0.1  [DBTDL]/[HDI]  0.5
mol %), keeping constant the ratios [PC]/[DBTDL]
¼ 2mol/mol and [APS]/[HEA]¼ 5mol %.
Tmax and Vf as functions of the [DBTDL]/
[HDI] ratio are reported in Figures 2 and 3,
respectively.
Pure FP was observed only in the range 0.1 
[DBTDL]/[HDI]  0.5 mol %. For [DBTDL]/
[HDI] < 0.1 mol %, FP did not self-sustain while,
when [DBTDL]/[HDI] > 0.5 mol %, simultane-
ous SP occurred. CAUTION: particular attention
has to be paid when high concentrations of APS
or DBTDL are used (i.e., when [APS]/[HEA] 
0.6 mol % or [DBTDL]/[HDI]  0.2 mol %): in
fact, due to the elevate temperature reached,
tube explosion may occur.
In the composition range explored, Tmax has a
maximum at [DBTDL]/[HDI] ¼ 0.17 mol %;
beyond this value, a temperature drop is
evident, it reaching a plateau for [DBTDL]/
[HDI]  0.34 mol % (Fig. 2).
In Figure 3 and 5, Vf shows a decreasing
trend. Starting from the maximum value of 3.0
cm/min for [DBTDL]/[HDI] ¼ 0.1 mol %, it
decreases down to 2.0 cm/min for [DBTDL]/
[HDI] approaching 0.5 mol %.
The effect of the [PC]/[DBTDL] ratio on Tmax
and Vf is reported in Figures 4 and 5.
Pure FP was found in the range 3  [PC]/
[DBTDL]  48 mol/mol ([DBTDL]/[HDI] ¼ 0.2
mol %; [APS]/[HEA] ¼ 0.6 mol %). For relatively
low inhibitor concentration (i.e., [PC]/[DBTDL]
< 3 mol/mol) simultaneous SP was present
while, when [PC]/[DBTDL] > 48 mol/mol, no FP
occurrence was observed (the urethane forma-
tion was completely inhibited and, consequently,
no heat was released by this reaction).
As can be seen in Figure 4, Tmax decreases as
[PC]/[DBTDL] increases. In particular, as stated
earlier, the reason of no FP occurrence for [PC]/
[DBTDL] > 48 mol/mol can be attributed to the
excessively large inhibiting PC effect on catalyst
reactivity.
In Figure 5, Vf is reported as a function of the
[PC]/[DBTDL] ratio. As observed for Tmax, also
Vf decreases as the [PC]/[DBTDL] ratio
increases, from 4.0 cm/min for [PC]/[DBTDL] ¼
3 mol/mol to 1.4 cm/min for [PC]/[DBTDL] ¼ 48
mol/mol ([DBTDL]/[HDI] ¼ 0.17 mol %; [APS]/
[HEA] ¼ 0.6 mol %).
Moreover, we varied the [APS]/[HEA] ratio to
assess the effect of radical initiator concentra-
tion.
Figure 2. In situ formation of DD and its FP: Tmax
as a function of the [DBTDL]/[HDI] ratio. ([APS]/
[HEA] ¼ 0.6 mol %, [PC]/[DBTDL] ¼ 6 mol/mol).
Figure 3. In situ formation of DD and its FP: Vf as
a function of the [DBTDL]/[HDI] ratio. ([APS]/[HEA]
¼ 0.6 mol %, [PC]/[DBTDL] ¼ 6 mol/mol).
Figure 4. In situ formation of DD and its FP: Tmax
as a function of the [PC]/[DBTDL] ratio. ([APS]/[HEA]
¼ 0.6 mol %, [DBTDL]/[HDI] ¼ 0.17 mol %).
3348 MARIANI ET AL.
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DOI 10.1002/pola

In Figures 6 and 7, Tmax and Vf are, respec-
tively, reported as functions of the [APS]/[HEA]
ratio by keeping constant [DBTDL]/[HDI] ¼ 0.17
mol % and [PC]/[DBTDL] ¼ 6 mol/mol.
The influence of the [APS]/[HEA] ratio on
Tmax seems to be negligible (Tmax always
 190 8C). In any case, the presence of APS was
essential for promoting the FP of HEA. As a
consequence, when APS was absent, lower tem-
perature values and no self-sustaining fronts
were found. In detail, it should be highlighted
that an increase of temperature was actually
observed (Tmax ¼ 132 8C) but, since the corre-
sponding front was not self-sustaining, its Tmax
and Vf (which gradually decreases from 0.5 to 0
cm/min in less than 2 cm from the ignition point
and along the reactor) are not respectively,
reported in Figures 6 and 7. By contrast, the
increase of Tmax (above 132 8C) and the occur-
rence of self-sustaining fronts having Vf  1.5
cm/min observed in the presence of APS are the
results of the larger amount of heat release due
to HEA radical polymerization which happens
together with the urethane linkage formation.
Vf significantly increases with APS concentra-
tion. In particular, when [APS]/[HEA] < 0.12 mol
% no FP occurred but, when this ratio was over-
come, a Vf increment was evident up to 3.6 cm/
min ([APS]/[HEA] ¼ 0.96 mol %, [DBTDL]/[HDI]
¼ 0.17 mol % and [PC]/[DBTDL] ¼ 6 mol/mol).
The pure FP threshold value was individuated
at [APS]/[HEA] ¼ 0.96 mol %: when this concen-
tration was got over, simultaneous SP was
observed but with a notable Vf value of 30 cm/
min, which is one of the highest found so far in
any FP system.
Figure 5. In situ formation of DD and its FP: Vf as
a function of the [PC]/[DBTDL] ratio. ([APS]/[HEA] ¼
0.6 mol %, [DBTDL]/[HDI] ¼ 0.17 mol %).
Figure 6. In situ formation of DD and its FP: Tmax
as a function of the [APS]/[HEA] ratio. ([PC]/[DBTDL]
¼ 6 mol/mol), [DBTDL]/[HDI] ¼ 0.17 mol %).
Figure 7. In situ formation of DD and its FP: Vf as
a function of the [APS]/[HEA] ratio. ([PC]/[DBTDL] ¼
6 mol/mol), [DBTDL]/[HDI] ¼ 0.17 mol %).
Table 2. In situ Formation of DD and Its FP: Effect
of Type and Concentration of Radical Initiator on
Tmax and Vf
Sample Initiator
[Initiator]/[HEA]
(mol %)
Tmax
(8C)
Vf
(cm/min)
FP2 APS 0.12 276 1.5
FP3 0.36 290 2.7
FP1 0.60 270 2.7
FP4a 1.20 – >30
FP5 AIBN 0.12 264 2.4
FP6 0.36 316 3.4
FP7 0.60 336 2.5
FP8 1.20 306 3.6
FP9 BPO 0.12 297 2.5
FP10 0.36 314 2.4
FP11 0.60 300 2.4
FP12 1.20 303 4.0
[DBTDL]/[HDI] ¼ 0.17 mol %, [PC]/[DBTDL] ¼ 6 mol/mol.
a Simultaneous presence of FP and SP.
POLYMERIZATION OF DIURETHANE DIACRYLATES 3349
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Finally, we investigated the effect of the type
of radical initiator on Tmax and Vf. The obtained
results are listed in Table 2.
Namely, APS, AIBN, and BPO were chosen as
representatives of different radical initiator fam-
ilies.
As will be shown, their effect on both Tmax
and Vf is quite different. At present we can not
provide an explanation for that and further
studies aimed to investigate this aspect are in
progress.
Namely, as can be seen from Table 2, the use
of AIBN resulted in an increment of Tmax as a
function of the [AIBN]/[HEA] ratio ([DBTDL]/
[HDI] ¼ 0.17 mol %, [PC]/[DBTDL] ¼ 6 mol/
mol). In particular, although the maximum tem-
perature was 336 8C for [AIBN]/[HEA] ¼ 0.6
mol %, when this ratio was exceeded, a slight
Tmax decrease was observed. CAUTION: particu-
lar attention during the performing of such reac-
tion is necessary due to the explosive behavior
of some of these samples, that is, when [AIBN]/
[HEA]  0.6 mol %.
Vf shows a plateau pathway. In fact, for
[AIBN]/[HEA] > 0.12 mol % just a little incre-
ment in Vf was observed and its average value
was found to be  3.0 cm/min.
BPO was chosen as a radical initiator belong-
ing to the class of peroxides. As for AIBN, Tmax
was poorly influenced by the [BPO]/[HEA] ratio
with values always close to 300 8C.
Vf remained almost constant (2.4–2.5 cm/min)
in the range 0.12  [BPO]/[HEA]  0.6 mol %
whereas, for [BPO]/[HEA] ¼ 1.2 mol %, it sud-
denly increased up to 4.0 cm/min.
FP of Preformed DD
Preformed DD (PDD) was prepared as described
in the Experimental section. Some preliminary
runs were unsuccessfully carried out using APS,
BPO or AIBN as radical initiators. However,
since PDD is a powder, homogeneous mixtures
with the above initiators could be obtained only
after heating at T > 80 8C (melted mixture),
which is a temperature too high to guarantee
that no initiator dissociation occurs before igni-
tion. For such a reason, higher dissociation tem-
perature radical initiators were chosen, namely
Luperox1 TBCE and t-BP, both belonging to the
class of peroxides.
PDD was molten at  90 8C, rapidly mixed
with the radical initiator and immediately after
cooled down to  70 8C, thus obtaining the
required homogeneous solid mixture on which
FP was performed.
No FP reaction was obtained for initial tem-
perature lower than 70 8C. Indeed, unsuccessful
runs were carried out starting at temperatures
of 25, 50 and 60 8C.
As reported in Table 3, Tmax is hardly depend-
ent on the [Luperox]/[PDD] ratio, ranging from
between 168 to 186 8C (for 1.0  [Luperox]/
[PDD]  10 mol %). On the contrary, in the
same range Vf increases from 2.4 cm/min
([Luperox]/[PDD] ¼ 1.0 mol %) to 3.9 cm/min
([Luperox]/[PDD] ¼ 5.0 mol %). Overcome this
value, it remains almost constant (3.8 cm/min
for [Luperox]/[PDD] ¼ 10 mol %).
In the same Table, the results obtained by
using tBP as a radical initiator are also listed.
Similarly to what previously mentioned for
the Luperox/PDD system, Tmax was slightly
affected by the initiator concentration (185 
Tmax  210 8C). On the contrary, Vf showed an
almost linear increment as [tBP] increased,
ranging from 0.6 cm/min when [tBP]/[PDD] ¼
1.0 mol % up to 1.4 cm/min when that ratio was
equal to 10 mol %.
Thermal Characterization
Since no significant differences were found along
each series, only some selected, significant
results are collected in Table 4.
As can be evinced by the reported data, the
highest Tg values of FP samples were obtained
by using APS as radical initiator. In particular,
such values result higher than both those of the
corresponding batch samples (thus probably
indicating a larger conversion of the former
ones) and of those obtained by using BPO or
AIBN. This result is quite unexpected; indeed,
Table 3. FP of PDD: Effect of Type and Concentra-
tion of Radical Initiator
Sample Initiator
[Initiator]/
[PDD] (mol %)
Tmax
(8C)
Vf
(cm/min)
FP13 LUPEROX 1.0 186 2.4
FP14 3.0 185 3.0
FP15 5.0 168 3.9
FP16 10 172 3.8
FP17 tBP 1.0 185 0.6
FP18 3.0 204 0.8
FP19 5.0 210 1.0
FP20 10 190 1.4
3350 MARIANI ET AL.
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DOI 10.1002/pola

since APS has a large molecular weight (930), it
may act also as a possible plasticizer. For
instance, sample FP1, which is characterized by
the highest Tg, contains an amount of APS as
high as 5 wt % to be compared with 1.2 wt % of
initiator when the same BPO concentration (0.6
mol %) is present.
As already mentioned, Tg data do not differ
significantly. However, a possible correspondence
between Tmax and Tg can be proposed. In fact, it
looks like the higher Tmax, the lower Tg, thus
probably indicating that the high temperature
reached by the front has a negative effect on the
conversion. The high temperature reached dur-
ing FP reactions could promote some thermal
degradation phenomena, with a subsequent low-
ering of the Tg values. However, the possible
occurrence of these phenomena should be con-
sidered acceptable in that their effect on Tg is
quite small when compared with the analogous
samples prepared by the classical method.
CONCLUSIONS
In the present work, a thorough study on the
possibility of using FP as an alternative tech-
nique for the synthesis of poly(urethane acryl-
ates) has been performed. These macromolecular
compounds have been obtained: (i) directly start-
ing from HDI and HEA, thus performing ure-
thane formation and vinyl polymerization at the
same time, and (ii) in two steps by allowing to
react the corresponding preformed diurethane
diacrylate. The first method has the advantage
of being faster but some caution is necessary
due to the excessive heat that is generated if
the reaction conditions are not properly chosen.
The second approach requires a further step but
has the advantage of being more controlled;
however, since the DD monomer is solid, to have
a homogenous monomer-initiator mixture, it has
to be heated and melted before polymerization
occurrence.
After a comparison between FP and the clas-
sical method (prolonged external heating at a
given temperature), no significant differences
have been found in terms of Tg. However, since
FP runs are carried out in minutes instead of
the hours typically necessary for the batch
methods, and required very easy protocols and
apparatuses, FP can be once again considered a
valid and convenient alternative way for the
preparation of polymer materials.
A careful study on thermal and mechanical
properties of all the obtained materials is in pro-
gress and will be reported soon.
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Table 4. In situ Formation of DD and Its FP: Tg Values (2nd Scan) of Some
Selected Samples
Sample Initiator
Initiator
Concentration (mol %)
Polymerization
Technique
Tg
(8C)
Tmax
(8C)
FP3 APS 0.36 FP 50.6 290
CP3 Batch 44.9 –
FP1 0.60 FP 60.0 270
CP1 Batch 45.5 –
FP11 BPO 0.60 FP 48.0 300
CP11 Batch 44.1 –
FP6 AIBN 0.36 FP 44.2 316
CP6 Batch 48.8 –
POLYMERIZATION OF DIURETHANE DIACRYLATES 3351
Journal of Polymer Science: Part A: Polymer Chemistry
DOI 10.1002/pola

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3352 MARIANI ET AL.
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DOI 10.1002/pola