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In this work, we developed a fast, highly efficient, and environmentally friendly catalytic system for classical free-radical polymerization (FRP) utilizing a high-pressure (HP) approach. The application of HP for thermally-induced, bulk FRP of 1-vinyl-2-pyrrolidone (VP) allowed to eliminate the current limitation of ambient-pressure polymerization of ‘less-activated’ monomer (LAM), characterized by the lack of temporal control yielding polymers of unacceptably large disperisites and poor result reproducibility. By a simple manipulation of thermodynamic conditions (p = 125–500 MPa, T = 323–333 K) and reaction composition (two-component system: monomer and low content of thermoinitiator) well-defined poly(1-vinyl-2-pyrrolidone)s (PVP) in a wide range of molecular weights and low/moderate dispersities (Mn = 16.2–280.5 kg mol⁻¹, Đ = 1.27–1.45) have been produced. We have found that HP can act as an ‘external’ controlling factor that warrants the first-order polymerization kinetics for classical FRP, something that was possible so far only for reversible deactivation radical polymerization (RDRP) systems. Importantly, our synthetic strategy adopted for VP FRP enabled us to obtain polymers of very high Mn in a very short time-frame (0.5 h). It has also been confirmed that VP bulk polymerization yields polymers with significantly lower glass transition temperatures (Tg) and different solubility properties in comparison to macromolecules obtained during the solvent-assisted reaction.
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Pressure-assisted solvent- and catalyst-free
production of well-dened poly(1-vinyl-2-
pyrrolidone) for biomedical applications
Paulina Maksym, *
ab
Magdalena Tarnacka,
ab
Dawid Heczko,
c
Justyna Knapik-
Kowalczuk,
ab
Anna Miela´
nczyk,
d
Roksana Bernat,
be
Grzegorz Garbacz,
f
Kamil Kaminski*
ab
and Marian Paluch
ab
In this work, we developed a fast, highly ecient, and environmentally friendly catalytic system for classical free-
radical polymerization (FRP) utilizing a high-pressure (HP) approach. The application of HP for thermally-
induced, bulk FRP of 1-vinyl-2-pyrrolidone (VP) allowed to eliminate the current limitation of ambient-
pressure polymerization of less-activatedmonomer (LAM), characterized by the lack of temporal control
yielding polymers of unacceptably large disperisites and poor result reproducibility. By a simple manipulation
of thermodynamic conditions (p¼125500 MPa, T¼323333 K) and reaction composition (two-
component system: monomer and low content of thermoinitiator) well-dened poly(1-vinyl-2-pyrrolidone)s
(PVP) in a wide range of molecular weights and low/moderate dispersities (M
n
¼16.2280.5 kg mol
1
,Đ¼
1.271.45) have been produced. We have found that HP can act as an externalcontrolling factor that
warrants the rst-order polymerization kinetics for classical FRP, something that was possible so far only for
reversible deactivation radical polymerization (RDRP) systems. Importantly, our synthetic strategy adopted for
VP FRP enabled us to obtain polymers of very high M
n
in a very short time-frame (0.5 h). It has also been
conrmed that VP bulk polymerization yields polymers with signicantly lower glass transition temperatures
(T
g
) and dierent solubility properties in comparison to macromolecules obtained during the solvent-assisted
reaction.
Introduction
Sustainability-related studies in the polymer chemistry eld
have gained increasing attention and directed the main
research focus to the production of well-dened materials using
non-toxic solvents and catalytic systems. In fact, reversible
deactivation radical polymerization (RDRP) of pseudo-living/
controlled features allowed signicant progress in the devel-
opment of novel greenestpolymerization strategies induced by
externalstimuli (e.g., light, ultrasound).
1,2
Of these methods,
the light-mediated processes have become an interesting
alternative to thermally-induced ones, where the contribution
of metal-based compounds has been signicantly reduced and
replaced by organocatalysts.
3,4
It needs to be stressed that both
photo-induced RDRPs namely Atom Transfer Radical Polymer-
ization (photo-ATRP)
5
and Reversible Addition-Fragmentation
Chain-transfer Polymerization (photo-RAFT)
6
gained a special
interest and became universal and robust methods to poly-
merize a broad range of functional monomers. However, the
main drawbacks of photo-RDRP strategies are related to the
long reaction time, the high risk of degradation of reagents
(during long-time irradiation), and, importantly, the dicult to
scale polymerization process. In addition, the utilization of
RDRP methods to polymerize several monomers, especially the
less-activatedmonomers (LAMs) (e.g., 1-vinyl-2-pyrrolidone
(VP), vinyl chloride) result in lack of temporal control, poor
system livingnessand production of polymers of broad
molecular weight distributions (dispersity, Đ).
7
Finally, due to
some thermodynamic and kinetic limitations of RDRP
approaches, the production of polymers of higher molecular
weight (M
n
) and predetermined structural parameters (narrow
Đ) within a reasonable time-frame is still hardly possible. In this
context, novel synthetic strategies providing fast polymerization
a
Institute of Physics, University of Silesia, ul. 75 Pułku Piechoty 1, 41-500 Chorz´
ow,
Poland. E-mail: paulina.maksym@smcebi.edu.pl; kamil.kaminski@smcebi.edu.pl;
Tel: +48323497610
b
Silesian Center of Education and Interdisciplinary Research, University of Silesia, ul.
75 Pulku Piechoty 1A, 41-500 Chorzow, Poland
c
Department of Pharmacognosy and Phytochemistry, Medical University of Silesia in
Katowice, School of Pharmacy with the Division of Laboratory Medicine in
Sosnowiec, Jagiellonska 4, 41-200 Sosnowiec, Poland
d
Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry,
Silesian University of Technology, ul. M. Strzody 9, 44-100 Gliwice, Poland
e
Institute of Chemistry, University of Silesia, ul. Szkolna 9, 40-007 Katowice, Poland
f
Physiolution GmbH, Walther-Rathenau-Str. 49a, 17489 Greifswald, Germany
Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra02246b
Cite this: RSC Adv., 2020, 10,21593
Received 10th March 2020
Accepted 15th April 2020
DOI: 10.1039/d0ra02246b
rsc.li/rsc-advances
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RSC Advances
PAPER
in environmentally friendly media (without the use of toxic
reagents) should be sought.
One of the ways to address these issues is moving towards
fast, highly ecient, and non-toxic strategy that uses a high-
pressure (HP) as an externalstimulus. This interesting
approach allows achieving control over polymerization due to
changes in the density, intermolecular interactions, or viscosity.
Since diusivities are inversely proportional to the medium
viscosity, the rate of termination can be reduced in the
compressed systems. Importantly, the overall progress of poly-
merization also depends on the value of activation volume (DV),
which in the majority cases is negative. Thus, at some ther-
modynamic conditions, polymerization is favored and proceeds
much faster with respect to the reactions carried out at ambient
conditions.
8
Herein one can stress that such results were ob-
tained for the HP controlled ATRP,
9,10
RAFT
1115
and also
FRP.
16,17
Moreover, we have also applied high-pressure to
control, pseudo-livingRAFT polymerization of sterically
hindered imidazolium-based ionic liquids (IL) LAMs.
18,19
The
polymers produced in this way were characterized by tailored
properties and excellent group delity with a wide range of M
n
(up 530 kg mol
1
) and narrow Đ1.10 that was not achievable
under ambient-pressure
20
RAFT. Interestingly, we also proved
that by appropriate selection of pressure (p¼5001200 MPa)
and system composition (a type of initiator, presence or absence
of solvent) control over classical thermally-induced FRP could
be achieved. Consequently, for FRP systems, the rst-order
polymerization kinetics and linear evolutions of the M
n
with
conversion were observed. Therefore, it seems to be crucial to
see whether the application of high-pressure in the case of VP
(LAM precursor of materials of high industrial importance),
where conventional polymerization by RDRP fails, will be
successful. It is worthwhile to note that VP possesses several
structural limitations, including very high reactivity and high
chain-transfer constant to the monomer that prevents the
production of the well-dened poly(1-vinyl-2-pyrrolidone) (PVP)
in a wide range of M
n
by RDRP. Besides, among them, RAFT is
the most eective VP polymerizing system.
21
However, it can be
stressed that despite the proper selection of chain transfer
agents (CTA, i.e., xanthates and dithiocarbamates), reactions
were poorly controlled, especially at higher monomer
consumptions. As a consequence, PVP characterized by both
moderate M
n
and Đ(M
n
¼6.453.0 kg mol; Đ¼1.132.30) have
been obtained.
22
In this work, we proposed a fast and versatile methodology
for HP thermally-initiated FRP of VP (see Fig. 1S in the ESI,
Scheme 1). Our motivation was to develop the most straight-
forward and greenestFRP strategy involving the use of only
the monomer and thermoinitiator (of very low concentration,
0.010.15 wt%), allowing to maintain high control and e-
ciency of the process. The rate of polymerization was modulated
by using a dierent initial concentration of thermoinitiator
or varying thermodynamical parameters (p¼125500 MPa,
T¼323333 K). We have found that well-dened PVP homo-
polymers in a wide range of molecular weight (M
n
¼16.2280.5
kg mol
1
,Đ¼1.271.45) could be prepared even within 0.5 h.
Additionally, also comprehensive thermodynamical and
rheological measurements were performed on the selected
synthesized system, and obtained results were compared to the
ones measured for the commercially available samples. The
characteristic of commercially supplied PVPs is presented in the
ESI.It can be stressed that although, at rst sight, the appli-
cation of HP seems to be a highly energy consuming process,
recently published reports indicated that is not true, and the
energy is consumed mostly during the compression stage.
23
One
can also notice that in industry pressure up to p¼1000 MPa is
routinely used to preserve food.
Results and discussion
VP polymerization at ambient- and elevated-pressures
A successful synthesis of well-dened PVP via HP FRP is strongly
related to the appropriate selection of polymerization conditions.
Lack of solvent, one type of initiator, low initiator concentration
(0.15 wt%, 0.10 wt% and 0.01 wt% in respect to VP), variation in
temperature (T¼323343 K) and pressure (p¼125500 MPa)
have been proposed to minimize/eliminate the occurrence of the
side reactions (e.g., chain transfer) and bimolecular termination
characteristic for ambient-pressure systems.
In preliminary experiments, ambient-pressure FRP of VP at
T¼333 K with the presence of 0.10 and 0.15 wt% of AIBN were
carried out. A kinetic study was performed by taking an aliquot
of the reaction mixture at the required time point and con-
ducting the
1
H NMR (see Fig. 2S, ESI) and SEC analysis. Note
that the
1
H and
13
C NMR spectra of PVP, as well as SEC traces
for polymers produced by both studied systems (ambient- and
high-pressure), are collected in the ESI.The data for the
homopolymerization of VP performed at ambient-pressure are
collected in Table 1. Fig. 1a and b show plots of ln([M]
t
/[M]
0
)
versus time and molar masses M
n
as well as plots of dispersity
indices Đagainst monomer conversion, respectively. As shown
in Fig. 1a, polymerizations showed a similar course irrespective
of the initial AIBN concentration. For the rst 2 hours, the
ln([M]
t
/[M]
0
) slop increased linearly, and then, the negative
curvature of the kinetic plot was noted, indicating the domi-
nation of the chain transfer side reactions and chain termina-
tion process. Consequently, polymerizations had stopped at
around 30% and 40% monomer consumption for 0.10 wt% and
0.15 wt% initial AIBN content, respectively.
Fig. 1b demonstrates the typical dependencies of increasing
M
n
with the VP conversion for the uncontrolled FRP. The ob-
tained polymers were characterized by M
n
in the range of 72.2
144.4 kg mol
1
and as expected by high dispersities (Đ¼1.72
2.21). Such results indicated the lack of control over the chain-
ends. One can note that our results are in good agreement with
other data regarding conventional FRP of VP performed in bulk,
an aqueous solution, or in organic solvents at atmospheric
pressure conditions. A few examples of VP FRP, yielded PVP
with relatively high molecular weight (M
n
¼2.5164.0 kg mol
1
)
and the high dispersity (Đ¼1.804.79).
2427
In this context, the application of elevated pressure as
externalreaction force and controlling factor seems to be an
excellent alternative to the current ambient pressure strategies.
In order to nd the optimal conditions for HP VP
21594 |RSC Adv.,2020,10,2159321601 This journal is © The Royal Society of Chemistry 2020
RSC Advances Paper
polymerization including acceleration of the process and gain-
ing the control over the reaction, we have studied systems of the
same content of AIBN (0.15 wt% and 0.10 wt% of AIBN) as in the
case of synthesis carried out at p¼0.1 MPa also at dierent
pressures (p¼125, 250, 500 MPa). Our preliminary measure-
ments indicated that p¼250 MPa and T¼333 K are the most
optimal conditions for VP polymerization. Besides, we also
carried out reactions with very low AIBN concentration
(0.01 wt%) to study the catalytic eect of the system's
compression. The high-pressure polymerizations were con-
ducted with the reaction time from 0.5 h up to 120 h depending
on the initiator concentration (see Table 2). Note that the
limiting value of conversion of VP for bulk processes vary
between 6070%. These values were reached aer 5 h and 8 h
for reactions with 0.15 wt% and 0.10 wt% AIBN respectively.
However, the polymerization performed with the lowest AIBN
content has stopped at 28% VP consumption.
As can be seen from the data presented in Fig. 2a and b, HP
FRP of VP shows rst-order kinetic plots as ln([M]
t
/[M]
0
)increased
linearly with a conversion for 0.15 wt% and 0.10 wt% AIBN
concentration. Consequently, PVPs of M
n
¼37.2231.6 kg mol
1
(Đ¼1.271.54) and M
n
¼66.2246.6 kg mol
1
(Đ¼1.481.54) for
systems with 0.15 wt% AIBN and 0.10 wt% have been obtained,
respectively. Interestingly, the system with the lowest AIBN
content showed negative curvature in the kinetic plot indicating
theoccurrenceoftheterminationprocess(Fig.2c).Takinginto
account very high M
n
¼220280.5 kg mol
1
and low/moderate
dispersities (Đ1.4) of produced in that way PVPs, we assume
that too high system viscosity contributed to the end of this
process rather than bimolecular termination. Fig. 3 presents the
M
n
and Đof PVPs as a function of conversion. From the systems
studied so far, the FRP with 0.10 wt% AIBN showed a linear
relationship of M
n
vs. conversion. It is worthwhile to mention that
presented herein pressure-controlled strategy enabled us to
produce PVP of very high M
n
200 kg mol
1
, relatively narrow
dispersity (up to Đ1.40) within a very short reaction time (2 h,
see example 1b in Table 2). On the other hand, PVPs obtained at
ambient-pressure were characterized by much lower M
n
(100 kg
mol
1
)andsignicantly broader dispersity indices (Đ¼1.70
2.21).
Also, from the SEC chromatograms (see Fig. 4), it can be seen
that there is a huge dierence in M
n
and Đvalues of the polymer
produced via HP FRP and commercially available ones. The SEC
trace of PVP obtained via high-pressure FRP revealed mono-
modal and symmetric shape indicating good control over the
polymer characteristics. On the other hand, samples prepared
via ambient-pressure FRP or commercial possessed
unsymmetrical/bimodal shapes of SEC traces (see Fig. 5S in the
ESI).
Scheme 1 Synthetic route to produce well-dened PVPs under elevated-pressure.
Table 1 FRP of VP performed at ambient pressure with 0.15 wt% of AIBN initial content
Ambient-pressure polymerization of VP
0.15 wt% of AIBN 0.10 wt% of AIBN
No. Time [h] Conv.
a
[%] M
nb
[kg mol
1
]Đ
b
No. Time [h] Conv.
a
[%] M
nb
[kg mol
1
]Đ
b
Ia 0.5 0.09 72.5 1.87 IIa 0.5 0.06 88.9 1.96
Ib 1 0.17 97.9 1.70 IIb 1 0.15 130.1 1.61
Ic 2 0.41 111.2 1.72 IIc 2 0.31 144.3 1.61
Id 3 0.43 109.4 1.78 IId 3 0.35 136.8 1.70
Ie 4 0.44 108.9 2.21 IIe 4 0.36 144.4 1.64
a
Estimated by
1
H NMR (600 MHz, CDCl
3
).
b
Estimated by SEC (DMF + 10 mM LiBr).
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To further investigate the impact of dierent temperature
and pressure on the polymerization rate and properties of
produced polymers, additional short-term reactions (0.5 h) for
a selected system with 0.10 wt% AIBN were carried out (see
Table 3). Reactions performed at T¼+10 higher (T¼343 K),
and p¼125 MPa allowed to obtain PVP with a very high M
n
¼
230.4 kg mol
1
and moderate dispersity Đ¼1.55 (65% VP
conversion). On the other hand, lowering the temperature to T
¼323 K and compression to p¼500 MPa resulted in a low VP
conversion aer 0.5 hours (4%) and production of well-dened
polymer with a low M
n
¼16.2 kg mol
1
(Đ¼1.38). Thus,
collected data showed that we could modulate the reaction rate
and molecular weight of polymers by changing both the ther-
modynamic parameters (p,T) or the concentration of the
thermoinitiator.
Concluding this stage of our investigations, we found that
the FRP process performed at ambient pressure proceeded with
the uncontrolled path at the beginning of polymerization (fast
termination, bimodal SEC traces of PVP samples). On the other
hand, the high-pressure VP polymerization revealed the
controlled nature of the reaction up to the end of the process.
For HP systems, we observed a decrease in the termination rate
(diusion-controlled process) that resulted from the increase in
system viscosity. Importantly, transfer reactions have a negative
value of DV. Therefore, the pressure should have an increasing
eect on the rate of these reactions. However, the proper
selection of reaction conditions (lack of solvent) allowed to gain
control over the FRP path and signicantly reduce the transfer
reactions and the biomolecular termination.
Thermal, rheological and solubility properties of synthesized
PVPs
As a nal point of our investigations, we characterized the
dynamical and rheological properties of both produced and
commercial PVPs with the use of DSC and rheological
measurements. To compare properties between PVP samples
one of the synthesized polymer (i.e., sample 2c presented in
Table 2, M
n
¼246.4 kg mol
1
,Đ¼1.46) and two commercially
Fig. 1 (a) Pseudo-rst-order kinetic plot versus conversion for FRP of VP performed at varied AIBN initial content, at ambient-pressure; (b)
Dependence of M
n
vs. conversion and Đvs. conversion of PVP produced at ambient-pressure.
Table 2 FRP of VP performed at 250 MPa with varied initial content of AIBN (0.150.01 wt%)
High-pressure polymerization of VP (250 MPa)
0.15 wt% AIBN 0.10 wt% AIBN 0.01 wt% AIBN
No. Time [h] Conv.
a
[%]
M
nb
[kg mol
1
]Đ
b
No. Time [h] Conv.
a
[%]
M
nb
[kg mol
1
]Đ
b
No. Time [h] Conv.
a
[%]
M
nb
[kg mol
1
]Đ
b
1a 0.5 0.23 37.2 1.27 2a 0.5 0.16 66.2 1.48 3a 22 0.17 91.1 1.47
1b 2 0.42 209.5 1.40 2b 2 0.39 199.9 1.54 3b 48 0.28 253.7 1.54
1c 5 0.64 231.6 1.54 2c 8 0.63 246.6 1.45 3c 120 0.29 280.5 1.45
a
Estimated by
1
H NMR (600 MHz, CDCl
3
).
b
Estimated by SEC (DMF + 10 mM LiBr).
21596 |RSC Adv.,2020,10,2159321601 This journal is © The Royal Society of Chemistry 2020
RSC Advances Paper
available PVPs (Sigma-Aldrich; PVP K30 M
nSEC
38.4 kg
mol
1
,Đ¼1.89 (sample C1) and PVP K90 M
W
360 kg mol
1
,
M
nSEC
¼108.6 kg mol
1
,Đ¼1.78 (sample C2), see ESI)were
examined.
Fig. 5a, c and e present the mechanical storage (G0) and loss
(G00) modulus spectra of the measured polymers, which were
obtained from the oscillation frequency sweep tests performed
at dierent temperatures (426452 K, 437449 and 450458 K
Fig. 2 Pseudo-rst-order kinetic plots versus conversion for FRP of VP at 250 MPa.
Fig. 3 Dependence of M
n
vs. conversion and Đvs. conversion of PVP produced at varied AIBN concentration at 250 MPa.
This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10,2159321601 | 21597
Paper RSC Advances
for 2c, C1, C2, respectively). To better visualize the obtained
data, the master curve was constructed for each sample using
the timetemperature superposition (TTS) (see Fig. 5b, c and d).
As a reference temperature (T
ref
) we have chosen Tat which G0
and G00 intersect at u¼14 rad s
1
. As can be seen, the
mechanical response of the synthesized PVP polymer is more
like commercial C2 sample. Both these samples exhibit (i) the
glassy plateau (high frequency) (ii) three decades long transition
regime (intermediate frequency) and (iii) the entanglement
regime, also called rubbery plateau (low frequency). In the
glassy and rubbery regime, G0(storage or elastic moduli)
surpasses G00 (loss or viscous moduli) exhibiting a typical solid-
like rheological behaviour. On the other hand, in the transition
regime, the samples show a uid-like behaviour since G00 >G0.
28
Intersections of the master curves of the G0and G00 delineate the
boundaries between these regimes and correspond to the two
important relaxation times.
29
First, marked as s
seg
, is the
relaxation time of one segment of the equivalent freely jointed
chain, while the second is Rouse relaxation time of chain
segments between entanglements (s
e
). The segmental relaxa-
tion times of the investigated polymers were estimated from the
frequency of the crossover point of G0and G00 by employing the
following relation s
seg
¼1/f
cross
. The temperature dependence of
s
seg
obtained in this way is displayed in Fig. 6a. Since the s
seg
(T)
usually follow the VogelFulcherTammann (VFT) equation,
30,31
we employed it to parametrized the obtained temperature
dependences. The empirical VFT is expressed as follows:
3234
log10sðTÞ¼log10 sNþB
TT0
(1)
where T
0
is the so-called Vogel temperature, B¼DT
0
, and s
0
provides the high temperature limit of the relaxation time.
The solid lines in Fig. 6a correspond to VFT ts. From the
extrapolation of the ts to temperatures at which s
seg
¼100 s,
the glass transition temperatures of the examined polymers
were estimated to be 419, 431 and 449 K for sample 2c, C1 and
C2, respectively. It is worth mentioning that these values are in
good agreement with the T
g
s obtained from the DSC measure-
ment see the DSC thermograms presented in Fig. 6bd.
Literature data reported that T
g
of PVP increases with the
molecular weight towards a limiting value (of 453 K) according
to the FoxFlory relation.
35
Besides, T
g
values decrease with
a decrease in viscosity average molecular weight.
20
Surprisingly,
comparing examined T
g
values, we found that synthesized
herein PVP sample of M
n
246 kg mol
1
(2c) presents much
lower T
g
with respect to the commercial ones (C1, M
n
30 kg
mol
1
, and C2, M
n
108.6 kg mol
1
). Such a scenario might be
a result of reaction conditions in which the polymer was
synthesized. Note that commercial samples were produced
using solvent polymerization, whereas our samples from the
monomer alone. As reported, PVPs prepared by g-irradiation
and thermal-decomposition of the initiator with the presence of
water or aqueous solution have a much higher T
g
than the ones
produced in bulk (449 K vs. 405 K for g-irradiation; 440 K vs. 386
K for thermal-decomposition of AIBN). Explanations of this
phenomena should be sought in strong interactions between
monomer and water molecule or other polar solvent forming
both dipole and hydrogen bonds during polymerization.
Another possible elucidation is related to the residual monomer
that fails to polymerize in the glassy state and decreases the T
g
value. However, in our case, each reaction mixture was puried
by the highly-ecient ultraltration method following by
precipitation, which allowed for the separation of the unreacted
monomer from the reaction, mixture which was conrmed by
1
H NMR measurements. Therefore, we assume that the main
factor aecting the T
g
value of produced PVP is connected to the
condition applied for the polymerization (bulk polymerization).
Interestingly, synthesized herein PVPs were characterized by
dierent solubility properties than commercially supplied. It is
well-known that PVP is soluble in water and other organic
solvents, including alcohols (i.e., methanol, ethanol), and some
chlorinated compounds such as chloroform or methylene
chloride. On the other hand, it is essentially insoluble in
hydrocarbons, ethers, ketones (i.e., acetone), and esters. Note
that PVP produced with the lack of solvent (water) can also be
dissolved in acetone, toluene or xylene. In our case, the main
dierence was in the solubility of synthesized polymers in
acetone (see Table 1S in the ESI). Nevertheless, our results are
consistent with literature data showing the signicant impact of
the polymer production method on its thermomechanical and
solubility properties.
36
Fig. 4 (a) SEC chromatograms of PVP produced under HP FRP
(sample 2c) and commercially available samples C2 (PVP K90) and C1
(PVP K30); DMF+10 mM LiBr.
Table 3 FRP of VP performed within 0.10 wt% of AIBN at varied
thermodynamic conditions
No. Time [h] T[K] p[MPa] Conv.
a
[%] M
nb
[kg mol
1
]Đ
b
I 0.5 343 125 0.65 230.4 1.55
II 333 250 0.16 66.2 1.48
III 323 500 0.05 16.2 1.38
IV 333 0.1 0.06 88.9 1.96
a
Estimated by
1
H NMR (600 MHz, CDCl
3
).
b
Estimated by SEC
(DMF+10 mM LiBr).
21598 |RSC Adv.,2020,10,2159321601 This journal is © The Royal Society of Chemistry 2020
RSC Advances Paper
It should also be stressed that strategy proposed in this
paper allowed us to obtain PVP homopolymers with a strictly
dened M
n
within the range of 16.2280.5 kg mol
1
, which
makes it very promising considering great exibility of these
macromolecules across multiple applications. Note that PVP
homopolymers, due to their unique combination of properties,
i.e. good solubility in water and many organic solvents, chem-
ical stability, biocompatibility, non-toxicity, and anity to
complex both hydrophobic and hydrophilic substances, are
widely used for designing materials for dierent applications.
One should mention the pharmaceutical industry and medi-
cine, optical and electrical applications, adhesives, coatings and
inks, agriculture, membranes, bres and textiles.
37
Among so
many, the biomedical and pharmaceutical applications are
particularly the most important. In fact, PVP is one of the best
additives and pore-former agents in ultra-,
38
macro-,
39
micro-
and nanoltration
40
membrane fabrication for biomedical
applications, e.g. hemodialysis
41
or drug release-controlling
membranes.
42
As polybase it can form interpolymer complexes
(IPC) with polyacids. Keeping in mind that this complexation
process is strongly dependent on the pH, these pH-sensitive
materials were designed for biomedical applications, e.g. pH-
controlled drug delivery.
43
For the latter use, the PVP with low
molecular weight is desirable since it forms complexes with
both low molecular weight compounds and polymers, making
insoluble substances soluble simultaneously improving their
biocompatibility.
36
On the other hand, PVP of high molecular
weight (higher than M
n
300 kg mol
1
) are less suitable for
biomedical application, because they have a high viscosity in
water and therefore dissolve too slowly, delaying dissolution of
Fig. 5 Mechanical loss G00 (solid lines) and storage G0(dashed lines) spectra of: (a) sample 2c, (c) C2 and (e) C1 from the temperature region
above T
g
, Master curves of G0(black symbols) and G00 (coloured symbols) of: (b) sample 2c, (d) C2 and (f) C1 obtained at a reference temperatures
of T
ref
¼T(s
cross
¼1.4 rad s
1
).
This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10,2159321601 | 21599
Paper RSC Advances
the active substance.
44
Nevertheless, these materials are oen
used commercially for the clarication of beverages such as
beer, vinegar, and grape wine.
37
High molecular weight PVPs are
also used as protective colloids and particle size regulators for
suspension polymerization of styrene, vinyl acetate, and vinyl
chloride.
Conclusions
In conclusion, the FRP of LAM monomer, VP, has been per-
formed under high-pressure conditions (p¼125500 MPa),
extending the pool of green, solvent- and metal-free synthetic
strategy of highly pure PVP. Our results demonstrated that
a careful selection of conditions (a type of initiator, low initiator
concentration, lack of solvent, pressure, temperature) is crucial
for achieving PVP of well-dened structural parameters in
a short reaction time-frame (0.52 h). Depending on the reac-
tion conditions, well-dened polymers in a wide range of M
n
¼
16.2280.5 kg mol
1
and the relatively low dispersities (Đ¼
1.271.55) have been obtained. Note that produced PVP has the
lowest dispersity values reported to date for FRP. We founded
that HP acts as an externalcontrolling factor that provides the
rst-order polymerization kinetics for classical FRP, something
that was possible so far only for RDRP systems. It was also
conrmed that the method of polymer preparation strongly
aects its physicochemical properties, including T
g
and solu-
bility. We also found that similar mechanical properties char-
acterize PVP of low dispersity indices (produced herein) to that
available commercially that are being sold as products of much
higher dispersity. Note that the proposed PVP synthetic strategy
is fully compatible with the current industrial polymerization
possibilities. In this context, one can remind about high-
pressure cells of large capacity that are used for the food
pasteurization process (p¼1 GPa). We believe that presented
herein the novel synthetic methodology opens an alternative
tool of preparation of well-dened PVP of high purity using the
greenestsynthetic strategy to date.
Conicts of interest
There are no conicts to declare.
Acknowledgements
P. M. and R. B. are thankful for nancial support from the
Polish National Science Centre within SONATA project (DEC-
2018/31/D/ST5/03464). D. H. is thankful for nancial support
from Medical University of Silesia within Research for Young
Scientists (Contract No. KNW-2-O-08/D/9/N).
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... Although significant progress has been made on a laboratory scale due to the use of CRP methods to 1-to reduce/eliminate limitations of ambient-pressure strategies, making it possible to obtain higher molecular weight PVP of linear and star-shaped topology, characterised by much better structural parameters than the commercial material. [27,28] Here, having the well-defined linear and four-arm star-shaped PVP produced via 'green' catalyst-and solvent-free HP strategies, we set out to check, whether the matrix topology can influence the ability to drug encapsulation (leading to API's amorphisation), the stability of drug-loaded structures, and ability to effectively release the drug from the carrier. Another aspect worth investigating is the difference in the ability to solubilise the drug, and its releasing between two linear matrices obtained by different polymerisation strategies (controlled, solvent-free vs commonly used uncontrolled solvent-assisted). ...
... Linear and star-shaped PVPs were obtained according to the previously described method. [28,29] Drug loading and micelle preparation J o u r n a l P r e -p r o o f Journal Pre-proof PVPs and MTZ were dissolved in methylene dichloride CH2Cl2 (or chloroform, depending on polymeric matrix solubility) with the following polymer/drug weight ratios: 2:1, 1:1 and 1:2 similarly to previously reported procedures. [30][31][32][33][34][35][36] Solutions were added dropwise into deionised water and stirred overnight to evaporate the organic solvent. ...
... Within this work, we selected three different PVP matrices i) linear (MnSEC=66.2 kg/mol; Đ=1.48) produced via HP thermally-induced free-radical polymerization, ii) unique, four-arm star-shaped PVP (MnSEC=175.6 kg/mol; Đ=1.81) synthesized via HP RAFT [28] that was previously not used as potential drug carrier, and iii) commercially supplied PVP (commercial name K90; MnSEC=108.6 kg/mol; Đ=1.78), which were applied as polymeric support for MTZ. ...
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