NMR study of vinylpyrrolidone polymerization in supercritical carbon dioxide

ISSN 0012�5008, Doklady Chemistry, 2009, Vol. 428, Part 2, pp. 246–249. © Pleiades Publishing, Ltd., 2009.Original Russian Text © A.A. Samoilenko, L.N. Nikitin, A.M. Lopatin, I.S. Ionova, Al.Al. Berlin, A.R. Khokhlov, 2009, published in Doklady Akademii Nauk, 2009, Vol. 428,No. 5, pp. 624–627.

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In the past decade, interest in spectroscopic studyof various chemical processes at elevated pressures hasbeen considerably increased. This especially pertainsto supercritical fluids and, in particular, to supercriti�cal carbon dioxide (sc�CO2). In study of chemicalreactions occurring in sc�CO2, various methods,including NMR, are used for determining molecularstructures. NMR was used for studying hydrogenbonding of methanol in sc�CO2 [1], dissolution of flu�orinated compounds and polymers in this mediumand solute–solvent interactions [2–4], as well as swell�ing of various polymers in sc�CO2 [5]. Spontaneousformation of microemulsions in sc�CO2 has also beenaddressed [6].

Supercritical CO2 is a good candidate as a solventfor polymerization and polycondensation reactions[7–10]. NMR can provide unique information on themechanisms and dynamics of these processes, espe�cially in in situ studies.

However, high pressures and temperatures neces�sary for studying such systems prevent the use of com�mercial NMR tubes for taking measurements; there�fore, researchers apply various approaches to designcells for such experiments with the use of a bronze res�onant cavity, tubes made of high�strength ceramics,sapphire, polyaryletheretherketone (PEEK), or con�tainers made of quartz capillaries [11].

This work deals with the design of a low�cost cellsuitable for routine measurements at high pressuresand elevated temperatures and demonstrates thepotential of NMR spectroscopy for in situ study ofpolymerization processes in sc�CO2 for polyvinylpyr�rolidone (PVP) as an example.

There are several constructions described in the lit�erature in which working cells are tubes made of thehigh�strength polymer PEEK. The prototype of thecell developed by us was a construction in which aPEEK tube was also used [11]. The polymeric tube wasreplaced by a heavy�walled capillary made of standardmolybdenum glass (Fig. 1). Advantages of this cell arethe ease of fabrication, high inertness, and higher ser�vice temperatures of glass as compared with a polymermaterial, which makes it possible to use this construc�tion for taking routine serial measurements.

The developed cell withstood working pressures upto 20 MPa and afforded satisfactory resolution in 1HNMR spectra (about 2 Hz).

A mixture of vinylpyrrolidone (Aldrich, 99%), 2 wt %of the polymerization initiator azoisobutyronitrile(Fluka, 98%), and 1 wt % of hexane as the internalNMR reference was placed into a tube (1.2 mm in i.d.,5.6 mm in o.d., 70 mm in length). After assembling thecomponents and sealing, the cell was filled with car�bon dioxide at a pressure of 6 MPa so that as tempera�ture increased to 63°C (the temperature of initiatordecomposition), the pressure increased to 8.5 MPaand the solvent (CO2) transformed into the supercriti�cal state. Carbon dioxide (State Standard GOST8050�85) was used in this study. The certified contentof water in CO2 was no more than 5 × 10–6%.

The cell was placed in a magnet of an NMR spec�trometer (AM�400 WB) in such a way that the tube waslocated in the working zone of the coil of a probe mod�ified to fit a tube 6 mm in o.d. 1H NMR spectra(400.13 MHz) were measured at intervals of a fewminutes over the entire period of vinylpyrrolidonepolymerization. In addition, an analogous experimentwith the use of heavy water as the solvent was carriedout at normal pressure in the same cell.

Figure 2 shows NMR spectra recorded in thecourse of PVP synthesis. These spectra reflect the evo�lution of the system due to the formation of a polymer.The upper spectrum was recorded 20 min after the

CHEMISTRY

NMR Study of Vinylpyrrolidone Polymerization in Supercritical Carbon Dioxide

A. A. Samoilenkoa, L. N. Nikitinb, A. M. Lopatinb, I. S. Ionovaa, Academician Al. Al. Berlina, and Academician A. R. Khokhlovb

Received April 9, 2009

DOI: 10.1134/S0012500809100048

a Semenov Institute of Chemical Physics, Russian Academyof Sciences, ul. Kosygina 4, Moscow, 119991 Russia

b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia

DOKLADY CHEMISTRY Vol. 428 Part 2 2009

NMR STUDY OF VINYLPYRROLIDONE POLYMERIZATION 247

specified temperature (63°C) was achieved. In thespectrum, new signals of oligomers are observed at1.76 and 3.27 ppm due to, respectively, СН2СН2СН2

and СН2СН2N in the pyrrolidone moiety and at 3.91ppm due to the СН group in the backbone; the signalat 2.2 ppm due to СН2СО is overlapped with thestrong signals of initial vinylpyrrolidone [12]; the sig�

nals at 7.03, 1.43, and 1.66 ppm arise from СН=СН2,cyclohexane, and azoisobutyronitrile, respectively.

The lower spectrum was recorded after 90 min ofthe reaction. It shows strongly broadened signals.However, it is still possible to determine the concen�trations of the protons in the residual liquid monomer.The spectral shape is caused by the poor solubility of

Valve

Fluoroplast�4 gasket

Molybdenum glass tube

Brass adaptor

(b)

9 5 48 7 6 3 2 1 0

(a)

−1 −2

6.95

7.03

3.91

3.27

1.76 1.

661.

43

Chemical shift, ppm

Fig. 1. Cell for NMR experiments at high pressures.

Fig. 2. 1H NMR spectra of the vinylpyrrolidone–initiator reaction mixture at 63°C in sc�CO2 after (a) 20 and (b) 90 min of thereaction.

248

DOKLADY CHEMISTRY Vol. 428 Part 2 2009

SAMOILENKO et al.

(b)

9 5 48 7 6 3 2 1 0

(a)

−1 −2

1.64

1.95

2.23

7.00

4.52

4.39 4.

33

3.74 3.

57 3.25

7.00

4.43

3.90

3.35

2.36

1.75

1.60

1.43

Chemical shift, ppm

Fig. 3. 1H NMR spectra of the vinylpyrrolidone–initiator reaction mixture at 63°C in D2O after (a) 20 and (b) 80 min of the reac�tion.

0.1

0 20

Vinylpyrrolidone in sc�CO2

100 120

1.0

40 60 80 140 160 180 200

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Con

cen

trat

ion

of C

H2=

CH

gro

ups,

mol

/L

Vinylpyrrolidone in D2O

Time, min

Fig. 4. Concentration of vinyl groups CH2=CH of monomeric vinylpyrrolidone vs. time at 63°C.

DOKLADY CHEMISTRY Vol. 428 Part 2 2009

NMR STUDY OF VINYLPYRROLIDONE POLYMERIZATION 249

PVP in sc�CO2 and the high viscosity of the reactionproduct.

The spectra in D2O (Fig. 3) are much betterresolved due to the high solubility of the polymer (oli�gomers), and all signals of the nascent molecular frag�ments are clearly pronounced.

The data obtained were processed and presented asthe plots of the concentration of vinyl groupsСH2=СH of the vinylpyrrolidone monomer versustime (Fig. 4).

A more than twofold increase in the rate of this pro�cess in D2O is noteworthy. This is explained by hydro�gen bonding of vinylpyrrolidone with water and, as aresult, an increase in the effective polymerization rateconstant [13]. A considerable change in the viscosityof the medium and its heterogeneity due to a decreasein the diffusion rate of molecules (initiator, monomer)should be taken into account in consideration of poly�merization kinetics in sc�CO2. However, the nearlycompletion of the vinylpyrrolidone polymerization insc�CO2 in 180 min is evidence of the high efficiency ofthe process. Based on the literature data, someresearchers have suggested a much longer duration(24 h) of the polymerization reaction under roughlythe same conditions [14].

Thus, we designed, fabricated, and tested an effi�cient cell for taking NMR measurements at high pres�sured and elevated temperatures.

The possibility of in situ 1H NMR monitoring ofpolymerization processes in supercritical carbon diox�ide was demonstrated for the first time.

We showed that the rate of PVP polymerization insc�CO2 decreases owing to an increase in the viscosityand heterogeneity of the reaction mass and, as a result,a decrease in the effective diffusion of the reactionmixture ingredients.

The vinylpyrrolidone polymerization efficiencywas shown to be significantly higher than the previ�ously determined one.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project nos. 07–03–91584,

08–03–00294, 08–03–12152, 08–03–90012), theDivision of Chemistry and Materials Science, RussianAcademy of Sciences (program “Design of NewMetallic, Ceramic, Glass, Polymeric, and CompositeMaterials”), and the Presidium of the Russian Acad�emy of Sciences (program P27 “Foundations of BasicResearch into Nanotechnologies and Nanomateri�als”).

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