Mechanochemically initiated polymerizations—5. Polymerization by vibratory milling of acrylamide and methacrylamide

Eur. Polym. d. Vol. 19, No. 6. pp. 525 528, 1983 Printed in Great Britain. All rights reserved

0014-3057/83/060525-(14503.00/0 (opyright © 1983 Pergamon Press Ltd

MECHANOCHEMICALLY

INITIATED

POLYMERIZATIONS--5 POLYMERIZATION BY VIBRATORY MILLING OF ACRYLAMIDE AND METHACRYLAMIDE CR. I. SIMIONESCU,l eL. VASILIU OPREA 1 and J. NtCOLEANt: 2 Department of Organic and Macromolecular Chemistry, Polytechnic Institute of Jassy and 2-p. Poni" Institute of Macromolecular Chemistry, Aleea Ghica Vod;~ 41-A, 6600 Jassy, Rumania (Received 16 Auqust 1982; in revised fi)rm 25 November 19821

Abstract- Mechanochemieal polymerization by vibratory milling of acrylamide and methacrylamide is discussed. The influences of duration, nature of medium and temperature are examined. The polymers are characterized by elemental analysis, ix:spectroscopy, X-ray and by determination of some properties. An anion-radical mechanism is proposed.

INTRODUCTION T h e possibility of initiating m e c h a n o c h e m i c a l l y polym e r i z a t i o n o f m o n o m e r s in the solid state has been r e p o r t e d [I 12]. Studies on m o n o m e r s of acrylamide (AA) a n d m e t h a c r y l a m i d e (MAA) types b e g a n with K a r g h i n and Plate [2 4], followed by B a r a m b o i m [7]; the p o l y m e r i z a t i o n s were p e r f o r m e d in the presence of s o m e inorganic activators (NaCI, BaSO,,, quartz etc.). M e c h a n i c a l dispersion o f these substances gave some new surfaces o f high chemical reactivity, able to activate h o m o p o l y m e r i z a t i o n and wtrious types of c o p o l y m e r i z a t i o n . P o l y m e r s have also been m a d e by vibratory milling of s o m e m o n o m e r s w i t h o u t a d d e d initiators [8-12]. T h e p r e s e n t p a p e r refers to the synthesis u n d e r similar c o n d i t i o n s of s o m e m a c r o m o l e c u l a r p r o d u c t s based on AA and MAA. EXPERIMENTAL

I. Apparatus, materials trod workinq methods

Syntheses were carried out on a laboratory mill LS-60 Labowi type (VEB Kefama Katzhtitte/Thfiringen) operating at 25 Hz frequency and 2 4 mm vibration amplitude. The reaction vessels and the working methods have been described [8 12]. The monomers (30 g}, purified by recrystallization from benzene and drying under low pressure, were introduced together with the grinding bodies (balls of 9 mm diameter) at a tilling ratio of ~l = m/p-100 = 0.5'~i, (m = monomer amount in g: p = grinding bull weight in g) in the reaction vessel. The grindings were performed in inert atmosphere with purified and dried N 2 after remowd of air by repeated flushings with inert gas. The reaction was carried out at 18 + 2 , thermostating being achieved by circulation of cooling water in the jacket. The products were separated from the grinding bodies after milling by sieving and immediately the unreacted monomer was removed by dissolving the sample in acetone (1:20 weight ratio polymer acetone). As a result of this treatment, the polymer remained insoluble and it was dried at 60' for 7 hr. The filtered product was slowly ew~porated in the atmosphere 525

and the residual solid was dried at 60 to constant weight. Part of it was dissolved in cold water and treated with methanol. As no precipitate was obtained, it was concluded that the fraction soluble in acetone is exclusively monomer. The polymer was purilied thus: dissolving in water, illtering, precipitation with methanol filtering and drying in vacuum. Poly(acrylamide) (PAA) wits dissolved in cold water (1 g 3 0 g water) while poly(methacrylamide)(PMAA) at the same sample/solvent ratio v, as treated at 60. The optimum solutionprecipitute ratios were found to be [ ml polymer s o l u t i o n 6 m l methanol for PAA and I I for PMAA. 2. ll!/hlence 0t some limdumemal tilctors o~1 1he mecluulochemical synthesis 2.1. Duration. The influence of duration of grinding on the conversion wits studied (see Fig. 1). It may be noticed that the polymerization is slow between 24 and 72 hr: then there is acceleration and linally u maximum at 96 hr. For longer durations, the conversion decreases. The shape of the conversion/time curve is typical R~r mechanochemically activated polymerizations. This is found for all monomers previously studied, vi:. acrylonitrile,

60

~ ._~

40

I

2o

-

2

O 48

96,

144

192

t(hr)

Fig. I. Influence o[" grinding duration on the conversion: ( 1) acrylamide : {2) methacrylamide.

CR. I. S1MIONESCUet al.

526

styrene, e-caprolactum, isoprene etc., and their copolymers; it must be related to the simultaneous development with the polymerization of some mechanodegradation processes [10-12]. When the mechanochemically synthesized macromolecules reach sufficient size to concentrate mechanical energy, they undergo homolytic scission to generate products of low molecular weight which go into the extraction agent during polymer purification. As in the case of AA and MA the soluble fraction represents the monomer itself, it must be accepted that the mechanogradation occurs at the ends of the chains and the resulting small molecules contaminate the polymer. Such a mechanodegradation process has been described for PMAA [13]. AA is more efficient in the mechanochemical synthesis than MAA; the difference is attributed to the presence in the second monomer of a side-group which modifies the polarity. 2.2. Medium nature. The nature of the medium has a decisive influence on polymerization efficiency. Important differences arise from the nature of the gas atmosphere and the presence of liquid or solid. In the present case, to obtain information on the reaction mechanism, polymerizations were carried out in the presence of radical acceptors such as phenol and hydroquinone introduced in a ratio of 1% based on monomer. The influence on the conversion is marked (Fig. 2). With inhibitor absent for a particular set of conditions, the maximum conversion for AA is 42% (Fig. 2, curve 1) whereas in the presence of phenol it is less than 12~ (Fig. 2, curve 2) and for hydroquinone ~ 2.5% (Fig. 2, curve 3). The same duration of 96 hr was used to obtain the maximum conversion. The results indicate a radical mechanism of the polymerization. 2.3. Temperature. Temperature is very important for the mechanochemical processes. To eliminate the influence of thermal factors on the mechanical activation of the chemical reaction, the influence of this factor has been studied. For crystalline monomers, it is appreciated that by decreasing the temperature the vibratory movement of the atoms in the network is decreased and the strength of intermolecular forces increased, the network becomes more rigid and less of the elastic shock provided by impact with the grinding bodies is taken up. As a result, internal tensions develop so increasing the number of active centres and the polymerization is accelerated.

I00 0

'" @ 5O L)

20

30

40

50

60

70

80

T ('C)

Fig. 3. Temperature influence on the conversion for acrylamide polymerization.

Starting from the idea that near the melting temperature of the monomer the lattice is weaker and under the action of mechanical shocks some chemical bonds can be broken, the influence of temperature was examined. The grindings were carried out for a fixed period of 48 hr in the temperature range 20 to 80 °. The results are given in Fig. 3 which shows that conversion increased with increasing temperature. Near the melting temperature of the monomer (T = 80°), the conversion is practically complete as no soluble product is obtained by washing the sample with acetone. Polymer was not produced in reference samples treated at the same temperatures under similar conditions but without mechanical energy.

3. Characterization of reaction products 3.1. Determination of chemical composition by elemental analysis. The results of elemental analyses for PAA and PAA mechanochemically synthesized samples at different grinding durations and for the unreacted monomer are shown in Tables 1 and 2. The C, H and N contents of polymers are close to those for the structural units: PAA: C--50.7% H--7.04~o N--19.7%

PMAA: C--56.35% H--8.25% N--16.46%

No residue is obtained on calcination of the mechanochemically synthesized polymers compared with those polymers synthesized mechanochemically from liquid monomers. The latter have a slight corrosive action on the vessel walls which, associated with mechanical shocks, caused release of very small amounts of metal in a colloidal

45

40

35 30

.~

25

i

2o-

o

Table 1. Elemental analyses of PAA and PMAA Content (~)

I

Duration (hr)

C

H

N

PAA

24 48 96 144 192

50.41 51.12 50.30 50.39 49.52

7.03 7.07 7.05 7.02 7.04

19.72 19.08 19.24 18.69 18.75

PMAA

24 48 96 144 192

55.56 55.21 55.72 56.21 54.74

8.10 8.12 7.98 8.24 8.31

16.41 16.20 16.10 15.21 15.44

Polymer

15 10

5 0

L 20

40

60

80

I00

120 140

160

180 200

t (hrl Fig. 2. Influence of some inhibitors on mechanochemica]

polymerization of acrylamide: (1) without inhibitors; (2) phenol; (3) hydroquinone.

Mechanochemical polymerization of AA and MAA Mx I0 3

Table 2. Elemental analyses of unreacted monomers

Duration (hr)

C

H

N

24 48 96 144 192

51.61 52.88 49.87 49.21 49.57

7.05 7.13 7.46 7.05 7.12

19.81 19.10 19.34 18.21 18.82

24 48 96 144 192

55.46 55.83 56.59 56.46 55.84

8,08 8,14 8.34 8.08 8.36

16.05 16.18 16.48 15.78 16.20

AA

MAA

M x I0 4

8-8

Content (%) Sample

527

6 -6

./* ,/

4-4

21-2

0 -0

I

1

I

48

96

144

L 192

t (hr) state which participated in the reaction. Consequently it may be concluded that these polymers contain metal units linked by coordination [%12]. Solid monomers of AA and MAA type did not show this behaviour. 3.2. Characterization by i.r. spectroscopy. I.R. characteristics are given in Table 3. The shape of spectra and the absorption peaks found for the characteristic groups of these polymers are very similar to those quoted in the literature [14]. 3.3. Determination oJ molecular weight. To determine molecular weights, an Ubbelohde viscometer was used (capillary No. 1) at 30~>with 0.2,% PAA and PMAA solutions in M. NaNO3 in water. The formula [r/] = 3.73 x 10 4 x ~0.66 was used [15, 16]. There seems to be a slight dependence of molecular weight on the duration of synthesis (Fig. 4). Figure 4 shows the molecular weight vs time; a slight dependence is seen. Existence of maximum molecular weights at 96 hr can be seen as for conversion. This fact can be accounted for by simultaneous polymerization and degradation. Within the period 24-96hr when there is a large amount of unreacted monomer, there is synthesis of polymer. When the maximum is reached, degradation becomes the main process and there is a decrease of molecular weight to a limiting value. 3.4. Characterization by X-ray diffraction. X-ray diffraction spectra for the mechanochemically synthesized poly-

Fig. 4. Influence of the duration on molecular weight of mechanochemically synthesized polymers: (I) PAA: (21 PMAA.

mers were compared with those for the monomers. From the spectra, destruction of the original crystalline lattice is clearly followed by the appearance of products of dominant amorphous structure. 3.5. Solubility. PAA is dissolved in water under all condilions: PMAA is soluble in water at 30". Both polymers gave viscous aqueous solutions of good stability and tolerance to electrolytes. Common organic solvents such as acetone, benzene, methylene chloride, ethyl acetate and dichlorethane do not dissolve the polymers. The present data are supported by published reports [15, 16].

DISCUSSION AND RESULTS Acryhtmide and methacrylamide have reactivities because of their structures. +8

CH2

_f'-8/H C\C

=~ ( - M )

different

+ ~H z =,~_C8fC(H: i ) \C ~ ( - M )

NH z

NH 2

(-I)

(-I)

ucrylo mide

methocrylomide

Table 3. I.R. absorption bands of PAA and PMAA Data given in the literature [14] (cm- 1)

Data obtained for PAA (cm i)

Data obtained for PMAA (cm ~)

For MAA, the effect of capture of the double bond electrons by the amide group is partially compensated by ( - - C H 0 group which induces an electron rejection

\

Attributed to effect (+ I); as a result

3498-3182 (fi) 3205 (fi) 2941 (m)

3325 3160 2920

2872 1620-1650 (fi)

1650

1435(m) 1351 (s)

1480 1400

650 (s)

1310 610

680-1000 (s)

3360 3170 2985 2920 2760 1655 1600

--NH 2

1400

--CH2 (shearing)

--CH2 --CH 3 - - C O (NH2)

--CHz (shearing) 1100 930 630

C--H (outside the plane)

/ C~-C

/

bond polarity is

\

decreased and thus also the reactivity of this monomer. Consequently conversions smaller than for acrylamide are expected. From the conversion curves, it can be seen that the polymerization process shows auto-acceleration. ]'here is a slow increase of conversion up to 48 hr and then over a short period (72-96hr) a j u m p is recorded. The auto-accelerating character of the polymerization and the amorphous character of the synthesized products show that the reaction is not controlled by the lattice. A matrix effect would generate stereoregular chains, not found experimentally. The polymer resembles a new phase in the mechanically destroyed surface layers of the crystal lattice.

528

CR. I. SIMIONESCUet al.

The amorphous character of the polymer results from unfavourable disposal of the monomer molecules in the lattice disturbing the course of the polymerization. Monomer molecules in the surface layers under mechanical actions are first released from the lattice and then they participate in the polymerization. Thus the reaction occurs at the surfaces where steric control by the lattice does not operate. Crystalline layers are distorted by the mechanical action of the accumulated polymer and extra bond breakages occur; they are responsible for autoacceleration of polymerization. Polymerization is initiated under the action of the electronic stream developed by mechano-emission under the conditions of the vibratory milling. CH2=CR + e-~ CONH2

I

C H 2 - - C R ~-~ (~H2--CR CONH2

CONH2

In principle, the growth of the chain may continue by anions or radicals. The effects of radical acceptors (Fig. 2) indicate the importance of the radical centers. Molecular and supramolecular characteristics of PAA and PMAA synthesized as before are determined by this particular type of reaction. Compared to the materials obtained by conventional methods, mechanochemically synthesized polymers have lower molecular weights. Chain growth is predominant at the beginning of the process when in the reaction medium there is mainly unreacted monomer and the synthesized molecules have not reached sizes ("critical length") sufficient to concentrate the mechanical energy. Molecular weight is also increased near the maximum of the conversion when most monomer is consumed as a result of the accelerating polymerization. In this stage of the reaction, the mechanosynthesis is opposed by mechanodegradation and it is also possible to obtain chains as a result of macroradicals recombining with those formed by homolytic splitting of preformed macromolecules. Above the maximum value of the conversion, the mechanodegradation becomes dominant so explaining the decrease in molecular weight,

CONCLUSIONS 1. Mechanical activation by vibratory milling leads to initiation of polymerization of some solid monomers such as AA and MAA. 2. Polymerization occurs by a radical mechanism in the monomer layers destroyed by dispersion. It is characterized by auto-acceleration and is accompanied by mechanodegradation. 3. The molecular weight of the polymer is of the order 10" for PAA and 103 for PMAA; the polymers have amorphous character. REFERENCES

I. Cr. Simionescu and CL Visiliu Oprea, Mechanochimia Compu~ilor Macromoleculari. Ed, Acad., Bucure~ti (1967). 2. V. A. Karghin and N. A. Plate, Vysokomolek. Soedin. I, 330 (1959). 3. N. A. Plate, V. V. Procopenko and V, A. Karghin, Vysokomolek. Soedin. l, 1713 (1959). 4. V. A. Karghin, N. I. Kobanov and N. I. Popoport, Vysokomolek. Soedin. 3, 787, 1091 (1961). 5. H. Grohn and R. Paudert, Chem. Technik. 12, 430 (1960). 6. R. Paudert, Dissertation. TH fiir chemie, Leuna-Merseburg (1962). 7. N. K. Baramboim, Mehanohim. Vysokomolek. Soedin., Moskva, lzd, Himija (1978). 8. C1. Vasiliu Oprea, I. Avram and R. Avram, Angew. Makromolek. Chem. 68, I (1978). 9. Cr. Simionescu, CI. Vasiliu Oprea and CI. Neguleanu, Eur. Polym. J. 15, 1037 (1979). 10. CI. Vasiliu Oprea and M. Popa, An qew. Makromolek. Chem. 90, 13 (1980). 11. CI. Vasiliu Oprea and M. Popa, An qew. Makromolek. Chem. 73, 92 (1980). 12. CI. Vasiliu Oprea, CI. Neguleanu, M. Popa and F. Weiner, Bull. I.P.I. XXVI (XXX) 1/2, 115 (1980). 13. N. A. Plate and V. A. Karghin. J. Polym. Sci. C 4, 1027 (1963). 14. M. Avram and Gh. D. Mateescu, Spectroscopia ~ IR. Applicatii ~n Chimia Organicd, Ed. Tehnicfi, Bucure~ti (1968). 15. W. M. Thomas, Encyclopedia of Polymer Science and Technology, Vol. I, p. 177. Wiley, New York (1964). 16. H. Rauch-Puhtingam and Th. V/51ker, Akr),l und Methakryl Verbindungen. Springer-Verlag, Berlin (1967).