Influence of Aromatic Nitro Compounds on Retarding of Radical Polymerization

ROCZNIKI CHEMII ANN. SOC. CHIM. POLONORUM 45, 1841 (1971)

INFLUENCE OF AROMATIC NITRO COMPOUNDS ON RETARDING OF RADICAL POLYMERIZATION. PART III. * INFLUENCE OF SOME METHYL DERIVATIVES OF NITROBENZENE, m-DINITROBENZENE AND sym-TRINITROBENZENE

by Tadeusz URBAŃSKI and Jan BUŻNIAK

Institute of Organie Chemistry and Technology, Polytechnical Uniyersity, Warszawa (T.U.) Technical Military College, Warszawa (J.B.)

Zbadano opóźniający wpływ orto i para metylowych pochodnych nitro benzenu, m-dwunitrobenzenu i sjym-trójnitrobenzenu na kopolimeryzację nienasyconej żywicy poliestrowej „Polimal 109". Najbardziej aktywne okazały się pochodne o-monometylowe. Stwierdzono jednak, że obecność dwóch grup metylowych w pozycji orto do grupy nitrowej osłabia opóź niające działanie tej grupy na przebieg kopolimeryzacji.

MccjieflOBano ona3flŁiBaiom,ee BjiMHHMe o- M n-MeTHJibHbix npoM3BOflHbix HMTpo6eH30Jia, •M-,ziByHMTpo6eH30Jia M cu.JUi-TpMHMTpo6eH30.rra Ha conoJiM- MepM3au,MK> HenpeflejibHoił nojiMScpMpHOM CMOJIM „IIojiMMaji 109". Hań- fleHO, HTO OTHOCMTCJIbHO CMJIbHbIM fleMCTBMeMOTMenajIMC b O-MOHOMeTMJIb- Hbie npoM3BOflHbie. OflHaKO npMcyTCTBMe flByx o-MeTMJibHbix rpynn ocjia6jiMBaeT ona3,n,biBaiomee ^eMCTBMe 3TMX HMTpocoeflwueHMM Ha cono- JlMMepM3ai4MIO.

The retarding action of ortho and para methyl derivatives of nitrobenzene, of m-dinitrobenzene and of siym-trinitrobenzene was examined on a copolymerization of unsaturated polyester resin „Polimal 109". It was found that ortho monomethyl nitro compounds are the most active retardants. On the contrary, two methyl groups in the ortho position to the nitro group reduce the retarding action of the nitro compounds.

This paper is the continuation of our examination on some properties of aromatic nitro compounds. We have tried here to elucidate the importance of a position of the methyl group in the aromatic ring. We have examined the nitro compounds in which the methyl group is situated in the ortho or para position in relation to the nitro group. In these compounds the methyl group produces a similar mesomeric effect on the nitro group contrary to the meta compounds. Derivatives of nitrobenzene, of m-dinitrobenzene and of sym-trinitrobenzene have been used. In these compounds the mesomeric effects, which result from mutual position of nitro group, are identical.

* Part II: Urbański T., Buźniak J., Roczniki Chem., 45, 789 (1971). 1842 T. Urbański and J. Buźniak

EXPERIMENTAL

We applied the method of investigation as presented previously 1.2). The unsaturated polyester resin „Polimal 109'' served as a standard polymerizable medium, the benzoyl peroxide in the form of a pastę in dibutyl phthalate in the ratio 50/50 was used as an initiator and the high purity nitrobenzene (m.p. 5.1—5.5°C, n$- 1.5527) was used as a standard nitro compound. In the polymerizable medium we examined the peak exotherm temperaturę

(tm), the maximum rate of the temperaturę increase (um), the gel time (T?) and the cure time (xu). The changes of value in these four parameters took place under the influence of added nitro compounds. The differences between the parameters of the standard resin and the parameters of the same resin with nitro compounds are given in the Table (A£m, Aum, ATŻ, ATU).

Table

Variation of the parameters tm, vm, T± and Tu of standard polyester resin under the influence of methyl isomers of nitrobenzene, m-dinitrobenzene and ^/n-trinitrobenzene of constant cdncentration

Examined compounds (2x 10~5

Avm AT No gram-equivalent of nitro group Ż Figurę per 1 g of the standard resin) °C °C/min s s

1 Nitrobenzene (standard compound) 10 16 15 25 1,2 2 2-Nitrotoluene 12 28 32.5 52.5 1,2 3 4-Nitrotoluene 9 4 5 7.5 1,2 4 2-Nitro-l,3-xylene 3.5 4 15 17.5 1,2 5 4-Nitro-l,3-xylene 10.5 16 22.5 40 1,2 6 Nitromcsitylene 2.5 0 10 12.5 1,2 7 1,3-Dinitrobenzene 21.5 48 75 130 3,4 8 2,4-Dinitrotoluene 25 56 70 137.5 3,4 9 2,6-Dinitrotoluene 34.5 72 145 280 3,4 10 2,4-Dinitro-l ,3-xylene 16.5 40 60 112.5 3,4 11 4,6-Dinitro-l,3-xylene 22 52 90 170 3,4 12 Dinitromesitylene 9.5 20 15 25 3,4 13 1,3,5-Trinitrobenzene 66 92 492.5 810 5 14 2,4,6-Trinitrotoluene 88 95 ..*) 1067.5 5 15 2,4,6-Trinitro-l ,3-xylene 43 98 362.5 637.5 5 16 Trinitromesitylene 13.5 36 40 67.5 5

The peak cxotherm temperaturę (tm) did not reach 90°C.

The following nitro compounds were examined: nitrobenzene, 2- and 4-nitrotoluene, 2- and 4-nitro-l,3-xylene, nitromesitylene, 1,3-dinitrobenzene, 2,4- and 2,6- -dinitrotoluene, 2,4- and 4,6-dinitro-l,3-xylene, dinitromesitylene, 1,3,5-trinitrobenzene, 2,4,6-trinitrotoluene, 2,4,6-trinitro-l,3-xylene, and trinitromesitylene. Ali nitro compounds were added in concentration 2X10~5 gram-equivalent of the nitro groups per 1 g of standard resin. The nitro compounds, if not commercially available, were obtained by methods described in the literaturę. The m. p's. (as a criterion. of purity) did not differ from those of the literaturę. Influence of aromatic nitro compounds 1843

RESULTS AND DISCUSSION

Our results from the experiments are given in the Table and in Figs 1—5. We limited ourselves to presenting the variation of the peak exotherm temperaturę (Atm) and of the cure time (AtJ, because the variation of the maximum ratę of the temperaturę increase (At>m) and of the gel time (AT*) gave almost analogous diagrams. We observed that in the case of mononitro derivatives of benzene the ortho methyl group activated the nitro group in retarding the radical po-

0 12 3

[-CH3]

Fig. 1. The effect of the position and of the number of methyl groups in derivatives of nitrobenzene on variation of the peak exotherm temperaturę (Atm) of standard polyester resin

NO?

FiS- 2. The effect of the position and of the number of methyl groups in derivatives

of nitrobenzene on yariation of the cure time (Atu) of standard polyester resin 1844 T. Urbański and J. Buźniak lymerization and the para methyl group only slightly deactivated the nitro group (Figs 1 and 2). So far, we found that 2-nitrotoluene proved the more active compound than nitrobenzene and 4-nitro-l,3-xylene than 4-nitrotoluene. We also found that 4-nitrotoluene was less active than nitrobenzene, 4-nitro-l,3-xylene and nitromesitylene less active than 2-nitrotoluene and 2-nitro-l,3-xylene, respectively. It is interesting that two methyl groups in the ortho position to the nitro group cause the decrease in the retarding of the radical polymeriza-

Nflj

[-CH3J

Fig. 3. The effect of the position and of the number of methyl groups in derivatives of m-dinitrobenzene on variation of the peak exotherm temperaturę (Atm) of standard polyester resin

0 I 2 3

[•CH3]

Fig. 4. The effect of the position and of the number of methyl groups in derivatives of m-dinitrobenzene on variation of the cure time (Atu) of standard polyester resin Influence of aromatic nitro compounds 1845

0 12 3 [-CHjJ Fig. 5. The effect of the position and of the number of methyl groups in derivatives of sj/m-trinitrobenzene on variation of the peak exotherm temperaturę (O) and of the cure time (®) of standard polyester resin tion to a great measure. This phenomenon is shown when comparing 2-nitrotoluene with 2-nitro-l,3-xylene and 4-nitro-l,3-xylene with nitromesitylene. It is known that two methyl groups in neighbouring nitro group cause a restricted rotation of one by reason of its magnitude and of impossibility to place in coplanar position with aromatic ring *>, We suppose that the retarding of the radical polymerization by the nitro group depends on the mutual position of this group and the aromatic ring. The observed phenomenon for mononitro compounds that the ortho nitro group to the methyl group increase the retarding of the radical polymerization was noticed by us in the case of polynitro derivatives, too. As may be seen from Figs 3 and 4 as welł as Table 2,6-dinitrotoluene show the most activity among dinitro derivatives. This is comprehensible because the methyl group is placed in the ortho position to both of the nitro groups. In the case of 2,4-dinitrotoluene the methyl group is in the ortho position to one and only nitro group and, therefore, this compound is less active that 2,6-dinitrotoluene. It is known that the second methyl group present in 2,4-dinitro-l,3- -xylene causes the blocking one of the nitro groups between two methyl groups and restricts the rotation. That causes a appreciable decrease of retarding of the radical polymerization by 2,4-dinitro-l,3-xylene when comparing with 2,6-dinitrotoluene. In the case of 4,6-dinitro-l,3-xylene a somewhat different situation has occurred. The second methyl group does not cause the change in the retarding properties of this compound in comparison with 2,4-dinitro toluene. This phenomenon may be explained by the fact that in 4,6-dinitro-l,3-xylene each methyl group is in ortho position to one of the nitro groups and in para position to another one. The para position re- duces partially the strong retarding effect of the ortho position. 1846 T. Urbański and J. Buźniak

The least active compound of dinitro derivatives proved to be dinitromesitylene. The fact that both nitro groups are placed between the methyl groups causes the decrease of the retarding properties to such an extent that dinitromesitylene is comparable with nitrobenzene, which comprises only one nitro group. The properties of the methyl derivatives of sym-trinitrobenzene are shown in the Table and in Fig. 5. One methyl group in 2,4,6-trinitrotoluene caused the increase in the retarding action of the radical polymerization in comparison with 1,3,5- -trinitrobenzene. Here the methyl group is in ortho position to the both nitro groups as in 2,6-dinitrotoluene and hence a similar action. Two methyl groups present in sym-trinitrobenzene derivatives con- stitute a steric hindrance of one of the nitro groups and three methyl groups, all of them. As a result of this phenomenon 2,4,6-trinitrotoluene, 2,4,6-trinitro-l,3-xylene and trinitromesitylene retard the radical poly merization in a decreasing order. It is necessary to mention that some observations about the participa- tion of certain nitro derivatives of benzene and toluene in retarding the radical polymerization were found by Szyc-Lewańska and Sy czewski a few years ago during investigation of the polymerization of methyl methacrylate 4). The results of our investigation point out that the position and the number of methyl groups in relation to nitro groups in aromatic ring is of great importance in retarding the radical polymerization by nitro compounds. Particularly we have noticed that the single methyl group in ortho position to one or two nitro groups always increases the retarding action of the radical polymerization. This phenomenon was noticed by us in our former paper 2) for a number of mononitro compounds. We have also observed that the steric hindrance of nitro group by two ortho methyl groups causes an appreciable decrease in retarding action of the radical polymerization in all cases. This phenomenon seems to depend on coplanarity of the nitro group and the aromatic ring. This will form a subject matter for our futurę research. To summarize, we can conclude, that the results of our investigation confirm the hypotheses: we believe that various activities of aromatic nitro compounds retarding the radical polymerization consist in different rates of the transfer of hydrogen atom between the polymerizing ma- croradicals with the help of catalytic intervention of nitro group, as previously suggested Received January 28th, 1971.

REFERENCES

1. Buźniak J., Urbański T., Polimery, 15, 333 (1970). 2. Urbański T., Buźniak J., Roczniki Chem., 45, 789 (1971). 3. Trotter J., Can. J. Chem., 37, 1487 (1959); Acta Cryst., 12, 605 (1959). 4. Szyc-Lewańska K., Syczewski M., Telrahedron, Suppl. I, 20, 257 (1964;. Influence of aromatic nitro compounds 1847

WPŁYW NITROZWIĄZKÓW AROMATYCZNYCH NA OPÓŹNIANIE POLIMERYZACJI RODNIKOWEJ. III. WPŁYW NIEKTÓRYCH POCHODNYCH METYLOWYCH NITROBENZENU, m-DWUNITROBENZENU I sym-TRÓJNITROBENZENU

T. URBAŃSKI i J. BUŻNIAK

Instytut Chemii i Technologii Organicznej Politechniki, Warszawa (T.U.) Wojskowa Akademia Techniczna, Warszawa (J.B.)

Zbadano opóźniający wpływ orto i para metylowych pochodnych nitrobenzenu, m-dwunitrobenzenu i sym-trójnitrobenzenu na przebieg kopolimeryzacji nienasy conej żywicy poliestrowej „Polimal 109" jako żywicy wzorcowej. Badane nitro- związki porównywano z nitrobenzenem przy stałym stężeniu 2X10_S gramorówno- ważnika grup nitrowych na 1 g żywicy. Stwierdzono zależność opóźniania polime ryzacji rodnikowej od wzajemnego położenia grup nitrowych i metylowych w pierś cieniu aromatycznym. Najbardziej aktywnymi nitrozwiązkami w opóźnianiu polimeryzacji rodniko wej okazały się pochodne o-metylowe, a najmniej aktywnymi — pochodne z grupą nitrową zablokowaną sterycznie pomiędzy dwiema grupami metylowymi. Pewne niewielkie zmniejszenie zdolności opóźniających polimeryzację zauważono również w przypadku mononitrozwiązków p-metylopochodnych. W szczególności zaobserwo wano, że wśród jedno-, dwu- i trójnitrozwiązków aromatycznych najbardziej aktyw nymi okazały się o-pochodne toluenu, a najsłabiej działającymi — pochodne mezy- tylenu. Wyniki obecnej pracy są zgodne z zaobserwowaną uprzednio regułą, że związ kami najbardziej aktywnymi są pochodne o-nitrowe 2>. Należy przypuszczać, że różnice aktywności poszczególnych nitrozwiązków aro matycznych w opóźnianiu polimeryzacji rodnikowej polegają na różnych szybkoś ciach w przekazywaniu atomu wodoru pomiędzy polimeryzującymi makrorodnikami z udziałem katalitycznego pośrednictwa grupy nitrowej. Hipotezę taką zasugerowa liśmy poprzednio ,

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