Crosslinking during radical polymerization of dodecyl methacrylate

88 Macromol. Mater. Eng. 2000, 283, 88 92 Full Paper: A much more efficient formation of crosslinks was observed in the free-radical polymerization of dodecyl methacrylate with respect to the amount of decomposed peroxide than it would correspond to the additional peroxide crosslinking of formed poly(dodecyl methacrylate). Polymer crosslinking also proceeds after using 2,29-azoisobutyronitrile as initiator of the polymerization of dodecyl methacrylate, although with a substantially lower efficiency compared to the initiation by peroxide under comparable conditions. The efficient formation of crosslinked structures can be explained by branching and copolymerization of monomer with multifunctional dead polymer. Multifunctionality of the formed macromolecules is a result of transfer and addition reactions of the present free radicals with the formed polymer. The difference in the influence of the initiator follows from the higher reactivity of oxy radicals in transfer reactions with monomer dodecyl methacrylate which results in a greater number of polymerizable double bonds built in the polymer chain. Dependence of the limiting viscosity number of the soluble part of poly(dodecyl methacrylate) on the conversion fraction X of the polymerized monomer at 608C at different concentration of dibenzoyl peroxide (curve 1 0.05 wt.-%, curve 2 0.15 wt.-%, curve 3 0.5 wt.-%); dashed line 4 is for 0.3 wt.-% of 2,29-azoisobutyronitrile as initiator. Crosslinking during radical polymerization of dodecyl methacrylate Milan Lazár, 1 Ludmila Hrčková, 1 Agnesa Fiedlerová, 1 Eberhard Borsig* 1, 2 1 Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 842 36 Bratislava, Slovak Republic Fax: ++421-7-54775923; E-mail: upolebor@savba.sk 2 Department of Fibres and Textile, Faculty of Chemical Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovak Republic Introduction Dodecyl methacrylate as a derivative of methacrylic acid has long been known as a radical-polymerizing monomer, [1 3] e.g., ref. In comparison with methyl methacrylate with the extension of alkyl groups the gel effect is gradually suppressed in the polymerization process [4]. On the other hand, the longer ester group increases the propagation rate constant, [5 9] which might indicate the participation of the more properly self-organized associates of monomer molecules in the addition reaction considered. Mainly steric hindrances of the two end alkyl groups around the radical center and also higher viscosity caused by longer alkyl chains in the monomer may contribute to a decrease in termination rate. [10] The long alkyl group can also be a site of the attack of radicals (by hydrogen atom abstraction). Transfer reaction can then cause a rise in the functionality of the monomer or higher sensitivity of building polymer units to transfer reactions compared to poly(methyl methacrylate). Both processes mentioned can be projected into the Macromol. Mater. Eng. 2000, 283 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1438-7492/2000/0111 0088$17.50+.50/0

Crosslinking during radical polymerization of dodecyl methacrylate 89 branching, even crosslinking, of poly(dodecyl methacrylate) macromolecules. A relatively significant branching of poly(dodecyl methacrylate) (as much as 8 branchings in a macromolecule) was observed [3] in solution polymerization using toluene as a solvent and 2,29-azoisobutyronitrile as an initiator at 60 8C. The branching of macromolecules is explained [3] either by transfer reaction to a polymer or by copolymerization of polymer molecules containing double C2C end bonds after disproportionation of propagating macroradicals. The aim of this paper is to investigate the influence of free radical initiation on the polymerization of dodecyl methacrylate and to get new data about the mechanism of the crosslinking reaction which as a side reaction usually accompanies the polymerization process in bulk. Results and discussion Formation of insoluble polymer The study of the conversion and of the insoluble part of the formed polymer in block homopolymerization of dodecyl methacrylate (Fig. 1) showed much higher polymer crosslinking efficiency when using dibenzoyl peroxide (DBP) as an initiator compared to 2,29-azoisobutyronitrile (AIBN). Experimental part Chemicals Dodecyl methacrylate (DMA), 96%, was a commercial product (Aldrich). It was shaken before polymerization with a water solution of sodium hydroxide and then dried over calcium chloride. Initiators used: dibenzoyl peroxide (Merck-Schuchardt), 98%, and 2,29-azoisobutyronitrile (Fluka), 98%, were recrystallized before use. Methods Determination of conversion in block polymerization of dodecyl methacrylate The initiator was weighed and placed into an ampoule, monomer (1 g) was added and the ampoule was sealed under nitrogen. After the reaction time at a given temperature, the ampoule was opened, the reaction mixture was diluted with a solvent (toluene), and the solution was slowly added to ethanol containing hydroquinone where the polymer was precipitated. The precipitated polymer was separated by filtration, washed and dried in a vacuum dryer to constant mass. Determination of insoluble portion in dodecyl methacrylate polymerization The isolated poly(dodecyl methacrylate) (PDMA) was packed into a glass cloth and extracted in a Soxhlet apparatus with xylene for 14 h. After extracting the soluble portion of polymer, the insoluble portion which remained in the glass cloth was dried to constant mass. The mass percent of the insoluble portion was calculated from the difference between the mass of the starting polymer packed in a glass fiber-cloth and the rest after extraction. Determination of the limiting viscosity number of poly(dodecyl methacrylate) The limiting viscosity number of PDMA was determined in xylene at 308C, using an Ubbelohde viscosimeter. Fig. 1. Dependence of the amount of insoluble poly(dodecyl methacrylate) (% gel) on the mass fraction of conversion X during polymerization of monomer (1 ml) initiated by dibenzoyl peroxide: + 0.5 wt.-%, 0 0.3 wt.-%, 9 0.15 wt.-% (5 ml), F 0.05 wt.-% and points f with 0.3 wt.-% of 2,29-azoisobutyronitrile at 608C. The simplest way to explain the observed difference in crosslinking of PDMA is indicated by the difference between the efficiency of benzoyloxy radicals that are more reactive than cyanopropyl radicals in hydrogen abstraction from hydrocarbon molecules. So the benzoyloxy radicals easier attack the PDMA chains under formation of polymer macroradicals. Their recombination (most probably of the free radicals on the dodecyl chain) would lead to the crosslinking of macromolecules, similarly as at peroxide initiated crosslinking of methacrylates with longer alkyl chain in the ester group [11]. But when we consider the obtained crosslinking efficiency as the ratio of the number of formed cross-links to the number of molecules of decomposed peroxide and if the crosslinks are formed only from species obtained after transfer reaction of initiator free radicals to the polymer chain, it would require to decompose a more than one hundred times higher molar amount of peroxide than it was used in this case of polymerization under simultaneous crosslinking of PDMA initiated by DBP.

90 M. Lazár, L. Hrčková, A. Fiedlerová, E. Borsig High crosslinking efficiency in the polymerization of DMA indicates that the main path for the formation of crosslinks between macromolecules is probably associated with the chain sequence of addition reactions between macroradicals and more than bifunctional macromonomers being formed. These macromolecules may contain more than one copolymerizable double C2C bond. The participation of the polymer with one double bond (it can react as a macromonomer) also contributes to the high efficiency of crosslinking: since in the copolymerization process, a great part of the non-crosslinked polymer chains is built into the insoluble gel fraction. Polyfunctional macromonomer can probably be formed so that the radical of the initiator abstracts hydrogen from neighboring carbons next to the multiple bond (mainly from the dodecyl group) and such a radical initiates the growth of the PDMA chain. Although such a transfer reaction is in general less probable than the addition of the radical to the vinylidene bond of a monomer, it is possible and it has been experimentally proved, in oxy radicals in particular. For instance, in the case of tertbutyloxy radical the ratio of the addition to hydrogen abstraction in the reaction with methyl methacrylate is 67:33. [12] The preference of hydrogen abstraction increases with the volume of alkoxy radicals. [13] An analogous ratio for benzoyloxy radical in the reaction with methyl methacrylate is definitely in favor of addition (99:1). [14,15] Hydrogen abstraction is much higher for the reaction between benzoyloxy radical and dodecyl methacrylate but particular data are lacking. The average functionality of additionally copolymerizing monomers can be estimated from a critical conversion of reacted monomers according to various approximative equations reported in a review. [16] Since the critical conversion at which the first insoluble gel fraction at 30 40% of the polymer is formed (Fig. 1), the average functionality of the formed macromonomers reaches the value of about 4. It follows from the relatively high portion of the gel insoluble fraction after reaching more than 90% conversion of the polymerized dodecyl methacrylate (Fig. 1) that the majority of the primarily formed macromolecules contains at least one polymerizable group. It is worth noticing that during initiation with 2,29-azoisobutyronitrile under the given conditions (Fig. 1 and the Figure caption) no crosslinked product is formed in poly- (dodecyl methacrylate), which points to the primary formation of macromolecules with no or with only one polymerizable group. The difference between the effect of the two different types of initiator observed indicates one of the possible mechanisms of the formation of double bonds at chain ends in a macromolecule. If a radical from the initiator is added to the monomer in the place of a double bond (this is how cyanopropyl groups function), then a polymerizable group can only be formed at the other end of the growing macromolecule during disproportion of radicals or monomolecular fragmentation of macroradicals or in transfer reaction to a monomer. The course of crosslink formation The mass fraction (s) of the formed soluble polymer can be used for calculation of the degree of crosslinking (c) in terms of the equation [17] c = (s 0.5 + s) 1 (1) that is valid for random distribution of the polymerization degree of macromolecules. The degree of crosslinking expresses the number of crosslinks per an average macromolecule (in our case in the limit of zero conversion). The dependence of the number of crosslinks on monomer conversions (Fig. 2 a) shows a permanent increase of c and of the slope dc/dx. The determined exponent n of the experimental plot c = const. N X n (Fig. 2b) reaches the value of 1.6. The observed experimental dependence of c on X can be interpreted by a model of simultaneous copolymerization of macromonomer (polymer with polymerizable double bond) with low molecular monomer (dodecyl methacrylate) and crosslinking of the formed macromolecules. The ratio dc/dx can be expressed as an increment of crosslinks in the process of copolymerization of the starting monomer (1 X) with a macromonomer being formed (X), so we get the first term of Eq. (2) and a mutual reaction of reactive polymer molecules X (X = quadratic term of Eq. (2)). If the concentration of macromonomer is proportional to the conversion fraction X of the polymerized monomer, then, analogously, the mass fraction of unreacted monomer can be expressed by the difference 1-X and the change of the number of crosslinks with conversion by Eq. (2), analogously to the immediate rate of the reaction of two reactants dc/dx = k (1 X) X + k9 N X 2 (2) and, after integration, by Eq. (3) c ˆ k k k9 2 N X2 3 N X 3 3 The experimental exponent n is approximately the same as the resulting exponent evaluated from Eq. (3) if k = 4 k9. Changes in molar mass The formation of chemical bonds between the formed macromolecules and the polymerizing monomer below the critical conversion of gel formation (formation of soluble polymer fractions) can be followed on the basis of the changes of the molar masses. A rather significant distinct maximum occurs in the dependence of the limiting viscosity number on the conversion of the polymerized monomer (Fig. 3) during initiation with dibenzoyl peroxide. The maximum is, as expected, in the range of the critical con-

Crosslinking during radical polymerization of dodecyl methacrylate 91 a) b) Fig. 3. Dependence of the limiting viscosity number of the soluble part of poly(dodecyl methacrylate) on the conversion fraction X of the polymerized monomer at 608C at different concentration of dibenzoyl peroxide (curve 1 0.05 wt.-%, curve 2 0.15 wt.-%, curve 3 0.5 wt.-%); dashed line 4 is for 0.3 wt.-% of 2,29-azoisobutyronitrile as initiator. Fig. 2. Dependence of the degree of crosslinking c on the conversion fraction X of the polymer formed (according to equation c = const. N X n ), a) in linear and b) in logarithmic coordinate system. The symbols are equal to the caption of Fig. 1. version where gel formation starts. The rise of the limiting viscosity number from the initial state of the polymerization corresponds approximately to the double average molecular mass in gel point, which corresponds to a change in the crosslinking degree from zero to 0.5. A decrease of the average molar mass (the limiting viscosity number of the soluble part of the polymer) after reaching the critical conversion reflects the fact that the longest and the most branched macromolecules are the first to enter the insoluble part of the crosslinked polymer. These macromolecules are most probable to undergo addition copolymerization and transfer reactions, which leads to the integration of the polymerizing monomer and macromonomer into the largest macromolecules. In the light of analogous considerations, the explanation of the slight effect of dibenzoyl peroxide concentration on the amount of insoluble polymer becomes more clear from Fig. 1 and 2. The most probable explanation of the small effect of various peroxide concentrations on the degree of crosslinking is the compensation of contradictory effects of the growing concentration of the initiator. The increase of dibenzoyl peroxide concentration leads to a decrease in molar mass of the formed polymer and thus probably also to a decrease of the average functionality of macromonomer. On the other hand, the increase of peroxide concentration raises the concentration of shorter copolymerizing macromonomer molecules. For shorter macromolecules we need then more crosslinks to build up a network than in the case of longer macromolecules

92 M. Lazár, L. Hrčková, A. Fiedlerová, E. Borsig built at lower peroxide concentration. The compensation is not necessarily complete, but the precision of the determination of the insoluble portion is not sufficient for finer separation of contradictory effects. As for the results of the changes in the limiting viscosity number of PDMA during its AIBN-initiated polymerization (line 4 in Fig. 3) we should say that a markedly greater scatter in the accuracy of measurements is associated with the solubility of AIBN that is much lower than that of the dibenzoyl peroxide in the monomer dodecyl methacrylate at 608C. In spite of the greater scatter of results, also in the initiation by the less reactive cyanopropyl radicals, the molar mass of PDMA increases with the increasing conversion, although no evident crosslinking of the formed polymer takes place. The increase in molar mass of the polymer during radical polymerization can be explained by the laws of the [18 20] gel (Trommsdorff) effect (ref. and numerous citations therein). The gel effect shows a more marked lowering of the termination rate constant compared to the propagation rate constant as a result of the decrease in the mobility of macroradicals with the increasing amount of soluble polymer in the polymerization system. However, in the case of dodecyl methacrylate the gel effect is very limited [3] as is also seen from the absence of the acceleration of polymerization as early as during the block polymerization of hexyl methacrylate. [21] The effect of temperature On the basis of our present state of knowledge about elementary reactions in the polymerization process, i. e., that the transfer reaction between propagating macroradicals and a monomer or the polymer has a higher activation energy than the propagation reaction in terms of the foregoing simplified idea, the polymerization might show a greater portion of insoluble polymer at higher temperature. For evaluating the temperature effect, the polymerization of dodecyl methacrylate initiated by 2,29-azoisobutyronitrile (0.3 wt.-%) was carried out at 50 8C (150 h) and at 908C (3 h). In contrast to the original estimate, we found a formation of crosslinked (insoluble) PDMA at the polymerization temperature of 508C but not at 908C. The molar mass of the macromonomer was shown to have an evidently greater effect on its polyfunctionality and, during its formation at lower temperature, the recombination of macroradicals is more probable than the extent of transfer reactions at higher temperature. Conclusions The finding that much more crosslinks were formed at free radical polymerization of DMA than it was expected from the decomposition of the peroxide used indicated a more complicated mechanism of crosslinking. Also an initiator with lower free radical activity caused crosslinking at the polymerization of DMA but with lower efficiency. A comparison of the effect of DBP and AIBN on the polymerization of DMA showed that not only primary oxy radicals but also propagating macroradicals play a role at the crosslinking reaction (at transfer reaction to monomer and polymer). Acknowledgement: This work was supported by the Project VEGA 2/5035/20, Slovak Republic. Received: April 26, 2000 Revised: August 28, 2000 [1] H. 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