Kemija u Industriji (May 2006)
Polimerizacija, toplinska stabilnost i mehanizam razgradnje kopolimera (met)akril-dicikloheksiluree i (met)akril-diizopropiluree sa stirenom i α-metilstirenom (Polymerization, Thermal Stability and Degradation Mechanism of (Meth)acryl-Dicyclohexylurea and (Meth)acryl-Diisopropylurea Copolymers with Styrene and α-Methylstyrene)
Abstract
This paper describes the polymerization of N-acryl-N,N'-dicyclohexylurea (A-DCU), N-methacryl- N,N'-dicyclohexylurea (MA-DCU) and N-methacryl-N,N'-diisopropylurea (MA-DiPrU) monomers with styrene (St) and α-methylstyrene (α-MeSt), thermal stability and degradation mechanism of prepared copolymers. Free-radical initiated polymerization was performed to low conversion by using dibenzoyl peroxyde (Bz2O2) in butanone at 70 °C under nitrogen stream. It was found that the pendant group in (meth)acrylic monomers have high influence to the polymerization as well as to the copolymer properties. A-DCU readily homopolymerized and copolymerized with St and r1,A-DCU = 0.72 and r2,α-MeSt= 0.07, while MA-DCU does not homopolymerized or copolymerized with α-MeSt under the same conditions, but copolymerized with St to randomly composed copolymers after a long heating of comonomers. Copolymers A-DCU with α-MeSt prepared under different monomer-to monomer-ratios in the feed have random composition with an azeotropic point at ratio of 0.75 (A-DCU) to 0.25 (St). The initial rate of copolymerization indicates that the rate increases almost linearly with the increase of ratio of A-DCU in the comonomer feed. Reactivity ratios determined by the Kelen-Tüdös method are: r1,A-DCU = 0.72 and r2,α-MeSt = 0.07. Molar mass of copolymers increased from 8.5 to 30 (kg mol-1) when mole ratio of A-DCU to α-MeStin the feed increased from 0.1 to 0.9. Poly(A-DCU) and copolymers with α-MeSt decomposed by two-step mechanism. Under TGA (nitrogen,10 °C min-1) conditions in the first step between 180 °C and 250 °C a quantitative yield of cyclohexylisocyanate (C6H11NCO) separated by a decomposition of dicyclohexylurea (DCU). The thermally stable residue represented poly(acryl-cyclohexylamide), poly(A-CHA), and copolymer with α-MeSt, poly(A-CHA-co-α-MeSt). Glass transition temperature (Tg) of poly(A-DCU) was at 184 °C and Tg of residue, poly(A-CHA), was at 161 °C. Tg's of the copolymers are higher for the copolymer with higher A-DCU content. Tg's of residue are increased also when the content of A-CHA in copolymer increased. Copolymers of A-DCU with St have a random composition with an azeotropic point at a ratio of 0.73 (A-DCU) to 0.27 (St). The rate of copolymerization of A-DCU with St increases by the increase of A-DCU in the feed. The reactivity ratio are: r1,A-DCU = 0.80 and r2, St = 0.50. Molar mass of copolymer, prepared at equimolar ratio of monomers in the feed, is Mw = 78.6 kg/mol-1. These copolymers decompose in TGA conditions by two-step mechanism, which correspond to the mechanism explained for the copolymers of A-DCU with α-MeSt. MA-DCU copolymerized with St to randomly composed copolymers. The reactivity ratio determined by the KT method are: r1, MA-DCU = 0.18 and r2, St = 4.84. These values indicate that MA-DCU favors cross-propagation over homopolymerization, while St favors homopolymerization as opposite to cross-propagation. It also shows, that since St is more reactive than MA-DCU, copolymers contain a higher proportion of St units. It was found, that the rate of copolymerization of MA-DCU with St is slower than the rate of copolymerization of A-DCU with St, and that the rate of copolymerization decreases by increasing the amount of MA-DCU in the feed. Molar mass of copolymer prepared at equimolar ratios of comonomers in the feed is Mw=12 kg mol-1. Thermal properties of poly(MA-DCU-co-St) are similar to those previously described in the decomposition of poly[A-DCU-co-St(α-MeSt)]. The results have also shown quite a big influence of DiPrU group in MA-iPrU to the polymerization with St and α -MeSt. Namely, MA-DiPrU does not homopolymerized or copolymerized with α -MeSt, but can polymerize with St only after a long heating of comonomers at 70 °C. The copolymerization of MA-DiPrU with St is a statistical reaction in which regardless of monomer-tomonomer ratios in the feed, an excess of St was in the copolymer. The reactivity ratios (KT method) are: r1, MA-DiPrU = 0.39 and r2, St = 1.03. The obtained data indicate a monomer tendency to alternating structure. The initial rate of copolymerization decreases with increasing the content of MA-DiPrU monomer in the feed. Molar mass of all copolymers are approximately of the same values, Mw = 12 - 8 kg mol-1 and Mn = 8.7 - 6.2 kg mol-1. Thermal behavior of those copolymers correspond to the behavior of acrylic and methacrylic polymers containing DCU as pendant group. Namely, all copolymers decompose under TGA conditions by a two-step mechanism. In the first step between 180 °C and 250 °C isopropylisocyanate (C3H7NCO) separates by degradation of diisopropylurea (DiPrU) in the side chain. The thermally stable residue represents the copolymer of methacryl-isopropylamide with St, which decompose by one-step mechanism between 280 °C and 450 °C.
