Persistent Radicals and Transfer Reactions in the Postpolymerization of Methyl Methacrylate
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- 作者：Nuria Garcí ; a ; Pilar Tiemblo ; Laura Hermosilla ; Paloma Calle ; Carlos Sieiro ; Julio Guzmá ; n
- 刊名：Macromolecules
- 出版年：2007
- 出版时间：November 13, 2007
- 年：2007
- 卷：40
- 期：23
- 页码：8168 - 8177
- 全文大小：152K
- 年卷期：v.40,no.23(November 13, 2007)
- ISSN：1520-5835
文摘
The posteffects in the polymerization of methyl methacrylate at low monomer conversions arestudied. The experimental two-step method consists of a short initial photoinduced polymerization period andsubsequent register of the monomer concentration evolution in the dark. After switching off the light, the radicalconcentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerizationreaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units).By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values ofthe termination rate coefficients, <kt>, scarcely exceeding 10 000 L mol-1 s-1, and radical concentrations at thebeginning of the postpolymerization about 10-8 mol L-1 are determined. The reliability of these fitting results ischecked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecularweight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomerand to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of theinstantaneous chain length distribution. The extremely low <kt> values obtained by the analysis suggest that thepower-law exponents which describe the chain-length dependency of kt should be higher than the unity for longradicals even in dilute solution (i.e., at low monomer conversions).