Mass Spectrometric Analysis for High Molecular Weight Synthetic Polymers Using Ultrasonic Degradation and the Mechanism of Degradation

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Abstract
We have investigated ultrasonic degradations of poly(ethylene oxide) (PEG) and poly(methyl methacrylate) (PMMA) in aqueous media by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The ultrasonic degradation of polymers was monitored as a function of ultrasonication duration to examine the structural details of ultrasonic degradation polymers. PEG solution ultrasonication produced five types of oligomers (M̄ ∼1000 Da) with different end groups, irrespective of the initial average molecular masses (M̄ = 2, 6, 20, and 2000 kDa). Several degradation pathways with free radical reactions have been suggested to explain these degradation products: the ultrasonic degradation of PEG is initiated by breaking of the C−O bond in the PEG chain, generating polymeric radicals with two terminal groups, i.e., X•(∼CH2CH2•) and Y•(∼CH2CH2O•), followed by termination with extraction or release of a hydrogen atom. However, PMMA (M̄ = 1630 Da) ultrasonication generated only one type of degradation oligomer, which has a hydrogen group at both ends, the same as that of the original oligomer. It has been suggested that the presence of the radical terminal groups X•(∼CH2•) and Y•(∼(CH3)CCOO(CH3)C•) is due to selective C−C bond breaking in the chain during the ultrasonic degradation of PMMA. The MALDI-TOFMS combined with the ultrasonic degradation technique (UD/MALDI-TOFMS) developed in this study could be extended to the analysis of synthetic polymer structures with high molecular weights.