Ring-Opening Oligomerization of Propylene Carbonate Initiated by the Bisphenol A/KHCO 3 System: A Matrix-Assisted Laser Desorption/ Ionization Mass Spectrometric Study of the Oligomers Formed Sa ´ ndor Ke ´ ki, Ja ´ nos To 1 ro 1 k, Gyo 1 rgy Dea ´ k, and Miklo ´ s Zsuga* Department of Applied Chemistry, University of Debrecen, H-4010 Debrecen, Hungary Received January 8, 2001 ABSTRACT: The decarboxylation upon oligomerization of propylene carbonate in the presence of the bisphenol A/KHCO 3 initiating system was studied by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Five different series of peaks appeared in the MALDI- TOF MS spectra of the oligomerization mixture. In addition to the chain-extension reaction with the propylene oxide units, the oligomers of all of the propylene oxide units and oligomers containing carbonate end groups condensed. The product of the oligomerization reaction was hydrolyzed under alkaline conditions. It was found that hydrolyzed oligomers do not carry carbonate groups. The results of the MS studies agreed well with those obtained by 1 H and 13 C NMR spectroscopies and size-exclusion chromatography (SEC). On the basis of the results of the present investigation, a mechanism for the oligomerization is also proposed. Introduction The ring-opening polymerization of the five-mem- bered cyclic carbonates (e.g., ethylene and propylene carbonate) occurs in the presence of Lewis acids or bases or transesterification catalysts at elevated temperatures (>100 °C), and the process is always accompained by decarboxylation to yield poly(ether carbonates) 1-8 (R ) H or CH 3 ), as shown below where x is the mole fraction of carbonate units. For ini- tiation by Lewis acids or transesterification catalysts, 1-6 x ) 0.4-0.5, and for initiation by base, x ) 0.1-0.3. 7,8 The oligomerization of ethylene (EC) and propylene carbonates (PCs) in the presence of bisphenol A (BPA) to yield oligo(ethers) is of great industrial importance. 9 This is because ethylene and propylene oxides can be replaced with their carbonates, thereby ensuring a more convenient synthesis of the corresponding oligo(ethers). However, our previous SEC studies on the oligomeriza- tion of PC in the presence of the BPA/base system showed that dimeric and trimeric products of the oligomers also formed. 10 This inspired us to perform a detailed matrix-assisted laser desorption/ionization time- of-flight mass spectrometry (MALDI-TOF MS) 11-14 study on the mixtures formed in the oligomerization reaction of PC in the presence of the BPA/KHCO 3 system. Identification and establishment of the microstructure of the oligomers formed should elucidate the mechanism of the oligomerization. Results and Discussion The reaction mixture was warmed continuously up to 160 °C within 2 h. Intense CO 2 evolution was observed. To obtain information on the CO 2 evolution during the reaction, the reaction volume was reduced to 100 mL. Figure 1 shows a typical amount of CO 2 evolved versus time curve, together with a reaction temperature versus time plot. The amount of CO 2 evolved versus time plot demon- strates that the intense CO 2 evolution starts when the reaction temperature reaches 140 °C. The total amount of CO 2 evolved level off at the final value of 100% conversion. A typical MALDI-TOF MS spectrum of the reaction mixture obtained after a 24-h reaction time is shown in Figure 2. Five different series of peaks can be identified. The legend and possible chemical structures of the series identified are summarized in Scheme 1. The first series of peaks (a) corresponds to the oligomers containing propylene oxide units with the BPA moiety. The mass numbers for series a can be expressed by where 228, 39, and 58 Da are the mass of the BPA, potassium (potassium cationized peaks), and propylene oxide units, respectively, and n is the number of the propylene oxide units. Series b corresponds to the oligomers containing BPA and propylene oxide units in which only one propylene carbonate unit is incorporated. The masses of series b components can be written as where 102 Da is the mass of a propylene carbonate unit. Series c is assigned as the peaks of the oligomers formed in the reaction of oligomers a with oligomers b via condensation by loss of H 2 O. The masses of series c components can be expressed by eq 3. Series d originates from the product of condensation of two b oligomers with each other. The masses of series M a ) 228 + 39 + 58n, i.e., M a ) 267 + 58n (1) M b ) 228 + 39 + 102 + 58n, i.e., M b ) 369 + 58n (2) M c ) M a + M b - 39 - 18, i.e., M c ) 579 + 58n (3) 6850 Macromolecules 2001, 34, 6850-6857 10.1021/ma0100302 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/29/2001