Structural characterization of poly(ε-caprolactone) and poly(ε-caprolactone-b-isobutylene-b-ε-caprolactone) block copolymers by MALDI-TOF mass spectrometry

Poly(epsilon -caprolactone) (PCI) and PCl-polyisobutylene-PCl (PCl-PIB-PCl) block copolymers were synthesized in anhydrous toluene by in situ conversion of 2- methyl-1-propanol (2MIP) and alpha,omega -dihydroxy PIB, respectively, to the corresponding aluminum alkoxide by reaction with a stoichiometric amount of triethylaluminum (TEA) followed by the addition of e-caprolactone. Structural characterization of 2MIP-initiated PCl by gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) demonstrated the presence of cyclic oligomers, which are produced by intramolecular transesterification reactions that become significant at high monomer conversions. A minor fraction of chains bearing carboxylic acid termini was also observed in the MALDI-TOF mass spectrum; however, carboxylic acid chain ends could not be detected by C-13 NMR analysis, Thus, the likely origin of the carboxylic acid termini is fragmentation of the initiator residue from the chain end during MALDI-TOF analysis. For PCl-PIB-PCl block copolymers, two different alpha,omega -telechelic PIE diols were used as macroinitiators, one derived from allyl and one from isopropenyl terminated PIB. Terminal olefins were converted to primary alcohols via regioselective hydroboration followed by alkaline hydrogen peroxide oxidation. After reaction with epsilon -caprolactone, formation of a block copolymer was evidenced by a shift of the polymer peak to lower elution volume in GPC analysis. Block copolymers possessed molecular weight distributions less than or equal to1.4, and molecular weights of the PCl blocks calculated from GPC were in excellent agreement with those found using MALDI-TOF MS. Structural analysis indicated that the PCl end blocks were severed from the crude block copolymer during MS analysis, for both allyl- and isopropenyl-derived materials. For allyl-derived materials the PCl blocks were found to predominantly carry a C-2 residue at the point of detachment of the PIB block; however, the isopropenyl-derived block copolymers showed a complex mixture of different residues suggesting a complex fragmentation mechanism during loss of the PIB block.