Structural characterization of poly(epsilon-caprolactone) and poly(epsilon-caprolactone-b-isobutylene-b-epsilon-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 conversi on of 2- methyl-1-propanol (2MIP) and alpha,omega -dihydroxy PIB, respectiv ely, to the corresponding aluminum alkoxide by reaction with a stoichiometr ic amount of triethylaluminum (TEA) followed by the addition of e-caprolact one. Structural characterization of 2MIP-initiated PCl by gel permeation ch romatography (GPC) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) demonstrated the presence of cycl ic oligomers, which are produced by intramolecular transesterification reac tions that become significant at high monomer conversions. A minor fraction of chains bearing carboxylic acid termini was also observed in the MALDI-T OF mass spectrum; however, carboxylic acid chain ends could not be detected by C-13 NMR analysis, Thus, the likely origin of the carboxylic acid termi ni is fragmentation of the initiator residue from the chain end during MALD I-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 t o primary alcohols via regioselective hydroboration followed by alkaline hy drogen peroxide oxidation. After reaction with epsilon -caprolactone, forma tion of a block copolymer was evidenced by a shift of the polymer peak to l ower elution volume in GPC analysis. Block copolymers possessed molecular w eight 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 al lyl- and isopropenyl-derived materials. For allyl-derived materials the PCl blocks were found to predominantly carry a C-2 residue at the point of det achment of the PIB block; however, the isopropenyl-derived block copolymers showed a complex mixture of different residues suggesting a complex fragme ntation mechanism during loss of the PIB block.