About formation of cycles in Sn(II) octanoate‐catalyzed polymerizations of lactides

Abstract

At first, formation of cycles in commercial poly(Llactide)s is discussed and compared with benzyl alcoholinitiated polymerizations performed in this work. This comparison was extended to polymerizations initiated with 4cyanophenol and pentafluorothiophenol which yielded cyclic polylactides via end-biting. The initiator/catalyst ratio and the acidity of the initiator were found to be decisive for the extent of cyclization. Further polymerizations of L-lactide were performed with various diphenols as initiators/co-catalysts. With most diphenols, cyclic polylactides were the main reaction products. Yet, only catechols yielded even-numbered cycles as main reaction products, a result which proves that their combination with SnOct 2 catalyzed a ring-expansion polymerization (REP). The influence of temperature, time, co-catalyst, and catalyst concentrations was studied. Four different transesterification reactions yielding cycles were identified. For the cyclic poly(L-lactide)s weight average molecular weights (M w 's) up to 120,000 were obtained, but 1 H NMR end group analyses indicated that the extent of cyclization was slightly below 100%. The influence of various parameters like structure of initiator and catalyst and temperature on the formation of cyclic poly(Llactide)s has been investigated. Depending on the chosen conditions, the course of the polymerization can be varied from a process yielding exclusively linear polylactides to mainly cyclic polylactides. Three different reaction pathways for cyclization reactions have been identified.

Cyclic poly(lactide)s via the ROPPOC method catalyzed by alkyl‐ or aryltin chlorides

“…Due to the high reactivity of Ph 2 SnCl 2 (relative to Bu 3 SnCl), it seemed to be reasonable to perform experiments at temperatures below 160 C to find out to which extent side reactions and equilibration reactions can be suppressed and the formation of cycles favored. In agreement with previous results, [28][29][30] it was expected that theses side reactions possess higher energies of activation. The experiments listed in Table 5 were conducted with L-lactide at 140, 120 and 102 C ( Table 5).…”

Synthesis of cyclic polymers and flaws of the Jacobson–Stockmayer theory

“…The message of eqn (1)-(3) is in perfect agreement with the mathematical treatment of irreversible polycondensations as presented by Stepto et al or Gordon and Temple (Fig. 2), [13][14][15][16] and with experimental results of this work and previous publications, [25][26][27] but it is in total contradiction to the Jacobson-Stockmayer calculation of a critical IMC above which total cyclization should be impossible.…”

“…Experimental evidence for this course of the ROPPOC approach was obtained when the in situ generated SnSPF catalyst (Scheme 2) was used for polymerizations in bulk at 160°C. 27 At a Lac/Cat ratio of 2000/1 and a reaction time of 1 h even-numbered linear chains having thioester (SC 6 F 5 ) end groups were the largely predominant species (Fig. 3A).…”

“…In another paper experiments with BuSnOBn, BuSnSBn and BuSnSPF were reported. 27 The BuSnOBn-initiated polymerization is an example of a normal ROP where the end group is less reactive than the ester groups in the backbone, so that etecyclization is absent or negligible. Correspondingly, the mass spectrum ( Fig.…”

“…At that time when the first experiments with BuSnSBn and BuSnSPF were performed, the authors were unable to perform MHS measurements. 27 Therefore, three more experiments with BuSnSPF were now conducted for his work and their results summarized in Table 3 (no. [2][3][4].…”