Dennis Langanke

Polylactones 54: ring-opening and ring-expansion polymerizations of ϵ-caprolactone initiated by germanium alkoxides

Et 3 GeOMe-initiated polymerizations of e-caprolactone were studied at six different temperatures, but even at 180 °C the conversion was below 50% after two days. Ge(OEt) 4 proved to be more reactive and a temperature of 120°C was found to allow for a nearly quantitative conversion after two days. All four ethoxy groups were active in the initiation process, but a broad molecular weight distribution was found and a low percentage (presumably < 1 mol%) of cyclic oligomers was formed. Ring-expansion polymerization initiated with a spirocyclic germanium alkoxide was feasible at 140 °C and yielded a molecular weight distribution quite different from that obtained with Ge(OEt) 4 . The monomer/initiator (M/I) ratio was varied, and it was found that the average degree of polymerization (DP) did not agree with the M/I ratio for M/Is above 200. Addition of 4-nitrobenzoyl chloride to the hot virgin reaction mixtures yielded poly(e-CL) chains having functional end groups but the degree of functionalization never exceeded 80%.

Polylactones, 52. Tin Carboxylates as Initiators ofɛ-Caprolactone

Eleven different tin carboxylates were used as initiators of the ring-opening polymerization of e-caprolactone. All polymerizations were conducted with 2 M solutions in chlorobenzene at 80°C and time/conversion curves were determined up to 96–99% conversion. The following structure-reactivity relationships were found. Noncyclic initiators, such as Bu2Sn(acetate)2 or Bu2Sn(benzoate)2 are more reactive than cyclic initiators such as Bu2Sn succinate and Bu2Sn phthalate, respectively. Electron withdrawing substituents enhance the reactivity. Bu2Sn(acetate)2 is more reactive than Bu3Sn acetate and Sn(II) bis(2-ethylhexanoate) is more reactive than Bu2Sn(2-ethyl-hexanoate)2. Traces of moisture to play the role of efficient co-initiators. This conclusion is supported by the following observations. The degrees of polymerization (DP) largely exceed the monomer/initiator (M/I) ratios. Furthermore, the DPs increase with the purity of e-CL suggesting that the monomer itself carries traces of moisture. The co-initiation with water and the incorporation of a few acyl endgroups were confirmed by MALDI-TOF mass spectroscopy. CH2OH endgroups were also found in all polyesters by 1H NMR spectroscopy, whereas only traces of acyl endgroups were detectable. Tetrabutyldistannoxane bisacetate is far more reactive than Bu2Sn bisacetate. Addition of an alcohol to a tin carboxylate allows a control of the molecular weight, and, as previously reported by several research groups, accelerates the entire polymerization process.

Macrocycles, 8. Multiblock copoly(ether-esters) of poly(THF) andε-caprolactone via macrocyclic polymerization

It has been demonstrated that the “polycondensation” of poly(tetrahydrofuran)-diols with dibutlytin dimethoxide in bulk yields tin-containing poly(THF) macrocycles and not linear polycondensates. These macrocycles were characterized by 1H and 119Sn NMR spectroscopy, by elemental analyses, viscosity measurements and mass spectroscopy. Furthermore, the stoichiometric insertion of γ-thiobutyrolactones was studied. The tin-containing macrocyclic poly(THF)-2 000 was used as macrocyclic initiator for the polymerization of e-caprolactone. The resulting macrocyclic block copoly(ether-esters) were reacted in situ with terephthaloyl chloride, succinyl chloride, or sebacoyl chloride. High temperatures proved to be advantageous for this polycondensation step. Number-average molecular weights (Mns) up to 50 000 were obtained with polydispersity indices in the range of 1.6–1.7. The resulting tin-free multiblock copoly(ether-ester)s have the properties of thermoplastic elastomers and show good film-forming properties.

Polylactones, 50. The Reactivity of Cyclic and Noncyclic Dibutyltin Bisalkoxides as Initiators in the Polymerization of Lactones

Dibutyltin bisethoxide and bis(2,2,2-trifluoroethoxide), the cyclic alkoxides derived from cis-1,4-butenediol, 1,4-butanediol, 1,8-octanediol and triethylene glycol as well as bis(4-nitrophenoxide), bis(4-meth-oxyphenoxide) and tetrabutylstannoxane bisacetae were used as initiators in the polymerization of e-caprolactone (e-CL) and β-BL). Most polymerizations were conducted at 60°C with 2 M solutions in chlorobenzene, and time-conversion curves were recorded. The less reactive initiators yielded sigmoidal time-conversion curves with autoacceleration during the first stage of polymerization. Dibutyltin bisethoxide and the cyclic 1,8-octane dioxide showed the highest reactivities. The less reactive initiators yielded degrees of polymerization (DPs) exceeding the monomer/initiator (M/I) ratios after short reaction times, but a longer reaction times, an equilibration between unreacted initiator and the poly(e-CL) chain took place. The slower polymerizing β-BL yielded DPs paralleling the M/I ratio even with less reactive initiators. The stannoxane and the dibutyltin bisphenoxides were considerably less reactive than the bisalkoxides were considerably less reactive than the bisalkoxides, and only traces of phenylester endgroups were detectable by 1 H NMR spectroscopy. MALDI-TOF mas spectrometry proved the formation of cyclic oligolactones with masses greather than 600 Da for reaction temperatures ≥60°C.

Macrocycles, 7. Cyclization of oligo- and poly(ethylene glycol)s with dibutyltin dimethoxide – a new approach to (super)macrocycles

When commercial oligo- or poly(ethylene glycol)s and dibutyltin dimethoxide were condensed in bulk with elimination of methanol, no linear polycondensates were obtained and tin-containing macrocycles were the only reaction products. In order to improve the hydrolytic stability, stoichiometric amounts of γ-thiobutyrolactone were inserted into the macrocyclic oligoethers, so that the Sn-O bonds were transformed into Sn-S bonds. The formation of monomeric (containing one Bu 2 Sn group) macrocycles was confirmed. The tin-containing macrocyclic polyethers were used as macrocyclic initiators for the ring-expansion polymerization of e-caprolactone. The resulting macrocyclic poly(ether-ester)s were isolated in the form of telechelic A-B-A block copolymers after precipitation into methanol.

Polylactones, 53. Formation of Cyclic Polyesters in the Combined Ring-Expansion Polymerization/Ring-Opening Polycondensation of Lactones

Ring-expansion polymerizations of β-D,L- butyrolactone (β-BL) or e-caprolactone (e-CL) were initiated with 2,2-dibutyl-2-stanna-1,3-dioxepane (DSDOP) and the monomer-initiator ratio (M/I) was varied. The resulting tincontaining polylactones were polycondensed in situ either with succinyl chloride (in the case of β-BL) or with suberoyl chlorid (for e-CL). The reaction conditions were optimized towards high molecular weights by the addition of bipyridine. The isolated tinfree polylactones were characterized by MALDI-TOF mass spectrometry. In the best spectra cyclic poly(e- caprolactone)s were detected up to masses around 10 600 Da and cyclic poly(β-butyrolactone)s up to masses around 17 000 Da. In addition to the cyclic polyesters linear chains having alcoholic OH and/or CO 2 H group were found. These results suggest that the chain growth is limited by cyclization and by incomplete conversion of the functional groups.

Ring-Closing Polycondensations

The role of cyclization in polycondensations is discussed for two different scenarios: thermodynamically-controlled polycondensation (TCPs) on the one hand and kinetically-controlled polycondensations (KCPs) on the other. The classical Carothers–Flory theory of step-growth polymerization does not include cyclization reactions. However, TCPs involve the formation of cycles via ‘back-biting degradation’, and when the ring–chain equilibrium is on the side of the cycles the main reaction products of the TCP will be cyclic oligomers. Two groups of examples are discussed: polycondensations of salicyclic acid derivatives (e.g. aspirin) and polycondensations of dibutyltin derivatives with long α-, ω-diols or dicarboxylic acids. Furthermore, various kinetically-controlled syntheses of polyesters and polyamides were studied and carefully optimized in the direction of high molecular weights. High fractions of cyclic oligomers and polymers were found by MALDI-TOF mass spectrometry, and their fractions increased with optimization of the process for molecular weight. These results disagree with the Carothers–Flory theory but agree with the theoretical background of the Ruggli–Ziegler dilution method (RZDM). When poly(ether-sulfone)s were prepared from 4,4′-difluorodiphenylsulfone and silylated bisphenol-A two different scenarios were found. With CsF as catalyst at a temperature of more than 145°C cyclic oligoethers were formed under thermodynamic control. When the polycondensation was promoted with K2CO3 in N-methylpyrolidone at ≤145°C the formation of cyclic oligoethers and polyethers occurred under kinetic control. A new mathematical formula is presented correlating the average degree of polymerization with the conversion and taking into account the competition between cyclization and propagation.

New Polymer Syntheses, 108. Controlled Degradation and In-Situ Functionalization of Polycarbonates by Means of Bu2SnO

It was found that diphenyl carbonate reacts with Bu 2 SnO above 140°C yielding CO 2 and Bu 2 Sn(OPh) 2 . This insertion reaction was also successfully applied to a polycarbonate prepared from polytetra-hydrofuran diol-1000 and Bu 2 SnO containing macrocyclic poly THF dioxides. Bu 2 SnO reacted analogously with poly(bisphenol A) carbonates yielding tin-containing rapidly crystallizing cyclic oligocarbonates. Their treatment with 1,2-ethanedithiol gave linear oligocarbonates having two OH end groups. Bu 2 Sn-containing macrocyclic oligocarbonates reacted in-situ with several carboxylic acid chlorides, forming telechelic oligocarbonates with functional ester end groups. Bu 2 Sn-containing macro-cycles also reacted in-situ with terephthaloyl chlorides yielding a poly(ester carbonate).

NEW POLYMER SYNTHESES 106. POLYCARBONATES BY RING-OPENING POLYCONDENSATIONS OF TIN-CONTAINING MACROCYCLES WITH BISCHLOROFORMATES

2,2-Dibutyl-2-stanna-1,3-dioxepane (DSDOP) was polycon-densed with bisphenol-A bischloroformate (BABC) in bulk. Regardless of the reaction temperature and time only number average molecular weights (Mns) around 8,000–11,000 were obtained. 13C NMR spectra proved that the isolated copoly-carbonates possessed a random sequence. MALDI-TOF spectra of the crude and of the fractionated samples revealed that all samples contained macrocyclic polycarbonates. DSDOP was also used as initiator for the macrocyclic polymerization of-cap-rolactone. The resulting cyclic polylactones were then polycon-densed with BABC in situ. Mns in the range of 25,000–40,000 and Mws in the range of 40,000–65,000 were found based on polystyrene calibrated GPC measurements. Similar results were obtained when 1,6-hexanediol bischloroformate was used as electrophilic reaction partner in the polycondensation step.

Biodegradable Block Copolymers, Star-Shaped Polymers, and Networks Via Ring-Expansion Polymerization

Over the past decades biodegradable (or more precisely “resorbable”) polyesters have found rapidly increasing interest of scientists and chemical companies. Among the numerous potential applications which are under investigations certain medical and pharmaceutical applications have already proven their usefulness, for instance: devices for controlled drug release,1-4 resorbable medical sutures,4-6 resorbable and transparent wound dressings,7.8 tissue separating film for surgery,9 resorbable rods, pins, and screws for the fixation of bone fractures,10 scaffolds for surgery,4 and cell containers for osteosyntheses.4 This situation has stimulated intensive research activities in the field of synthesis and characterization of biodegradable polyesters.