Syntheses and polymerizations of α‐amino acid N‐carboxyanhydrides (NCAs) were reported for the first time by Hermann Leuchs in 1906. Since that time, these cyclic and highly reactive amino acid ...derivatives were used for stepwise peptide syntheses but mainly for the formation of polypeptides by ring‐opening polymerizations. This review summarizes the literature after 1985 and reports on new aspects of the polymerization processes, such as the formation of cyclic polypeptides or novel organometal catalysts. Polypeptides with various architectures, such as diblock, triblock, and multiblock sequences, and star‐shaped or dendritic structures are also mentioned. Furthermore, lyotropic and thermotropic liquid‐crystalline polypeptides will be discussed and the role of polypeptides as drugs or drug carriers are reviewed. Finally, the hypothetical role of NCAs in molecular evolution on the prebiotic Earth is discussed.
100 years young: In 2006 the chemistry of α‐amino acid N‐carboxyanhydrides (NCAs) marks a history of 100 years. The NCAs were used for syntheses of oligo‐ and polypeptides in numerous ways, and recently have played an important role in the hypothesis of molecular evolution. Peptide block copolymers, liquid‐crystalline polypeptides, and the application of polypeptides as drugs or drug carriers are also discussed.
This article reviews the usefulness of tin(
ii
) and tin(
iv
) salts and compounds as catalysts for the polymerization of lactides. The text is subdivided into nine parts mainly reflecting different ...polymerization strategies, such as ring-opening polymerization (ROP), ring-expansion polymerization (REP), ROP combined with simultaneous polycondensation (ROPPOC), various catalysts with unknown polymerization mechanisms, and polycondensation of lactic acid. Since the toxicity of tin salts and compounds is a matter of concern and frequently mentioned in numerous publications, the first section deals with facts instead of myths about the toxicity of tin salts and compounds.
Reaction mechanisms and synthetic methods used for the preparation of homo- and copolylactides based on tin(
ii
) and tin(
iv
) catalysts are reviewed.
Eco-friendly polymers synthesized using bismuth catalysts are explored. Such biodegradable and biocompatible compounds are catalyzed from cyclic esters.
A new concept called ring‐opening polymerization–polycondensation (ROPPOC) is presented and discussed. This synthetic strategy is based on the intermediate formation of chains having two end groups ...that can react with each other. The ROPPOC syntheses are subdivided into three groups according to the nature of the chain ends: two ionic end groups, one ionic and one covalent chain end, and a combination of two reactive covalent end groups may be involved, depending on the catalyst. The usefulness for the preparation of cyclic polymers is discussed with a review of numerous previously published examples. These examples concern the following classes of cyclic polymers: polypeptides, polyamides, and polyesters, including polycarbonates and cyclic polysiloxanes. It is demonstrated that the results of certain ROPPOC syntheses are in contradiction to the Jacobson–Stockmayer theory. Finally, the usefulness of ROPPOCs for the detection of polydisperse catenanes is discussed.
Ring‐opening polymerization combined with simultaneous polycondensation (ROPPOC) is a new group of polymerizations that enables access to various classes of cyclic polymers such as polypeptides, biodegradable polyesters, polyamides, and polysiloxanes.
l -Lactide (LA) was polymerized with neat tin( ii ) 2-ethylhexanoate (SnOct 2 ) in toluene at 115 °C at low concentration with variation of the LA/Cat ratio. Cyclic polylactides (cPLAs) with number ...average molecular weights ( M n ) between 7000 and 17 000 were obtained. MALDI-TOF mass spectrometry also revealed the formation of a few percent of linear chains. Crystalline cPLAs with M n around 9000 and 14 000 were annealed at 140 °C in the presence of ScOct 2 or dibutyl-2-stanna-1,3-dithiolane (DSTL). Simultaneously, crystallites of extended linear chains and crystallites of extended cycles were formed regardless of the catalyst, indicating that transesterification reaction proceeded different for linear chains and for cycles, governed by thermodynamic control. The formation of extended chain crystallites with low dispersity indicates the existence of symproportionation of short and long chains. A complementary experiment was carried out with a PLA ethyl ester composed mainly of linear chains with a small fraction of cycles.
l
-Lactide was polymerized in bulk at 120, 140, 160 and 180 °C with neat tin(
ii
) 2-ethylhexanoate (SnOct
2
) as the catalyst. At 180 °C the Lac/Cat ratio was varied from 25/1 up to 8000/1 and at ...160 °C from 25/1 up to 6000/1. The vast majority of the resulting polylactides consist of cycles in combination with a small fraction of linear chains having one octanoate and one COOH end group. The linear chains almost vanished at high Lac/Cat ratios, as evidenced by MALDI-TOF mass spectrometry and measurements of intrinsic viscosities and d
n
/d
c
values. At Lac/Cat ratios <1000/1 the number average molar masses (
M
n
) are far higher than expected for stoichiometic initiation, and above 400/1 the molar masses vary relatively little with the Lac/Cat ratio. At 180 °C slight discoloration even at short times and degradation of the molar masses were observed, but at 160 °C or below colorless products with weight average molar masses (
M
w
) up to 310 000 g mol
−1
were obtained. The formation of high molar mass cyclic polylactides is explained by a ROPPOC (Ring-Opening Polymerizatiom with simultaneous Polycondensation) mechanism with intermediate formation of linear chains having one Sn-O-CH end group and one mixed anhydride end group. Additional experiments with tin(
ii
)acetate as the catalyst confirm this interpretation. These findings together with the detection of several transesterification mechanisms confirm the previous critique of the Jacobson-Stockmayer theory.
l
-Lactide was polymerized in bulk at 120, 140, 160 and 180 °C with neat tin(
ii
) 2-ethylhexanoate (SnOct
2
) as the catalyst and a cyclic topology was detected.
l
-Lactide was polymerized with tin(
ii
)acetate, tin(
ii
)2-ethyl hexanoate, diphenyltin dichloride and dibutyltin bis(pentafluorophenoxide) at 130 °C in bulk. When an alcohol was added as ...initiator, linear chains free of cycles were formed having a degree of polymerization (DP) according to the lactide/initiator (LA/In) ratio. Analogous polymerizations in the absence of an initiator yielded high molar mass cyclic polylactides. Quite similar results were obtained when -caprolactone was polymerized with or without initiator. Several transesterification experiments were conducted at 130 °C, either with polylactide or poly( -caprolactone) indicating that several transesterification mechanisms are operating under conditions that do not include formation of cycles by back-biting. Furthermore, reversible polycondensations (revPOCs) with low or moderate conversions were found that did not involve any kind of cyclization. Therefore, these results demonstrate the existence of revPOCs, which do neither obey the theory of irreversible polycondensation as defined by Flory nor the hypothesis of revPOCs as defined by Jacobson and Stockmayer. A new concept encompassing any kind of revPOCs is formulated in the form of a "polycondensation triangle".
Reversible polycondensations exist involving intermolecular transesterification in the absence of back-biting, which may be understood as part of a so-called 'polycondensation triangle' based on the definition of three polycondensation classes.
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•A new synthetic method for a convenient preparation of novel tin(II) catalysts.•Two of the new catalysts are extraordinarily efficient for REPs of l-lactide.•Yield colorless, ...optically pure, cyclic poly(l-lactide)s having Mw’s up to 250,000 g mol−1.
Five new cyclic catalysts were prepared by a new synthetic method from tin(II)-2-ethyhexanoate and silylated catechols, silylated 2,2′dihydroxybiphenyl or silylated 1,1′-bisnaphthol. These catalysts were compared with regard to their usefulness as catalysts for the ring expansion polymerization (REP) of l-lactide in bulk at 160 °C, and with two different tin(IV) derivatives of 1,1′bisnaphthol. Best results were obtained using seven-membered cyclic tin(II)bisphenoxides, which yielded colorless cyclic poly(l-lactide)s free of racemization with weight average molecular weights (Mw) up to 305,000 g mol−1. Furthermore, these catalysts were active even at a lactide/catalyst ratio of 20,000/1. Our new results were superior to those obtained from all other previously published catalysts yielding cyclic poly(l-lactide). The seven-membered cycles also proved to be more active than tin(II) 2-ethylhexanoate with and without the addition of alcohol.
The purpose of this work is to demonstrate the suitability of a new class of cyclic tin catalysts for a potential racemization-free, technical production of high molar mass cyclic pol(l-lactide). ...Furthermore, an experimental evidence for the assumed ring-expansion polymerization is elaborated.
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•New cyclic tin catalysts allow for preparation of high molar mass cyclic poly(l-lactide)s.•Experimental evidence for ring-expansion polymerization has been elaborated.•The new catalysts allow for easy upscaling of the polymerization process.
Two new catalysts (SnNa and SnBi) were prepared from dibutyltin oxide and 2,2′-dihydroxybiphenyl or 2,2′dihydroxy(1,1′-binaphtyl). These catalysts enabled rapid polymerizations of l-lactide at 160 or 180 °C in bulk, whereby almost exclusively cyclic polylactides were formed. These polymerizations were free of racemization and yielded pol(l-lactide)s having weight average molecular weights (Mw's) up to 140 000 g mol−1. The Mw's varied little with the Lac/Cat ratio as expected for a ring expansion polymerization (REP). Polymerizations performed in bulk at 140, 120 and 102 °C yielded cyclic polylactides with lower molecular weights. At 102 °C a strong predominance of even-numbered cycles was found with SnNa as catalyst. SnNa can also catalyze alcohol-initiated ROPs yielding linear poly(l-lactide) free of cyclics.
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•The established mechanism of SnOct2-catalyzed and alcohol-initiated ROPs of L-lactide needs modification.•The structure of the initiator has strong influence on transesterification ...in the melt and thus on the molecular weight distribution.•The end groups formed by the initiators also have an influence on transesterification reactions in the solid state.•These transesterification reactions involve the mobile and immobile amorphous phases, but not the interior of the crystallites.
SnOct2 (Sn(II) 2-ethylhexanoate) catalyzed ROPs of L-lactide were performed in bulk with eight different alcohols as initiators. The time was varied between 1 h and 24 h for all initiators. For two initiators the temperature was also lowered to 115 °C. Even-numbered chains were predominantly formed in all polymerizations at short times, but the rate of transesterification (e.g. even/odd equilibration) and the molecular weight distribution were found to depend significantly on the nature of the initiator. Observed transesterification reactions also continued in solid poly(L-lactide), and with the most active initiator, almost total equilibration was achieved even at 130 °C. This means that all chains including those of the crystallites were involved in transesterification reactions proceeding across the flat surfaces of the crystallites. The more or less equilibrated crystalline polylactides were characterized by DSC and SAXS measurements with regard to their melting temperature (Tm), crystallinity and crystal thickness.
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