Mechanistic studies on cationic ring-opening polymerisation of cyclodextrin derivatives using various Lewis acids

In order to investigate the potential of cyclodextrins for the preparation of block-like substituted polysaccharides, we submitted mixtures of heptakis[2,3,6-tri-O-methyl]-β-cyclodextrin (Me₂₁-β-CD, 1) and heptakis[2,3,6-tri-O-methyl-d ₃]-β-cyclodextrin ((Me-d ₃)₂₁-β-CD, 2) to cationic ring-opening polymerisation (CROP). Reactions were performed with BF₃·OEt₂, methyl triflate (MeOTf), and Et₃OSbCl₆. Products were compared with respect to their degree of polymerisation (DP) and the average block length (BL). Highest DP was observed with BF₃·OEt₂, while Et₃OSbCl₆ was the most active initiator. Average block length decreased from 14 in the early stage of product formation to about 2 due to competing chain transfer reaction. ¹H NMR spectroscopy, GLC, GLC–MS, ESI-MS and MALDI-TOF-MS were applied for detailed investigations of side reactions. During incubation with BF₃·OEt₂, a stereroisomeric β-CD with one β-glucosidic linkage (Me₂₁-β-CD_6α1β, 3a (Me-d ₃)₂₁-β-CD_{6α 1β}, 3b) is formed as an intermediate, while linear Me₂₁- and (Me-d ₃)₂₁-maltoheptaose (4a/b) was detected in the early stage of the reaction promoted by MeOTf. In the case of Et₃OSbCl₆, both intermediates (3a/b, 4a/b) can be observed during the lag phase of polymerisation, but to a very low degree. End group analysis by GLC reveals that some alkyl exchange occurs at position 3 and 6 in the presence of Et₃OSbCl₆, and that polymerisation is also initiated by protons. Copolymerisation of heptakis[2,3,6-tri-O-benzyl]-β-cyclodextrin (Bn₂₁-β-CD, 5) and Me₂₁-β-CD (1) and subsequent debenzylation yielded a polymer of only 1,4-glcp-Me₃- and 1,4-glcp-residues. Reactivity of Bn₂₁-β-CD was significantly lower than of Me₂₁-β-CD, resulting in higher average block length of 1,4-glcp-Me₃-units.     

Photoinitiated cationic polymerisation of multifunctional systems

Different types of tridimensional polymer networks have been synthetised by photoinitiated cationic polymerisation of vinyl ether and epoxy-functionalised oligomers and polymers. The polymerisation kinetics was followed by real-time infrared (RTIR) spectroscopy, a technique that records directly conversion versus time profiles in a timescale as short as 1 s. The addition of a diacrylate monomer was shown to accelerate the ring-opening polymerisation of epoxidized polyisoprene, with formation of interpenetrating polymer networks having well contrasted properties. A dual polymer network has been generated by photocrosslinking of a polyisoprene functionalised with both epoxy and acrylate groups.     

Kinetic features of cationic oligomerization of epoxides

The kinetic features of the cationic oligormerization of epoxides of various structures influenced by Lewis acids (BF₃OEt₂, SnCl₄) were investigated. It was shown that the systems studied could he divided into three groups, based on the nature of conversion-time kinetic curves of oligomerization: (1) (Epoxypropane-BF₃OEt₂, 3,4-epoxybutene-1-BF₃OEt₂) oligomerization stops before the monomer is exhausted completely. (2) (1-Chloro-2,3-epoxypropane-BF₃OEt₂ or SnCl₄, 2-methyl-2,3-epoxybutane-BF₃OEt₂) oligomerization can be carried out almost up to complete conversion of the monomer. The reaction rate, however, decreases more rapidly than that expected from the kinetics of monomer consumption. (3) (1-tert-Butylperoxy-2,3-epoxypropane-SnCI₄) oligomerization kinetics are described by a simple exponential function with regard to monomer concentration. Determination of the propagating species concentration in the systems studied suggests that the initiation of cationic oligornerization of epoxides with Lewis acids is an instantaneous process and the discrepancy observed is a result of the different nature of kinetics of the active site destruction. In the first group of systems the rapid destruction of the propagating species prevents the oligomerization to complete conversion. In the second group a somewhat lower rate of decrease in concentration of the propagating species with time is explained by their regeneration during the process. In the third system, which is a unique case, the concentration of the propagating species remains constant with time. It is assumed that these features are the result of the stability or pseudostability of the active sites which depends on the nature of the epoxide and initiator.     

New reactivity of the uranyl ion: ring opening polymerisation of epoxides

The uranyl aryloxide, [UO(2)(OAr)(2)(THF)(2)], and uranyl chloride, [UO(2)Cl(2)(THF)(3)] or [UO(2)Cl(2)(THF)(2)](2) act as pre-catalysts for the ring opening polymerization of propylene oxide and cyclohexene oxide. Coordination of the monomers has been investigated using (1)H EXSY spectroscopy and kinetic and thermodynamic parameters reported. NMR analyses of the polymers suggest a bimetallic mechanism for the polymerization.     

Living cationic polymerisation of styrene in an ionic liquid

For the first time, living cationic polymerisation of styrene has been carried out in room temperature ionic liquids under mild reaction conditions and using mild acid catalysts (e.g. organoborate acids) to obtain polymers of narrow polydispersity.     

Synthesis of branched polystyrenes via cationic polymerisation

Branched polystyrenes have been prepared rapidly (within 15 min) and in good yields (40-99%) in dichloromethane solution at 0 °C via cationic copolymerisation of styrene (St) with divinylbenzene (DVB) using SnCl₄ as the initiator. All reaction components were deliberately used as supplied to evaluate whether such a simple approach could provide a facile synthesis of branched polymers and this has proved to be the case. The only additional experimental precaution was to avoid condensation of atmospheric moisture during the reactions. No additional chain regulating species was required to avoid crosslinking providing the St/DVB mole feed ratio was ?100/5. The intrinsic chain transfer to monomer reaction seems to be sufficient to reduce the length of the primary polymer chains and hence inhibit crosslinking and gelation in the case of the above mole feed ratios. The branching architecture of the products has been evaluated by ¹H NMR and MALS-SEC analyses.     

Dark cure studies of cationic photopolymerizations of epoxides: Characterization of kinetic rate constants at high conversions

The effective propagation rate constant (k_p; averaged over all the propagating active centers) was characterized for solvent‐free cationic photopolymerizations of phenyl glycidyl ether over the entire range of conversions, including the high conversion regime in which mass transfer limitations become important. The profile for the k_p as a function of conversion was found to exhibit a constant plateau value at low to intermediate conversions, followed by a monotonic increase above a threshold value of conversion. To explain this trend, it is proposed that at high conversion the diffusional mobility of the photoinitiator counterion is reduced whereas the mobility of the cationic active center remains high because of reactive diffusion. Therefore, with increasing conversion, the average distance between the active centers and counterions may increase, resulting in an increase in the propagation rate constant. The profiles for the k_p values were investigated as a function of the temperature, photoinitiator anion, and photoinitiator concentration. As the photoinitiator concentration was increased, the plateau value of the effective propagation rate constant decreased whereas the threshold conversion increased. All of the experimental trends are consistent with the proposed increase in ion separation at high conversions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4409–4416, 2004     

Some transformations of acetylenic alcohol epoxides

It is shown that the hydration of acetylenic alcohol epoxides containing an epoxide ring in the position relative to the triple bond in the presence of sulfuric acid leads to the formation of acetylenic triols: 6-methyl-4-heptyne-1,2,6-, 7-methyl-5-octyne-2,3,7-, 2,7-dimethyl-5-octyne-2,3,7-, and 2,3,7-trimethyl-5-octyne-2,3,7-triols. It was established that the hydration process is hampered as the number of alkyl substituents attached to the epoxide ring increases. The triols were subjected to intramolecular cyclization under the influence of mercuric chloride to the corresponding chloromercurated derivatives of furan: 2-(1-chloromercuri-2-methyl-1-propenyl) and 5-methyi-2-(1-chloromercuri-2-methyl-1-propenyl)furans. The IR and PMR spectral data are presented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1164–1167, September, 1977.     

Bismuthonium- and pyrylium-based radical induced cationic frontal polymerization of epoxides

Cationic polymerization is a powerful tool when it comes to adhesive, coating, composite, and bulk material production. A variety of monomers is suitable for this technique, however epoxy-based systems are used in many applications, due to their high reactivity and versatility. With the introduction of the radical induced cationic frontal polymerization (RICFP), an even more efficient pathway to cure epoxides and with it a variety of new applications was obtained. The prevailing initiator class for RICFP applications is the iodonium salt. Decades of research and fine-tuning of the formulations lead to a highly effective and versatile initiator system. With the introduction of bismuth- and oxygen-based onium salts for frontal polymerization, the well-known iodonium salt is challenged. Bismuthonium hexafluoroantimonates shows high rate of polymerization and conversions of 84% in an epoxy system. The advantage of bismuthonium-based systems is the pot life of the formulations, which can be cured at the press of a button. In this study, iodonium salts last around 3 days before a significant viscosity increase is measured, while the bismuthonium initiator lasts over 10 times as long with no significant drop in reactivity or frontal velocity.     

Dark‐cure studies of cationic photopolymerizations of epoxides: Characterization of the active center lifetime and kinetic rate constants

We have characterized the effective rate constants for termination/trapping (k_t/t) and propagation (k_p) for solvent‐free cationic photopolymerizations of phenyl glycidyl ether for conversions up to 50%. We have performed dark‐cure experiments in which active centers are produced photochemically for a specified period of time until the initiating light is shuttered off, and then the polymerization rate is monitored in the dark. This method is especially well suited for characterizing cationic polymerizations because of the long active center lifetimes. Our analysis provides profiles of the instantaneous kinetic rate constants as functions of conversion (or time). For photopolymerizations of phenyl glycidyl ether initiated with iodonium photoinitiators, k_t/t and k_p remain essentially invariant for conversions up to 50%. For the photoinitiator (tolycumyl) iodonium tetrakis (pentafluorophenyl) borate (IPB), the values of k_t/t at 50 and 60 °C are 0.027 and 0.033 min^?1, respectively. The corresponding values of k_t/t for diaryliodonium hexafluoroantimonate (IHA) are 0.041 and 0.068 min^?1. The values of k_p at 50 °C for IPB and IHA are 0.6 and 0.4 L mol^?1 s^?1, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2064–2072, 2003     

The synthesis and study of the photoinitiated cationic polymerization of novel cycloaliphatic epoxides

The synthesis of a series of difunctional epoxides bearing two epoxycyclohexyl groups linked together by an alkylene ether group has been carried out. Subsequently, the reactivities of these novel monomers was investigated and compared to the reactivity of the cycloali-phatic epoxide, 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexane carboxylate ( I ) in pho-toinitiated cationic polymerization. It was observed that alkylene oxide linking the two epoxycyclohexyl groups was short and the monomers are more reactive than I . The effects of the photoinitiator structure and the experimental conditions of the cationic photopo-lymerization on the rates was also studied using real-time infrared spectroscopy. © 1995 John Wiley & Sons, Inc.     

Syntheses of cardanol-based cationic surfactants and their use in emulsion polymerisation

Six quaternary ammonium salts were designed and synthesised with moderate to high yields in three steps, based on cardanol, a low-cost and abundant renewable resource. The new ammonium salts can act as reactive surfactants due to their having both a hydrophilic ammonium group and a hydrophobic unsaturated alkyl chain. The gemini surfactants with a linker of a linear saturated aliphatic hydrocarbon chain exhibited a relatively low CMC value (≤ 0.2 mmol L^?1) and surface tension (≤ 27 mN m^?1), signifying that this kind of amphiphile exhibited good surface active properties. The photo-active gemini surfactant with critical micelle concentration (CMC) of 0.05 mmol L^?1 was used successfully as the sole emulsifier in the emulsion polymerisation of methyl methacrylate (MMA). In addition, a benzyl bromide-containing surfactant can act as both an atom transfer radical polymerisation (ATRP) initiator and an emulsifier in an activator generated by the electron transfer atom transfer radical polymerisation (AGET ATRP) of MMA in emulsion. The value of the number-average molecular mass of the resulting cardanol-end poly(methyl methacrylate) (PMMA) is M_n,GPC = 45.1 kDa, with polydispersity of 1.39.     

A new visible light sensitive photoinitiator system for the cationic polymerization of epoxides

This communication reports the development of an efficient three‐component visible light sensitive photoinitiator system for the cationic ring‐opening photopolymerization of epoxide monomers and epoxide functional oligomers. The photoinitiator system consists of camphorquinone in combination with a benzyl alcohol to generate free radicals by the absorption of visible light. Subsequently, the radicals participate in the free radical chain induced decomposition of a diaryliodonium salt. The resulting strong Brønsted acid derived from this process catalyzes the cationic ring‐opening polymerization of a variety of epoxide substrates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 866–875, 2009     

The applicability of DTG to polymerisation studies

The application of derivativ thermogravimetry (DTG) to investigate polymerisation mechanisms is illustrated using the radical polymerisation of di-n-alkyl itaconates as well as the photoinitiated polymerisation of methyl methacrylate by colloidal CdS. The thermal properties of the resulting polymers are described. The advantages and disadvantages of DTG are discussed.

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Zusammenfassung Die Anwendung von DTGA zur Untersuchung von Polymerisationsmechanismen wird anhand der radikalischen Polymerisation von Di-n-alkyl-itakonaten und der fotoinitiierten. Polymerisation von Methylmetacrylat mittels kolloidalem CdS dargestellt. Die thermischen Eigenschaften der erhaltenen Polymere werden beschrieben. Es werden Vor-und Nachteile von DTGA diskutiert.

     

High molar mass polymers by cationic polymerisation in emulsion and miniemulsion

The generation of poly(p-methoxystyrene) with a molar mass of several thousand g mol(-1) by cationic polymerisation in emulsion, is described here for the first time. Such a striking result was achieved by carrying out the polymerisation inside monomer droplets, thus preventing fast transfer reactions with water.     

Effect of oligo(spiroorthocarbonate)s on the volume shrinkage of epoxides during crosslinking by sulfonium salt‐initiated cationic polymerization of epoxides

Oligo(spiroorthocarbonate)s 1 , which were synthesized by the polycondensation of pentaerythritol derivatives with tetraethylorthocarbonate, were employed as comonomers in the cationic polymerization of epoxide initiated by sulfonium salt. In the copolymerization, the spiroorthocarbonate moiety of 1 underwent double ring‐opening reaction, leading to the efficient diminution of the volume shrinkage upon the copolymerization. Thermal properties of the resulting networked polymers were evaluated by TGA. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1564–1568