A method of determining the active centers in cationic polymerization, applied to polymerization of dioxolane, trioxane, and formaldehyde

Abstract

I n order to determine the concentration of active centers, polymerizations are terminated by addition of sodium alkoxide. This yields alkoxy endgroups which after acid hydrolysis of the isolated and purified polymers are determined by gaschromatography of the produced alcohol.I n cationic polymerization of anhydrous formaldehyde a t -78 "C under certain reaction conditions a fast and quantitative initiation reaction and no kinetic chain termination have been observed. Presumably living polymers are obtained. The unexpected decrease in the rate of polymerization after a certain conversion is ascribed to hindrance of monomer diffusion in the crystalline polymer in which the active centers are imbedded.Cationic chainends have been detected in the 6oCo y-ray induced polymerization of crystalline trioxane. This proves a cationic mechanism of chain propagation in the radiation-induced polymerization of trioxane.I n polymerization of 1.3-dioxolane by HC104 a t 20°C a gradual but quantitative initiation reaction and no kinetic termination reaction have been observed. Termination by addition of alkoxide and determination of alkoxy endgroups also helped to distinguish between several different mechanisms of chain propagation proposed in the literature. It is assumed that the active center in dioxolane polymerization is a tertiary oxonium ion which propagates by an SN 2 reaction with monomer. Z U S A M M E N F A S~U N G :Zur quantitativen Bestimmung der aktiven Zentren werden die Polymerisationen durch Zugabe eines Natriumalkoholates abgebrochen und die so eingebauten AUcoxy-Endgruppen analytisch bestimmt. Dies erfolgt durch saure Hydrolyse der isolierten und gereinigten Polymeren und gaschromatographische Bestimmung des entstandenen Alkohols. Bei der kationischen Polymerisation von Formaldehyd bei -78 "C erfolgt unter gewissenReaktionsbedingungen eine schnelle und vollstandige Initiierung und kein Kettenabbruch, so da13 man lebende Polymere erhdt. Das starke Absinken der Bruttopolymerisationsgeschwindigkeit nach einem gewissen Umsatz ist vermutlich auf Diffusionsbehinderung des Monomeren auf seinem Wege zu den im kristallinen Polymeren eingeschlossenen aktiven Zentren zuriickzufiihren.Bei der durch Wo-y-Strahlen induzierten Polymerisation von kristallinem Trioxan wurden durch Abbruch mit Natriumathylat kationische Kettenenden in geringer Konzentration festgestellt. Dadurch wird fiir das Kettenwachstum bei der strahleninduzierten Polymerisation von Trioxan ein kationischer Mechanismus nachgewiesen. A Method of Determining the Active Centers in Cationic PolymerizationBei der mit HClO4 bei 20 O C initiierten Polymerisation von Dioxolan wurde durch Bestimmung der Konzentration der aktiven Zentren festgestellt, d& im Laufe der Polymerisation der Initiator langsam, aber quantitativ zur Reaktion kommt und da13 keine kinetische Abbruchsreaktion erfolgt. Auaerdem konnte qualitativ zwischen verschiedenen in der Literatur vorgeschlagenen Polymerisationsmechanisrnen unterschieden werden. Es wird angenommen, da13 das Kettenwachstum ub...

Attempted copolymerization of 1.3‐dioxolane with tetrahydrofuran by triethyl oxonium tetrafluoroborate

“…It is generally accepted that the propagating species in the cationic polymerization of tetrahydrofuran (THF) is a tertiary oxonium ion which reacts with THF by the S N~ mechanism. On the other hand, the mechanism of the polymerization of 1.3-dioxolane (DOL) seems still to be rather uncertain, because three different mechanisms have been proposed for the propagation reaction: (1) The propagating species is a tertiary oxonium ion which grows by the S N~ mechanism as in the polymerization of THF. (2) The propagating species is a tertiary oxonium ion, but it propagates by the S N~ mechanism, t h a t is, it is converted to a linear carbonium ion before the addition of DOL, as suggested for the polymerization of trioxane.…”

“…(3) The propagating species is an oxonium ion forming part of a large ring which grows by inserting DOL monomer into the ring by a four centered reaction. JAACKS et al 1) asserted that the mechanisms (2) and (3) should be ruled out on the basis of end-group analysis, while PLESCH and WESTERMANN 2) based their ring expansion mechanism (3) on the fact that poly-1.3-dioxolane, even of a very low molecular weight, was devoid of end-groups if prepared under strictly anhydrous conditions. Apart from the mechanism, the propagating species of THF and DOL have markedly different reactivities.…”

Some Aspects of the Polymerisation of 1,3-Dioxacycloalkanes

“…A simpl~ and useful method of distinguishing between secondary and tertiary oxonium ians is due to Jaacks's group and consists of neutralising the reaction mixture with an alkoxide. 12 Details of the method have been described, but it is worthwhile repeating here exactly what it shows. lf Etois added to a reaction mixture containing secondary oxonium ions, whatever their structure may be, it abstracts the proton, forms ethanol, and leaves the oxygen compound:…”

NMR studies on the growing species of cyclic acetals in their cationic polymerization. 1H‐NMR spectra of oxycarbenium ions formed in some model reactions

“…2, the chemical shift of the end-methylene protons of 4 is considerably smaller Tab. 2. in the reaction of 1,3-dioxane (1) with triethyloxonium salts"' Chemical shifts 6 of the end-methylene protons of the ionic species produced Solvent 11,2 11,l (9,2)h' 9,9 (9,4)…”