Halogen End Groups Research Articles

Synthesis of polymers with hydroxyl end groups by atom transfer radical polymerization

Polymers prepared by atom transfer radical polymerization (ATRP) contain end groups defined by the initiator used. Alkyl halides, used as initiators, lead to polymers with an alkyl group at one end and a halide as the other chain end. Using functionalized initiators such as 2-hydroxyethyl 2-bromopropionate, hydroxyl groups can be directly incorporated at one polymer chain end while the other end functionality remains a halogen. The direct displacement of the halogen end groups with hydroxyl groups was unsuccessful due to side reactions such as elimination (for polystyrene) or hydrolysis of ester functions (for polyacrylate). Another approach to generate hydroxyl end groups was based on the substitution of the halogen end groups by ethanolamine. This was successful for polystyrene but additional substitution at the backbone esters was observed in polyacrylates. Multiple substitution reactions could be avoided by using 4-aminobutanol instead of 2-aminoethanol. Hydroxyl terminated polyacrylates were also obtained by extending the polyacrylate chain end with one allyl alcohol unit in a one-pot process by adding an excess of allyl alcohol at the end of e polymerization of acrylate.

Dehalogenation of polymers prepared by atom transfer radical polymerization

Polymers prepared by atom transfer radical polymerization (ATRP) have well-defined end groups, predetermined by the initiator used. A typical initiator is an alkyl halide from which the halogen is transferred to one chain end. To remove the halogen end group, dehalogenation with trialkyltin hydride has been used. Procedures for the removal of the polymer halogen end groups are described, one of them being a one-pot reaction where the dehalogenation of the polymer chain ends occurs immediately after polymerization.

Synthesis of azido end-functionalized polyacrylates via atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) was demonstrated to yield well-defined polyacrylates with halogen end groups. These halogen end groups were transformed to azide groups which were subsequently reduced into amino groups. The replacement of the halogen end groups by azide groups was obtained either by using sodium azide or by treatment with trimethylsilyl azide in the presence of both stoichiometric and catalytic amounts of tetrabutylammonium fluoride (TBAF). The azide end groups were reacted with triphenylphosphine in order to obtain iminophosphorane groups which were then hydrolyzed yielding amino terminated polyacrylates.

Synthesis of Well-Defined Allyl End-Functionalized Polystyrene by Atom Transfer Radical Polymerization with an Allyl Halide Initiator

The allyl chloride and allyl bromide initiated atom transfer radical polymerization, ATRP, of styrene was conducted to afford α-allyl-ω-halopolystyrene with molecular weights predetermined by Δ[M]/[I]0 and with low polydispersities, Mw/Mn≍1.2 for allyl bromide and Mw/Mn≍1.3 for allyl chloride. The structure of the α-allyl-ω-halopolystyrene was confirmed by 1H NMR spectra. The halogen end-groups were transformed to azido group quantitatively with trimethylsilyl azide in the presence of tetrabutylammonium fluoride.

A chlorophobic pocket in the p-tert-butylcalix[4]arene cavity: a test site for molecular recognition investigated by 13C CP MAS NMR and X-ray crystallography†

Five-membered aliphatic guests X(CH2)3Y (X, Y = Me, OH, Cl, Br) adopt equilibrium positions in the asymmetric p-tert-butylcalix[4]arene cavity formed in the solid state and reveal a marked aversion of the host pocket for halogen end-groups.

Mechanistic and Synthetic Aspects of Atom Transfer Radical Polymerization

Mechanistic and synthetic aspects of atom transfer radical polymerization (ATRP) are reviewed. This controlled/“living” system polymerizes many monomers including styrenes, (meth)acrylates, acrylonitrile and dienes. The halogen end groups can be converted to other functional groups such as amines and azides. In addition to producing well-defined linear homopolymers, statistical copolymers, block copolymers, and gradient copolymers, ATRP can be used to synthesize graft and hyperbranched copolymers through copolymerization with functionalized monomers. Selection of appropriate conditions for ATRP depends on targeted molecular weight and degree of polymer chain end-functionality and includes considering the monomer(s) to be polymerized, initiator structure/reactivity, amount of catalyst/deactivator used, halogen end-group used, and temperature.

Synthesis of End-Functionalized Polymers by Means of Living Anionic Polymerization. 8. Reactions of Living Anionic Polymers with α,ω-Dihaloalkanes

In order to prepare well-defined polymers with halogen end groups, anionic living polymers of isoprene and styrene were allowed to react with excess amounts of α,ω-dihaloalkanes in THF at −78 °C. The monosubstitution reactions of 1,3-dichloropropane with these anionic living polymers proceeded cleanly to afford quantitatively the polymers with chlorine end groups that possessed controllable molecular weights and narrow molecular weight distributions. On the other hand, the reactions of the anionic living polymers with either α,ω-dibromoalkanes or α,ω-diiodoalkanes proceeded competitively with side reactions leading to dimer formation of the starting living polymers. Thus, the bromine- and iodine-terminated polymers were obtained in 12−93% yields along with undesirable dimers. As side reaction candidates, we speculated on metal−halogen interchange and/or single electron transfer pathways followed by coupling reactions between the reaction intermediates generated by both pathways. By end-capping the polysty...

Endgroups in Poly (methyl Methacrylate) Initiated by Photosensitized Ferric Salts in Aqueous Media

AbstractPhotosensitized ferric salts (ferric chloride, as well as bromide and perchlorate) have been used as initiators of aqueous polymerization of methyl methacrylate under varied conditions. Analysis of endgroups in the resulting polymers have been made by the application of dye techniques. Both hydroxyl and halogen endgroups have been traced in the polymers in varying proportions except in case of perchlorate where only hydroxyl endgroups have been detected. Some of the results are in sharp contrast with those reported by Evans et al. While increasing concentration of both ferric chloride and bromide was associated with a sharp fall in the total endgroup content of the polymers, ferric perchlorate did not show such effect. Specific action of some anions derived from additives such as salts and acids on the total endgroup content of the polymer was also investigated. Total endgroup content of the polymer decreased appreciably when a halide or a halogen acid was used in the polymerization medium, the effect of bromide being more predominant than the chloride. No such effect was observed in the case of potassium perchlorate, perchloric acid, and potassium sulfate as the additive. An explanation has been suggested for such specific action of anions.

Studies in some new initiator systems for vinyl polymerization. Part I. Molecular halogens or halates as one component

AbstractSeveral new initiating systems involving a molecular halogen, halates, or perhalate as one of the components and a reducing metal salt, ammonia, an amine or an organic compound as the other component, have been introduced for the aqueous (and in a few cases the non‐aqueous) polymerization of methyl methacrylate. Some of the resulting polymers have been subjected to endgroup analysis by the application of the ultrasensitive dye techniques recently developed in our laboratory. Reducing metal salt—halogen‐initiated polymers are found to incorporate hydroxyl and halogen endgroups, while polymers initiated by amine—halogen systems incorporate hydroxyl and halogen as well as amino endgroups. However, in the ammonia—halogen system polymers incorporate only halogen endgroups. On the basis of the results of the endgroup analysis of the polymers an attempt has been made to explain the probable initiation mechanism with regard to the nature and identity of the initiating species involved in such processes.

Incorporation of halogen endgroups in polymers and their detection by the dye test

AbstractThe incorporation of halogen endgroups in polymers has been established in the following cases for polymerization of styrene, methyl methacrylate and methyl acrylate: (1) chain transfer with halogenated hydrocarbons (chloroform, benzyl chloride, bromoform, ethylene dibromide, octadecyl bromide, ethyl iodide, cetyl iodide); (2) copolymerization with suitable components (vinyl bromide, tetrachloroethylene, trichloroethylene); (3) use of halogen atoms as initiators, e.g. FeCl3 (+hv), FeBr3 (+hv), HCl (+hv) and KBr + H2O2 (+hv); (4) chemical transformation of a group already present in a polymer, e.g., conversion of hydroxyl endgroup to halogen endgroup by a usual method; addition of hydrogen bromide to double bond in polymers. The method of detection consists in converting a polymer with halogen endgroups to a quaternary pyridinium halide by refluxing with pyridine, a few drops of glycerol being added in the case of chlorine endgroup to facilitate the reaction. The converted quaternary halogen endgroup is detected independently by two tests developed in this laboratory: the dyeinteraction test with rhodamine 6GX conc. (Calco) dye and also the dye‐partition test for cationic group with Bromophenol Blue or Disulfine Blue VN 150 as reagents.