Ivan Gitsov

Self-condensing vinyl polymerization : an approach to dendritic materials

Self-condensing vinyl polymerization was used to produce dendritic polymers with both highly branched structures and numerous reactive groups. A vinyl monomer will undergo self-polymerization if it contains a pendant group that can be transformed into an initiating moiety by the action of an external stimulus. The self-polymerization combines features of a classical vinyl polymerization process with those of a polycondensation because growth is accomplished by the coupling of reactive oligomers. Highly branched, irregular dendritic structures with a multiplicity of reactive functionalities are obtained by polymerization of 3-(1-chloroethyl)-ethenylbenzene.

Importance of active‐site reactivity and reaction conditions in the preparation of hyperbranched polymers by self‐condensing vinyl polymerization: Highly branched vs. linear poly[4‐(chloromethyl)styrene] by metal‐catalyzed “living” radical polymerization

The self-condensing vinyl polymerization of 4-(chloromethyl)styrene using metal-catalyzed living radical polymerization catalyzed by the complex CuCl/2,2′-bipyridyl has been attempted. Given the unequal reactivity of the two potential propagating species in this system, a variety of polymerization conditions were tested to optimize the extent of branching in the products. Typical reaction conditions included polymerization in the bulk, or preferably in chlorobenzene solution, with catalyst to monomer ratios in the range 0.01–0.30, temperatures of 100–130°C, and reaction times from 0.1 to 32 h. Polymers with weight average molecular weights between 3 × 103 and 1.6 × 105 and different extents of branching are formed as evidenced by size-exclusion chromatography, light scattering, and NMR analysis of the reaction products. The influence of reaction conditions on the molecular weight and branching of the resulting polymers is discussed in detail. In sharp contrast to an earlier report, the weight of evidence suggests that, at a catalyst to monomer ratio of 0.01, an almost linear polymer is obtained, while a high catalyst to monomer ratio favors the formation of a branched structure. As a result of the unequal reactivity of the primary and secondary benzylic halide reactive sites, growth occurs by a modified self-condensing vinyl polymerization mechanism that involves incorporation of the largely linear vinyl-terminated fragments formed early on in the polymerization into the vinyl polymer, to afford an irregularly branched structure. Chemical transformations involving the numerous benzylic halide functionalities of the highly branched polymer have been investigated.

Dendrimers and Hyperbranched Polymers: Two Families of Three-Dimensional Macromolecules with Similar but Clearly Distinct Properties

Abstract Dendrimers and hyperbranched polymers are globular macromolecules that are characterized both by a highly branched structure, in which all bonds converge to a focal point or core, and a multiplicity of reactive chain-ends. Because of the obvious similarity of their building blocks, many assume that the properties of these two families of dendritic macromolecules are almost identical and that the terms “dendrimer” and “hyperbranched polymer” can be used interchangeably. This assumption is incorrect because only regular dendrimers have a precise end-group multiplicity and functionality and exhibit properties that are totally unlike those of all other families of macromolecules. For example, regular dendrimers display a maximum in the relationship between their intrinsic the applications that are contemplated for the dendritic polymers. Using living systems as a model, one must anticipate that the regular placement of reactive groups at a precise location, as opposed to throughout a structure, has g...

Micelles with highly branched nanoporous interior : Solution properties and binding capabilities of amphiphilic copolymers with linear dendritic architecture

This article describes the solution behavior of model amphiphilic linear-dendritic ABA block copolymers that self-assemble in aqueous media and form micelles with highly branched nanoporous cores. The materials investigated are constructed of poly(ethylene glycol), PEG, with molecular weight 5,000 or 11,000 as the water-soluble B block and poly(benzyl ether) monodendrons [G] of second and third generation as the hydrophobic A fragments. The process of self-assembly in aqueous media and the character of the micellar core are investigated by fluorescence spectroscopy using pyrene as the molecular probe. The data obtained by different methods indicate that the critical micelle concentrations (cmc) for these systems are between 1.1 × 10−5 and 2.0 × 10−5 mol/L for [G-2]-PEG5000-[G-2] and between 7.08 × 10−6 and 7.94 × 10−6 mol/L for [G-3]-PEG11000-[G-3]. It is found that the ratio of the first and third vibronic bands (I1/I3 ) in the fluorescence spectrum of the encapsulated pyrene changes from 1.77 to 1.32 when the concentration of [G-2]-PEG5000-[G-2] increases from 1.1 × 10−6 mol/L to 1.1 × 10−4 mol/L. For [G-3]-PEG11000-[G-3] these changes are between 1.77 and 1.17 in the same concentration range. The hybrid copolymers form host-guest complexes with several polyaromatic compounds (phenanthrene, pyrene, perylene and fullerene, C60) that are stable over extended periods of time (more than 12 months).

Linear-dendritic supramolecular complexes as nanoscale reaction vessels for "green" chemistry. Diels-Alder reactions between fullerene C60 and polycyclic aromatic hydrocarbons in aqueous medium.

This study describes the first Diels-Alder (DA) reaction performed in aqueous medium with highly hydrophobic compounds-fullerene (C 60) as the dienophile and anthracene (An) or tetracene (Tet) as the dienes, respectively. The reactions are performed in nanocontainers, constructed by self-assembly of linear-dendritic amphiphilic copolymers with poly(ethylene glycol), PEG or poly(ethylene oxide), PEO as the hydrophilic blocks and poly(benzyl ether) monodendrons as the hydrophobic fragments: G3PEO13k, dG3 and dG2. Comparative studies under identical conditions are carried out with an amphiphilic linear-linear copolymer, poly(styrene)1800- block-PEO2100, PSt-PEO, and the nonionic surfactant Igepal CO-720, IP720. The binding affinity of supermolecules built of these amphiphiles toward the DA reagents decreases in the following order: G3PEO13k > dG3 > PSt-PEO > dG2 > IP720. The kinetic constant of binding is evaluated for tetracene and decreases in a similar fashion: 5 x 10 (-7) M/min (G3PEO13k), through 4 x 10 (-7) M/min (PSt-PEO) down to 1.5 x 10 (-7) M/min for IP720. The mobility of substrates encapsulated in the micellar core, estimated by pyrene fluorescence decay, is 95-121 ns for the micelles of the linear-dendritic copolymers and notably higher for PSt-PEO (152 ns), revealing the much denser interior of the linear analogue. The apparent kinetic constant for the DA reaction of C 60 and Tet within the G3PEO13k supermolecule in aqueous medium is markedly higher than in organic solvent (toluene), 208 vs 1.82 M /min. With G3PEO13k the conversions reach 49% for the DA reaction between C 60 and An, and 55% for C 60 and Tet. Besides the monoadduct (26.5% yield) the reaction with An produces exclusively increasing amounts of D 2 h -symmetric antipodal bis-adduct, whose yield reaches up to 22.5% after 48 h. In addition to the environmentally friendly conditions notable advantages of the synthetic strategy described are the extended stability of the linear-dendritic nanovessels, the easy collection of the products formed, and the recovery and reuse of unreacted reagents and linear-dendritic copolymers.

Hydrolysis of biodegradable polymers by superoxide ions

The objective of this study was to examine the reactivity of superoxide ion as an oxygen nucleophile towards biodegradable biomaterials having an aliphatic polyester structure, such as poly(D,L-Lactide) (PDLLA). The changes in molecular weights and thermal properties of PDLLA were studied as a function of the superoxide ion concentration, hydrolysis time, and hydrolysis temperature. The superoxide ion induced fragmentation of PDLLA yielded a mixture of various species with different chain length. A combined size exclusion chromatography (SEC) method with a chemical tagging by phenyl isocyanate revealed that the structure of the oligomer species with the lowest molecular weight formed in the superoxide ion induced degradation mixture was linear. The significant reduction in Tg was consistent with the significant reduction in the molecular weight of PDLLA. The linear low molecular fragments (dimers and trimers) in the reaction mixture could act as an internal plasticizers to enhance the Tg reduction by increasing the free volume of chain ends. The relationship of Tg with molecular weight of PDLLA followed the general theory of Fox–Flory. The mechanism of simple hydrolysis of ester by superoxide ion proposed by Forrester et al. was modified to interpret the data obtained from the synthetic biodegradable polymers.

A novel depolymerization route to phosphorus-containing oligocarbonates

Abstract The interaction of poly[2,2-bis(4-hydroxyphenyl)propanecarbonate] (PC) with phosphonic acid dialkyl esters [(RO) 2 P(O)H, where R CH 3 or CH 3 CH 2 ] or with phosphoric acid triethyl ester (CH 3 CH 2 O) 3 P(O) was studied. Treatment of PC pellets with phosphonic acid dialkyl esters at 160°C or with triethyl phosphate at 180°C yielded phosphorus-containing oligocarbonates. The structure of the phosphorus-containing oligocarbonates was studied by 31 P NMR, 1 H NMR and 13 C NMR spectroscopy.

Star-graft copolymers. Synthesis of amphiphilic graft copolymers with star-branched poly(oxyethylene) side chains

The synthesis of novel amphiphilic star-graft (SG) copolymers containing hydrophilic poly(oxyethylene) (PEO) side chains attached to a hydrophobic backbone by multifunctional entity is reported. In a first step poly(phthalimidoacrylate-co-styrene) was converted into polymers containing different number of multifunctional branching cites distributed along the main chain by partial aminolysis of the phthalimidoacrylate units with tris(hydroxymethyl)aminomethane. In the second step, these reactive copolymers yielded SG copolymers with different number of star-shaped PEO side groups by reaction with isocyanato terminated methoxy–PEO. The copolymers were characterized by size-exclusion chromatography, IR-, and NMR-spectroscopy. Their thermal properties were examined by thermal gravimetric analysis and differential scanning calorimetry. The studies indicate that the grafting degree and hydrogen bonding determine to a great extent the behavior of the SG copolymers in solid state and in solution.

Separation and characterization of ɛ-caprolactone oligomers by gel permeation chromatography

SummaryIt is shown that the combined use of gel permeation chromatography and the isocyanate method of determination of hydroxyl end groups extends the possibilities for studying complex oligomer mixtures of ɛ-caprolactone. Oligomer mixtures of ɛ-caprolactone obtained by (C6H5)3CSbCl6 and by (C6H5)3CK are investigated. At initiation by both the initiators — the cationic and the anionic one — a covalent bond between the initiator and the polymer chain is formed. In the case of the initiation by (C6H5)3CK intramolecular transesterification proceeds which results in cyclic oligomers. At initiation by (C6H5)3CSbCl6 linear oligomers are formed. It is assumed that the ɛ-caprolactone polymerization by (C6H5)3CSbCl6 proceeds by alkyl-oxygen bond scission.

Mechanism of the anionic polymerization of lactones, initiated by intercalation graphite compounds

SummaryThe anionic polymerization of β-propiolactone (PL)oxethane-2-one, pivalolactone (PVL) — 3,3-dimethyloxethane-2-one and ɛ-caprolactone (CL) — oxepane-2-one, initiated by binary and ternary intercalation compounds (IC) of lithium and potassium in graphite is investigated. The polymerization is carried out in bulk or in xylene. It is established that the lactones penetrate in IC and polymerize in their interlayer spacings. The polymerization causes a delamination of IC. High polymers of the lactones can be obtained by the action of some IC investigated.