Search results for "Tacticity"

A Deeper Insight into the Postpolymerization Modification of Polypenta Fluorophenyl Methacrylates to Poly(N -(2-Hydroxypropyl) Methacrylamide)

2014

This work provides a detailed insight into the synthesis of N-(2-hydroxypropyl)methacrylamide (HPMA) polymers employing the activated ester approach. In this approach, polypenta fluorophenyl methacrylate (PFPMA)-activated ester polymers are synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization and transferred into HPMA-based systems by the use of 2-hydroxypropylamine. To prove quantitative conversion in the absence of side reactions, special attention is devoted to investigate different reaction conditions by different analytical methods ((1) H, (19) F, inverse-gated (13) C NMR, and zeta potential measurements). Furthermore the influence of common solvent…

Polyreactions in ordered systems: Polymerization of octadecyl methacrylate in monolayers at the gas–water interface

1977

The ultraviolet-initiated polymerization of octadecyl methacrylate (ODMA, octadecyl 2-methyl-2-propenoate) as a monomolecular layer at the gas-water interface was studied. The polymerization was carried out at 27°C at the nitrogen-water interface; air inhibits the polymerization. At 27°C the ODMA monolayer exhibits three different states which were characterized by surface pressure-area diagrams and by surface potential measurements. The ODMA monolayer was polymerized under constant surface pressure in the range between 0 and 10 dyne/cm. The polymerization was followed by recording the contraction of the film. The conversion was determined by comparison of the area per monomer unit during p…

Crystallization Behaviour at High Cooling Rates of Two Polypropylenes

1993

Phase distribution of quenched samples of two isotactic polypropylenes, having different molecular weight distributions, was evaluated by a deconvolution procedure of WAXD spectra. The dependence on cooling rate of the two resins shows the low molecular weights rich polymer is characterized by a faster kinetics with an α-monoclinic to mesomorphic transition taking place at higher cooling rates.

1982

The stress relaxation behaviour after compression yielding of two semicrystalline polymers, a low density polyethylene and an isotactic polypropylene, is investigated. Data taken at several strains, reached with different sample loading rates, are considered. Master curves are obtained by means of dimensionless stress and time measures, similar to those recently adopted for some amorphous polymers. Die Spannungsrelaxation nach Druckbeanspruchung wurde an zwei semikristallinen Polymeren, Polyethylen neidriger Dichte und isotaktischem Polypropylen, untersucht. Die bei unterschiedlichen Deformationen, welche mit unterschiedlichen Beanspruchungsgeschwindigkeiten erreicht wurdern, erhaltenen Erg…

Effect of bulkiness and lewis acidity of aluminium compounds on the anionic polymerization of methyl methacrylate in toluene

1996

The bulkiness and the Lewis acidity of added aluminium compounds strongly affect the polymerization of methyl methacrylate in toluene at −78°C. The polymerization strongly deviates from ‘ideal’ first-order kinetics, i.e. the first-order time-conversion plots are kinked at low monomer conversions. Additionally, for the more bulky and more Lewis-acid aluminium alkyls, the time-conversion plots show a further downward curvature. This curvature is not the result of a termination reaction because the polymers are free of side products like β-ketoesters or vinyl ketone units. The molecular weight distributions and tacticities of the resulting polymers are also affected. Dependent on the aluminium…

Synthesis and Characterization of Syndiotactic Polystyrene-Polyethylene Block Copolymer

2019

The direct synthesis of syndiotactic polystyrene-block-polyethylene copolymer (sPS-b-PE) with a diblock structure has been achieved. The synthetic strategy consists of the sequential stereocontrolled polymerization of styrene and ethylene in the presence of a single catalytic system: cyclopentadienyltitanium(IV) trichloride activated by modified methylaluminoxane (CpTiCl3/MMAO). The reaction conditions suitable for affording the partially living polymerization of these monomers were identified, and the resulting copolymer, purified from contaminant homopolymers, was fully characterized. Gel permeation chromatography coupled with two-dimensional NMR spectroscopy COSY, HSQC, and DOSY confirme…

Theoretical study of asymmetrically substituted poly(organosilanes)

1994

Summary form only given. Poly(organosilanes) [-SiRR'-] where R and R' represent various alkyl and aryl groups, are the subject of intense scientific and technological interest because of the interesting properties such as, intense near-UV absorption, highly efficient photoluminiscence, non-linear optical properties, these polymers present. In this contribution, we present the electronic structure of some asymmetrically substituted poly(organositanes). We have focused the study on the effect of the tacticity on the electronic structure of the polymer. We have employed ab initio methods to obtain reliable geometrical parameters. The band structure calculations have been performed using the va…

Structure and phase transitions of a liquid crystalline polymer

1981

The structures, textures as well as thermodynamic properties of a side chain polymer exhibiting a liquid crystalline phase in addition to a partially crystalline state and the isotropic fluid state were investigated. Furthermore the kinetics of phase transitions between these states were analyzed. It was found that the properties of this polymer are intermediate between that of low molecular weight liquid crystals and common polymers. In particular it was observed that the relation between the liquid crystalline texture and the structure is different from that of low molecular weight liquid crystals and that the properties of the crystalline and liquid crystalline state depend strongly on t…

Crystallization of polymer melts under fast cooling. II. High-purity iPP

1992

SYNOPSIS Samples of a high-purity isotactic polypropylene (iPP) were quenched from the melt so as to monitor cooling history. A continuous variation of morphology and crystal structure was obtained with cooling rate. This is discussed in relation to sample thermal history evidencing that cooling history relevant to quenched samples is in the neighborhood of 90°C. In particular the samples are essentially mesomorphic when at this temperature cooling rates larger than 80°C/s were adopted, while below a few tens of °C/s only a­ monocline form is obtained. Densities of quenched samples were compared with predictions of an isokinetic extrapolation of Avrami model of polymer crystallization kinet…

Influence of the irradiation parameters on the molecular modifications of an isotactic polypropylene gamma-irradiated under vacuum

2000

Isotactic polypropylene was irradiated under vacuum in a complete set of experimental conditions. The influence of the irradiation parameters, total absorbed dose, D, and dose rate, I, was analysed in order to verify the theoretical predictions of a simple kinetic model, already presented (Sarcinelli L, Valenza A, Spadaro G. Polymer 1997;38:2307), based on the rates of the main reactions occurring during irradiation, i.e. β-scission, addition to double bonds and termination. The concentration of free radicals and double bonds formed during irradiation is detected together with a comparison to their dependence on the irradiation parameters. A quantitative determination of the “inversion curv…