Model compounds and 13C NMR investigation of isolated ethylene units in ethylene/propene copolymers (original) (raw)
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Polymer Bulletin, 1995
This study employed the~3C-NMR spectroscopy to investigate the influence of the increase of the comonomer concentration on the microstruture of ethylene/1hexene and ethylene/1-octene copolymers obtained by the use of MeSiCp2ZrC12, Cp2ZrC12,, Et[Ind]2ZrC12 and [Ind]2ZrCl2 catalysts. For both comonomers butyl or hexyl branches were isolated between ethylene blocks. As the ct-olefm concentration in the copolymer increased, butyl or hexyl branches became closer, some of them, separated by only one or two ethylene units. Incorporation of ct-olefm in the copolymer was higher for the bridged catalysts, MeSiCp2ZrCI2, and Et[Ind]2ZrC12 than for the unbridged ones. The ~-olefin size did not seem to effect its reactivity towards ethylene.
Solid state 13C NMR analysis of syndiotactic copolymers of propene with 1-butene
Polymer, 2000
An analysis of the structure of syndiotactic copolymers of propene with 1-butene prepared with a single-center metallocene-based catalyst in a whole range of comonomer composition, by solid state 13 C NMR CPMAS spectroscopy, is presented. The presence of resonances of carbon atoms of butene and propene units in the spectra of all copolymers, with intensities depending on the composition, possibly indicates the inclusion of the comonomeric units in the unit cells of both homopolymers. The splitting of the resonance of methylene carbon atoms in conformational environment TG·GT of propene units in propene-rich copolymers has been explained by a conformational effect, assuming that the butene units, included in the crystalline phase of syndiotactic polypropylene, produce a slight distortion of the local conformation close to the comonomeric units. The presence of additional signals at Ϸ35 and Ϸ45 ppm, attributed to the resonances of backbone GT·TG and TG·GT methylene carbon atoms, respectively, located at interfaces between propene (P) and butene (B) units, i.e. belonging to PB sequences, has been explained assuming that also in isolated butene units the lateral ethyl groups assume the "double" gauche conformation, typical of the chains of form I of syndiotactic poly(1-butene). ᭧
Macromolecules, 2005
The copolymerization of ethylene and norbornene by catalytic systems composed of i-Pr-[(Cp)(Flu)]ZrCl2 (1) and methylaluminoxane was investigated. Ethylene-norbornene (E-N) copolymers with 40.2 mol % of norbornene are highly alternating (NENE 50 mol %) and contain a significant amount of racemic ENNE (8 mol %) and no ENNN sequences. The microstructural analysis by 13 C NMR of such copolymers was completely obtained at the tetrad level by a methodology that exploits all the peak areas of the spectra and accounts for the stoichoimetric requirements of the copolymer chain. The analysis at the tetrad level allowed us to test the statistical model best describing E-N copolymerization with Cssymmetric catalyst 1 and to study the polymerization mechanism. The root-mean-square deviations between experimental and calculated tetrads demonstrate that the first-order Markov model is sufficient to describe the microstructure of E-N copolymers with 1. It is concluded that in E-N copolymerizations with this catalyst both N and E are inserted according to a Cossee's migratory insertion, and backskips of the copolymer chain to its original position occur, causing the formation of both meso and racemic NEN sequences. The probability of chain backskip is relatively high with respect to that observed in syndiotactic propylene polymerization under the same polymerization conditions. This effect seems to be due to norbornene strong coordinating ability which can influence the competition between site epimerization and chain propagation.
Structural analysis of copolymers of syndiotactic polypropylene with 13C-enriched ethylene
A structural analysis of copolymers of syndiotactic polypropylene with small amounts of 13 C-enriched ethylene, in the range 0.4-2.6 mol %, is reported. X-ray diffraction and solid-state 13 C NMR CPMAS data indicate that the as-prepared copolymer samples are crystallized in the conformationally disordered modification of form II of sPP containing kink bands. The disorder corresponds to the presence of portions of chains in the trans-planar conformation in chains having a prevailing 2-fold helical conformation. A direct evidence of the partial inclusion of the ethylene units in the crystalline regions of the copolymers is provided.
Triad sequence determination of ethylene–propylene copolymers – application of quantitative 13C NMR
Polymer Testing, 2009
A methodology for acquiring fully quantitative 13 C{ 1 H} NMR spectra for high performance ethylene-propylene copolymers has been proposed. To minimize the spectral acquisition time without sacrificing spectral quality, different amounts of chromium(III)acetylacetonate relaxation agent has been added to optimize its concentration. The study demonstrates the critical setting of delay time for determining six triad distributions from eight discrete set of resonances which otherwise leads to inaccurate determination of triad concentrations. It allows precise integral measurements of low intensity resonances depending on copolymer composition, and significant reduction of experimental time.
Structural Characterization of Syndiotactic Copolymers of Propene with 1-Butene
Macromolecules, 1998
A structural characterization of a propylene-styrene-ethylene terpolymer sample (sP/(S-E)), prepared with syndiospecific C s-symmetric metallocene catalyst, is presented. The study of the polymorphic behavior in as-prepared, melt-crystallized, and fiber samples has confirmed that ethylene units are mainly bound to styrene units arising from catalyst reactivation after the secondary insertion of styrene. Styrene-ethylene units are mainly segregated in the amorphous phase. The bulky styrene units, bound to ethylene, prevent the inclusion of ethylene units in the crystals of syndiotactic polypropylene (sPP), as generally occurs for copolymers of sPP with ethylene. This explains the experimental evidence that the influence of the presence of styrene-ethylene units on the polymorphism of sPP is smaller than that observed in copolymers of sPP containing only ethylene or butene comonomeric units.
Macromolecules, 2011
A thorough 13 C NMR analysis of a series of propene/4-methyl-1-pentene copolymers, prepared with the isoselective metallocene precatalyst [rac-(EBTHI)ZrCl 2 ], has paved the way to the first full description of such potentially interesting family of copolymers at the triad level. Several tetrads and even longer sequences were assigned and quantified as well. The main problem, consisting in the partial overlap of the resonances of the RR-methylenes of the chain and the R-methylenes of the branch, generally utilized for the microstructural description of propene/linear higher R-olefins copolymers, was overcome by analyzing other less informative regions of the spectra along with the application of two-dimensional heteronuclear ( 1 HÀ 13 C) correlation measurements. A computational procedure for checking the correctness of the proposed assignments was applied to the 13 C NMR observed integrals of two samples differing in comonomer compositions. In both cases, a correspondence between the observed and calculated values of the sequence molar fractions was found.
Macromolecules, 1988
Propylene has been polymerized in the presence of the catalyst 6-TiC13/Zn(13CH2CH3)2, used as a model system, at different concentrations of Et3N and of 2,2,6,64etramethylpiperidine. The 13C NMR analysis of the steric structure of the chain end groups of all the fractions of the polymers indicates that there are a variety of active sites, both isotactic and atactic, with different steric features, although producing polymer chains of the same tacticity. The distribution among the different types of active sites belonging to the same class changes at different concentrations of the free and/or complexed base. Moreover at concentration corresponding to the maximum of catalyst activity, the stereospecificity of the first step in the most isotactic fractions is higher than that observed in the absence of base and depends on its steric features. This fact provides evidence for the presence of the amines in the active-center environment.