Detailed investigations of the polymerization of ethylene by (α-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were [{(2,6-RR‘C6H3)−NC(Nap)−C(Nap)N−2,6-RR‘C6H3)}NiBr2] (Nap = 1,8-naphthdiyl) (4a, R = CF3, R‘ = H; 4b, R = CF3, R‘ = CH3; 4c, R = C6F5, R‘ = H, 4c, R = C6F5, R‘ = H; 4d, R = C6F5, R‘ = CH3; 4e, R = CH3, R‘ = H, 4f, R = R‘ = CH3; 4g, R = R‘ = CH(CH3)2), [{(2,6-C6H3(i-Pr)2)−NC(CH2CH2CH2CH2)CN−(2,6-C6H3(i-Pr)2)}NiBr2] (5), and [{(2,6-C6H3(i-Pr)2)−NC(Et)C(Me)N−(2,6-C6H3(i-Pr)2)}NiBr2] (6). Active polymerization catalysts were formed in situ by combination of 4−6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polyme...

Synthesis of branched polyethylene using (α-diimine)nickel(II) catalysts : influence of temperature, ethylene pressure, and ligand structure on polymer properties

Relaxation processes in crystalline polymers: experimental behaviour — a review

Relaxation processes in crystalline polymers: molecular interpretation — a review

A survey of recent high-resolution solid-state 13C NMR studies is given to illustrate how and to what extent 13C chemical shifts of a variety of molecular systems are displaced upon conformational changes. Examples are taken from studies of alkanes, polysaccharides, polypeptides, and naturally occurring and synthetic ionophores, in which more than single conformations are available owing to either crystalline polymorphs or a variety of metal-ligand interactions. The magnitude of the conformation-dependent 13C chemical shifts are in many instances as large as 8 ppm and are well related to a set of torsion angles of nearby single bonds. The application of this concept to conformational characterization is also discussed.

Conformation‐dependent 13C chemical shifts: A new means of conformational characterization as obtained by high‐resolution solid‐state 13C NMR

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers.

Relations force-longueur a temperature ambiante determinees pour une serie de polyethylenes d'une large gamme de masse moleculaire et contribution moleculaire

Influence of structural and morphological factors on the mechanical properties of the polyethylenes

Relaxations α et β d'une variete de polyethylenes etudiees en detail par des methodes mecaniques dynamiques a basse frequence

Dynamic mechanical studies of α and β relaxations of polyethylenes

The well-established data which gives a complex composition dependence for the relaxation of ethylene copolymers has been reexamined. Emphasis is placed on the β-transition. In the reanalysis of the data two new experimentally determined concepts are introduced. These are that this transition is definitely found in the homopolymer linear polyethylene, and that it is a reflection of segmental motions which take place within the interfacial region. With these principles it is possible to separate the β-transition from the glass temperature and thus explain the very unique composition relations.

The transition in ethylene copolymers: The β-transition

The carbon-13 spin-lattice relaxation times T1 of the crystalline portion of a set of polyethylenes have been studied. Chain structure and crystallization conditions have been varied over the widest possible extremes so that large differences are developed in the level of crystallinity, the supermolecular structure, and the crystallite thickness. Concomitantly, the observed crystalline T1 values cover the extraordinarily wide range of about 40–4500 s. They bear a one-to-one relation with the crystallite thickness, which is found to be the key structural variable determining this property. A correlation with the temperature for the α-transition can be established, which implies a similar type of segmental motions for the two phenomena. Major changes in the interfacial structure can also have a drastic influence on the value for the crystalline T1. Analysis of the magnetization decay curve also allows for a quantitative determination of the degree of crystallinity, which is found to be in excellent agreement with the corresponding value found from Raman spectroscopy.

Carbon‐13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure

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Papers

Influence of structural and morphological factors on the mechanical properties of the polyethylenes

Dynamic mechanical studies of α and β relaxations of polyethylenes

The transition in ethylene copolymers: The β-transition

Carbon‐13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure