Polymorphic Behavior and Mechanical Properties of Isotactic 1-Butene–Ethylene Copolymers from Metallocene Catalysts

Abstract

A study of the crystallization behavior and mechanical properties of random isotactic butene–ethylene copolymers prepared with a metallocene catalyst is presented. The use of the metallocene catalysis ensures a fine control over the molecular structure with low concentration of rr stereodefects (0.8%), negligible amount of regiodefects, and random and uniform distribution of ethylene constitutional defects. This molecular characteristic has allowed evidencing the only effect of the presence of ethylene units on the polymorphic behavior and mechanical properties of isotactic polybutene (iPB). The presence of ethylene accelerates the transition of form II into form I at room temperature and at concentration of nearly 6 mol % favors the direct crystallization from the melt of the stable form I. The presence of ethylene also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing ethylene content. A significant modification of the properties of iPB is observed for ethylene concentration higher than 8 mol %, with development of elastomeric properties, never observed for the iPB homopolymer prepared with Ziegler–Natta catalysts and not observed in the iPB homopolymer with similar content of stereo defects. In these samples, elastomeric properties are due to the low degree of crystallinity that develops upon aging at room temperature by direct crystallization of form I′ from the amorphous phase.

Crystallization behavior and mechanical properties of copolymers of isotactic poly(1-butene) with 1-octene from metallocene catalysts

“…The similar effect of rr triads and ethylene units has been interpreted as due to the increase of flexibility of iPB chains induced by both rr triads and ethylene units, making the crystallization kinetics of form I in nonisothermal conditions competitive with that of form II (intrinsic flexibility effect) [23,24,26]. Moreover, the presence of high concentration of these defects also produces a decrease of the thermodynamic stability of form II, through the reduced stability of the 11/3 helical conformation of form II, compared to the 3/1 helical conformation of form I [23,24,26]. In fact a random and uniform distribution of these defects along the chain reduces the average length of crystallizable butene units, and this decrease is expected to have a greater influence on the stability of the complex 11/3 helix of form II rather than on the 3/1 helix of forms I and I' (interruption effect).…”

“…It is worth noting that the pure form I' has been obtained by crystallization from the melt or from the amorphous phase in stereoirregular iPB homopolymer samples, containing concentration of rr stereodefects higher than 2e3 mol% [23,24], and in buteneeethylene copolymers with ethylene concentration higher than 6 mol% [26], prepared with metallocene catalysts. The similar effect of rr triads and ethylene units has been interpreted as due to the increase of flexibility of iPB chains induced by both rr triads and ethylene units, making the crystallization kinetics of form I in nonisothermal conditions competitive with that of form II (intrinsic flexibility effect) [23,24,26].…”

“…Only the recent availability of novel samples of iPB and its copolymers prepared with metallocene catalysts [22e26], characterized by a controlled molecular structure in terms of stereoregularity, random distribution of stereodefects and of comonomeric units along the polymer chains and uniform comonomer concentration, has allowed understanding the effect of stereodefects [23e25] and comonomer units [26], on the polymorphic behavior and mechanical properties of iPB [23e26]. In particular, it has been found that stereodefective iPB samples, containing rr stereodefects in concentration higher than 2e3 mol% crystallize from the melt directly in the form I [23,24].…”

“…Moreover, in buteneeethylene copolymers from highly isospecific metallocene catalysts [26], it has been found that the presence of ethylene accelerates the form II / form I transition at room temperature and produces increase of flexibility and ductility, and at concentration of nearly 6 mol% favors the direct crystallization from the melt of the form I' [26]. A significant modification of the properties of iPB is observed for ethylene concentration higher than 8 mol%, with development of elastomeric properties, never observed for the iPB homopolymer [26].…”

“…We found a surprising result that high concentrations of octene comonomer units induce crystallization of form I, contrary to expectation from the pioneer studies of TurnereJones [3], and development of elastomeric properties. While elastomeric properties in buteneeethylene copolymers are quite expected [26], development of elastic properties as a result of incorporation of long octene units is not obvious and deserves deep investigation.…”