Novel metallocene catalysts for the synthesis of ethylene/α‐olefin copolymers are reviewed here. The technology used—single‐site constrained geometry catalyst technology—is demonstrated to be useful for the preparation of a wide array of copolymers with unique materials properties, such as a high melt fracture resistance, as illustrated in the Figure.

ABSTRACT:Copolymerization of ethylene and styrene by the INSITE™ technology from Dow presents a new polymer family identified as ethylene-styrene interpolymers (ESI). Based on the combined observations from melting behavior, density, dynamic mechanical response, and tensile deformation, a classification scheme with 3 distinct categories is proposed. Polymers with up to 50 wt % styrene are semicrystalline and are classified as type E. The stress-strain behavior of low-crystallinity polymers at ambient temperature exhibits elastomeric characteristics with low initial modulus, a gradual increase in the slope of the stress-strain curve at higher strains, and large instantaneous recovery. The structural origin of the elastomeric behavior is probably a network of flexible chains with fringed micellar crystals serving as multifunctional junctions. Polymers with more than 50 wt % styrene are amorphous. Because the range of glass transition temperatures encompasses ambient temperature (nominally 25°C), it is useful to differentiate ESIs that are above the glass transition as type M and those that are below the glass transition as type S. Type M polymers behave as rubber-like liquids. They have the lowest modulus and lowest stress levels. Some elastic characteristics are attributed to the entanglement network. Type S polymers exhibit large strain rate sensitivity with glassy behavior at short times and rubbery behavior at longer times. The term ''glasstomer'' is coined to describe these polymers. The division between type M and type S is based on chain dynamics, rather than solid state structure, and thus depends on the temperature of interest. At ambient temperature, ESIs with 50 to 70 wt % styrene are classified as type M; polymers with more than 70 wt % styrene are classified as type S.

Using a combination of differential scanning calorimetry and quasi-isothermal temperaturemodulated calorimetry, we investigated the temporal evolutions of the melting temperature, degree of crystallinity, and excess heat capacity during crystallization of linear polyethylene and low styrene content ethylene-styrene copolymers. Describing isothermal crystallization as the succession of three stages (primary, mixed and secondary crystallization stages), we established the following correlations: (1) the evolution of the melting temperature with time parallels that of the degree of crystallinity, (2) the excess heat capacity increases linearly with degree of crystallinity during the primary stage, reaches a maximum during the mixed stage, and decays during the secondary stage, (3) the rate of decay of the excess heat capacity parallels the rate of secondary crystallization, and (4) the rates of shift of the melting temperature and decay of the excess heat capacity lead to apparent activation energies that are very similar to these reported for the crystal R c relaxation by solid-state NMR, dynamic mechanical, and dielectric spectroscopies. Strong correlations in the time domain for secondary crystallization by lamellar thickening and evolution of the excess heat capacity suggest that the reversible crystallization/melting phenomenon is associated with molecular events in the melt-crystal fold interfacial region. Specifically, we conclude that the excess heat capacity observed during the high-temperature crystallization of linear polyethylene and low styrene content copolymers is most likely to originate from the segmental processes in the crystal/ melt fold region that have been discussed by Fischer, Mansfield, and Strobl. These studies also provide preliminary indications that the excess heat capacity observed during crystallization at lower temperatures in the case of ethylene copolymers of high comonomer content is consistent with the lateral surface model proposed by Wunderlich.

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