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Summary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC‐MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples.

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Copolymer properties depend on several parameters and one of the most important is its composition. Typically, this composition is determined by 13C nuclear magnetic resonance (13C NMR) analysis, which requires a long acquisition time and a fine adjustment of parameters to obtain the desired resolution. In this article, a method to determine the comonomer content of polymers based on the analysis from gel permeation chromatography (GPC) with an infrared (IR) detector is described. This GPC‐IR method is faster than NMR and can be carried out routinely during the GPC analysis. Three sets of ethylene/propylene copolymers synthesized using metallocene and Ziegler–Natta catalyst over a wide composition range. Linear and second order polynomial equations were checked for the relation between the ethylene percent and the IR CH3/CH ratio. Average deviation between GPC‐IR and NMR composition values was about 1% and allow conclude that this technique can be considered to be a goodalternative. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers