A Critical Evaluation of Infrared Methods for Determination of the E/P Ratio of Ethylene Propylene Rubbers

A general method of analyzing ethylene—propylene (EP) copolymers for comonomer content is needed which is applicable to all types of copolymers: random copolymers (I), … EPEEPEPPEPEEE …; alternating copolymers (III), … EPEPEPEP …, block copolymers (II), … PEE … EEPP … PPEE … EEP …; and copolymers containing random (or alternating) segments together with blocks along the chain, i.e., mixed I or III, and II. Random copolymers can contain odd-numbered sequences of CH2 groups if the EP units are head-to-tail; if some head-to-head, tail-to-tail addition occurs, then even-numbered sequences of CH2 groups will also be present (IV; structures I—IV are shown on the next page). Block copolymers sometimes have crystalline polyethylene blocks and may also contain crystalline, isotactic polypropylene blocks. Mixed copolymers are more complex in structure than either random or block copolymers considered individually. In view of the lack of chemical methods for assessment of copolymer type or even for the determination of the propylene content of a copolymer, the most fruitful approach so far has been in the application of infrared spectroscopy. As CH2 groups in the copolymers can appear in many different environments, the method of analysis should be primarily concerned with the assay of CH3 groups. Even if appreciable head-to-head, tail-to-tail addition occurs, these methyl groups are separated by two carbon atoms, and little interaction should occur. Consideration has also been given to the measurement of ethylene content or total thickness in order to complete the basis for the calculation of the E/P ratio.