Analyses of short chain branches in polyolefins with improved 1H NMR spectroscopy.

Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The H-1 NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved H-1 pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 mu g of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.