Assignment of Regioirregular Sequences in the 13C NMR Spectrum of Syndiotactic Polypropylene

Abstract

The 13C NMR microstructure of a polypropylene (PP) sample is a fundamental source of information on its properties, and also a ‘fingerprint’ of the catalytic species used to produce it. Likely due to a much greater technological importance, isotactic polymers (i-PP) have been more thoroughly investigated that syndiotactic ones (s-PP). In this paper, we report the first full assignment of regioirregular sequences in s-PP samples made with two well-known molecular catalysts, namely a Cs-symmetric (cyclopentadienyl)(fluorenyl) ansa-zirconocene and a fluxional bis(phenoxyimine)Ti species. The results shed more light on the mechanism of chain propagation at the two catalysts, and open the door to the investigation of more elusive cases like the formation of s-PP blocks in the presence of multi-sited heterogeneous Ziegler-Natta systems.

Regioirregular Monomeric Units in Ziegler–Natta Polypropylene: A Sensitive Probe of the Catalytic Sites

“…Therefore, the 13 C NMR fingerprint of the 2,1 units in this fraction (Figure ) was compared with that observed in a model s-PP sample made with a molecular catalyst (D of Scheme ) whose peculiar mechanism of stereocontrol ends up with a perfect mimic of genuine chain-end control. − It should be noted that for the latter catalyst chain propagation occurs predominantly with 2,1 regiochemistry, and therefore the isolated regiodefects are 1,2-inserted units; however, far from the chain ends which of the two regiochemical insertion modes prevails is immaterial with respect to the 13 C NMR fingerprint. A recent study concluded that in s-PP samples made with catalyst D the regioirregular units are also stereoirregular (see insert in Figure and Scheme , left); the very close similarity of their 13 C NMR fingerprint with that of fraction 3 (Figure and Scheme , right) clearly indicates that the same holds for the regiodefects in the latter fraction. This is nicely consistent with the model of Figure C, and with the experimental finding that fraction 3 is even less regioregular than fraction 2 (0.20% vs 0.14% 2,1 units; Table ).…”

“…This is nicely consistent with the model of Figure C, and with the experimental finding that fraction 3 is even less regioregular than fraction 2 (0.20% vs 0.14% 2,1 units; Table ). It may be worthy to add here that no similarity instead was envisaged with the 13 C NMR fingerprint of the isolated regioirregular 2,1 units in a model s-PP sample prepared with a C s -symmetric ansa -zirconocene catalyst (E of Scheme ) under chain-migratory enantiotopic-sites control (Figure , top insert); 2,1 insertion for the latter catalyst is highly enantioselective …”

“…It may be worthy to add here that no similarity instead was envisaged with the 13 C NMR fingerprint of the isolated regioirregular 2,1 units in a model s-PP sample prepared with a C s -symmetric ansa-zirconocene catalyst (E of Scheme 2) under chain-migratory enantiotopic-sites control (Figure 6, top insert); 2,1 insertion for the latter catalyst is highly enantioselective. 24…”

“…Fractions 1–3 were isolated by exhaustive Kumagawa extraction with the appropriate solvents in sequence. Isotactic PP samples obtained with single center catalysts, used as model compounds, were obtained with a Freeslate PPR48 platform, using well established protocol procedures. , Syndiotactic PP samples were prepared according to the literature . For details, see the Supporting Information.…”

“…22,23 Syndiotactic PP samples were prepared according to the literature. 24 For details, see the Supporting Information.…”