Intramolecular hydride shift polymerization by cationic mechanism. II. Spectroscopic analysis of poly‐3‐methylbutene‐1

Abstract

The structures of cationically obtained poly‐3‐methylbutene‐1, isotactic poly‐3‐methylbutene‐, and hydrogenated 3,4‐polyisoprene have been investigated by nuclear magnetic resonance and infrared spectroscopy. These studies corroborate the previously proposed structure of an α,α′‐dimethylpropane repeating unit for the cationic polymer and they confirm the conventional 1,2 head‐to‐tail enchainment for the isotactic polymer and for the hydrogenated 3,4‐polyisoprene. The NMR spectra of cationic poly‐3‐methylbutene‐1 and polyisoprene are very similar and show only two peaks. They are quite different from the spectra of isotactic poly‐3‐methylbutene‐1 and hydrogenated 3,4‐polyisoprene. The isotactic product was solubilized by careful pyrolysis to render it amenable to spectroscopic studies. Unfortunately, present‐day NMR spectroscopy does not possess high enough resolution to be useful in the quantitative investigation of mixtures of 1,3 and 1,2 enchainments. The NMR spectrum of polyisobutylene oxide can be used to distinguish cationic poly‐3‐methylbutene‐1 from the isotactic modification and from the hydrogenated 3,4‐polyisoprene. The more important bands have been analyzed and assigned. Isotactic poly‐3‐methylbutene‐1 and hydrogenated 3,4‐polyisoprene both have a conventional 1,2 structure, and their infrared spectra are essentially identical.

gem‐Dimethyl Effects in the Thermal Behavior of Polyethylene

“…[24][25][26][27][28][29][30] Instead of yielding the expected product of olefin polymerization (polyethylene with an isopropyl group on every other carbon), the polymerization proceeded via a 2,3-type hydride shift to form a tertiary carbocation during each propagation step, yielding a gemdimethyl on every third carbon. They reported that if the Scheme 1.…”

“…[27] This was most likely due to the presence of non-hydrogen-shifted repeat units in the polymer, although the NMR techniques of the day were not resolved enough to determine the specific content of the two repeat units in the polymer samples. [30] However, when the polymerization was conducted at À130 8C, the polymer was exclusively hydrogen shifted and consequently was 25-30% crystalline. This semicrystalline polymer exhibited a viscosity average molecular weight of approximately 50 000 g Á mol À1 with a T m of %55 8C and a T g of À12 8C.…”

“…In related work conducted in the early 1960's, Kennedy and coworkers synthesized polyethlene with gem ‐dimethyl groups on every third carbon via a cationic polymerization of 3‐methyl‐1‐butene in the presence of aluminum chloride 24–30. Instead of yielding the expected product of olefin polymerization (polyethylene with an isopropyl group on every other carbon), the polymerization proceeded via a 2,3‐type hydride shift to form a tertiary carbocation during each propagation step, yielding a gem ‐dimethyl on every third carbon.…”

“…They reported that if the polymerization was conducted above −120 °C, high conversions could be achieved, but the polymer formed was completely amorphous and rubbery 27. This was most likely due to the presence of non‐hydrogen‐shifted repeat units in the polymer, although the NMR techniques of the day were not resolved enough to determine the specific content of the two repeat units in the polymer samples 30. However, when the polymerization was conducted at −130 °C, the polymer was exclusively hydrogen shifted and consequently was 25–30% crystalline.…”