Crystallinity in isotactic polyolefins with unbranched side chains

Abstract The crystalline forms of several isotactic unbranched polyolefins in the series polyhexene‐1 to polyoctadecene‐1 have been investigated by X‐rays. Polytetra‐, ‐hexa‐, and ‐octadecene‐1 each crystallise three‐dimensionally in two forms, Types I and II. Both types give characteristic diffraction patterns and also show strong low angle reflections whose spacings (lower in the Type II than in the Type I series) vary linearly with the length of the side chains. In both forms the main chains are aligned in planes and the side chains are fully extended on either side of the main chain axis and pack parallel to each other as in the n ‐paraffins. The unit cells of the more crystalline Type II forms are orthorhombic and it is deduced that the main chain configuration is a quaternary helix; the side chains are tilted by 130.6° to the axes of the helices and pack side‐by‐side as in orthorhombic polyethylene. In the Type I form the side chains are extended at right angles to the main chain. The spacings of the low angle reflections given by crystalline polydodecene‐1, polydecene‐1 and polynonene‐1 place these polymers also in the Type I series. Polyhexene‐1 crystallises with a 7 2 helical configuration of main and side‐chains. Polyheptene‐1 shows two forms but the type of crystallinity is uncertain. The changeover from crystallinity involving an entirely helical configuration, as in the series polybutene‐1 to polyhexene‐1, to the I/II type of crystallinity is reflected by a minimum in the curve of melting point vs. side‐chain length. The increasing melting points from polynonene‐1 to polyoctadecene‐1 lie close to those of corresponding n‐paraffins. A third partially ordered form of the three highest members, and the diffraction patterns of the amorphous polymers are briefly discussed.