A composition capable of being biaxially oriented over
a temperature range 98—125° C. without prior cross
linking consisting essentially of an ethylene-butene-l co
polymer having a density in the range 0.920—0.945 and
containing 2-10 percent by weight butene-l said copoly
mer being polymerized in the presence of a catalyst con
sisting essentially of an aluminum trialkyl and a mixture
of ammonium silico?uoride and chromium oxide on a
silica-alumina support said copolymer having the char
acteristic that at its crystallization temperature it crystal
lizes uniformly at a rate of increase in crystallinity with
decreasing temperature of greater than 10%/° C. in the
range 20-40% crystallinity and a rate of increase in
crystallinity with decreasing temperature of less than
10%/° C. thereafter.
This