Copolymers of ethylene and 1-heptene

1. A copolymer comprising units derived from ethylene and 1-heptene wherein the peak force at break per millimeter of film thickness P of a blown film made of the copolymer satisfies the following relationship:

P> 1.9 C− 127  (1)

wherein P is expressed in N/mm and determined on a 50 micrometer thick blown film according to ASTM-D5748-95; and

C is given by the relationship C=D(1−CFW), wherein D is the density of the copolymer measured in kg/m 3 according to ASTM-D-792 and CFW is the weight fraction of the copolymer having a crystallization point of 75° C. or higher as determined using crystallization analysis fractionation, and C is greater than or equal to 550.

2. The copolymer of claim 1 wherein P satisfies the following relationship:

P> 2.7 C− 563  (1a)

wherein P and C have the same meanings as in formula 1 in claim 1 .

3. The copolymer of claims 1 wherein P satisfies the following relationships:

P> 3.4 C− 944  (1b)

wherein P and C have the same meanings as in formula 1 in claim 1 .

4. A copolymer according to any of the claims 1 - 3 wherein the density D is from 890 to 935 kg/M 3 and the melt index as measured according to ASTM-D-1238 Procedure A, Condition E at 190° C./2.16 kg is from 0.3 to 25 g/10 min.

5. A copolymer according to claim 4 wherein the density D is from 895 to 930 kg/m 3 .

6. A copolymer according to claim 5 wherein the density D is from 901 to 928 kg/m 3 .

7. A copolymer according to claim 4 wherein the melt index is from 0.5 to 10 g/10 min.

8. A copolymer according to any of the claims 1 - 3 wherein the fraction CFW is from 0 to 0.5.

9. A composition comprising a copolymer of any of the claims 1 - 3 .

10. An article prepared from or containing a copolymer of any of the claims 1 - 3 .

11. An article of claim 10 which is a film.

12. An article of claim 11 which is a packaging film.