High-density polyethylene was thermolysed with short-chain organic compounds (additives) with an ester functional group in the presence of free radical initiator such as dicumyl peroxide at 160 degrees C. The experimental results inferred that an additive with a hydrogen donor containing a thiol group showed better efficiency toward functionalization and an additive with a methylene bridged group showed better efficiency toward cross-linking. The FTIR spectral results indicated that the area of the carbonyl peak at 1730 cm(-1) increased with the increase in percentage grafting of additives with simultaneous increase of cross-linking. The ester grafting was further confirmed by saponification reaction. The possibility of acid functionalization via intramolecular hydrogen transfer was also checked through acid hydrolysis of the saponified products. A suitable reaction mechanism was proposed in order to explain the experimental and spectral data obtained. (c) 2005 Wiley Periodicals, Inc.