Classical tie-molecule concentrations in a series of metallocene catalysed medium density polyethylenes have been computed and comparisons made with literature data on non-metallocene catalysed systems over a comparable density range. It is shown that a complex inter-relationship exists between the concentration of classical tie-molecules and molecular weight, lamellar thickness and crystallinity and the X-ray long period where the latter is influenced by the concentration of short chain branches. Tie-molecule concentration levels increase steeply beyond a short chain branch (SCB) content of approximately 4 butyls/1000 C atoms with a maximum value for a metallocene sample containing 9.7 butyls/1000 C atoms. Relative tie-molecule concentrations corrected for volume crystallinity decrease sharply with increasing X-ray long period and decreasing short chain branch content. Comparisons among the different catalyst systems revealed that a chromium catalysed polyethylene with 6.8 butyls/1000C atoms had a similar tie-molecule concentration to a metallocene catalysed polyethylene with 11.5 butyls/1000C atoms. Tear and impact strength data showed a continuous increase with increasing short chain branching, varying slightly with catalysts type but differing significantly at the highest concentration levels of short chain branching. These mechanical property - SCB content correlations are closely related to those between classical tie-molecule concentrations and the degree of short chain branching.