Conversely: for substances of similar structure and  comparable polarity  intermolecular attractive forces increase with increasing molecular weight. Thereby increasing melting point  with increasing molecular weight.

Depends on a few things. Firstly, how did you carry out the DSC exp? Did you take the data from the 2nd heating scan? when you say molecular mass, do you mean Mn or Mw? Is the material hygroscopic or hydrophillic?

What about the crystallization temperature and crystallinity?

Most importantly,  this kind of question is trivial if reactant and biproducts are not mentioned.

A little detail about the material would help.

About 5 mg of each sample was used for the analyses against a standard reference.  Both the 1st and 2nd heat scan show that the fatty betadiketone (reactant) had higher melting point than the product (after alkylation of the betadiketone with methylacrylate)

I am not yet certain of hygroscopic or hydrophilic nature of either of these materials

Kaana, many factors come into play when considering melting points  compounds. While molecular mass has effect on the melting point, melting pointing is not always directly related to molecular mass. Take a simple case of H2O and H2S. In facts many factors can be considered, but one clear thing is that these factors vary for compound(s).

When you have what you people have observed above, just sit down and look at the chemistry of the compounds involved and I guess you will be able to interpret the anomaly observed, as you have not given these compounds.

Thanks