No convincing experimental exists that high molecular weight polyethylene chains are biodegradable at any measurable rate by enzymatic action. Numerous studies have suggested that the upper limit for aliphatic hydrocarbon chain is 20 carbon atom.[1]

There are some enzymatic reactions that degrade hydrocarbon chains include enzymatic oxidation, dehydrogenation and carbon-carbon bond breaking (see Figure 1). These reactions can preseumably only be carried out on low molecular weight hydrocarbon chain.
hydrocarbon_degradation_1.gif

Figure 1[1] : Variety of ways in which enzymes can modify low molecular weight hydrocarbon chains. E1 to E7 are generic enzymes.
It is open question whether the enzymatic processes occur as cell surface or if the hydrocarbon needs to be taken inside the cell to carry out the enzymatic reaction. If the hydrocarbons need to be taken inside the cell, then that would explains why only low molecular weight hydrocarbon chains can be biodegraded.

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  1. ^ Lenz RW. 1993. Biodegradable polymers. Advances in Polymer Sciences. 107:1-40.