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For example, polyethene is a macromolecule. So each bag of polyethene should be one molecule. Doesn't tearing it therefore involve breaking strong covalent bonds? Shouldn't the energy needed to do this be very high? But it tears pretty easily, wondering why this is so.

Any insights welcome!

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PE/PP bags are not formed by a single macromolecule.

Their macromolecules are linear with very occasional branching. The mass of each such a macromolecule is many orders of magnitude smaller than the mass of the bag.

So called giant macromolecules aka network covalent bonding have mass/size equal or at least comparable to the mass/size of the solid object. Their atoms need bonding to 3-4 neighbor atoms, forming 3D structure of chemical bonds (diamond, quartz), or 2D structure (graphene, independent or as a part of graphite)

The tear beyond elasticity likely starts with breaking these intermolecular bonds keeping the individual molecules together; these are weaker than the covalent $\ce{C-C}$ and $\ce{C=C}$ bonds within a polymer chain.

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    $\begingroup$ @buttonwood I have taken the tearing question as conditional one, with the condition not met. :-) Thanks for improving my English and Android mistyping. $\endgroup$
    – Poutnik
    Oct 23 at 7:27
  • $\begingroup$ No worry. Both words probably are not used frequently enough to be considered in the default spell check, except if they offer to add words (like the ones for Firefox, or Emacs, etc.) because their user encounters them with intent. In a case of changing semantics (like calender/calendar/colander), the addition for subsequent automatic correction (flyspell like) might even be considered as the road not to be taken. $\endgroup$
    – Buttonwood
    Oct 23 at 9:35

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