It is well-established that micro and nanoplastics (MNPLs) are released from polymers through environmentally triggered bond breaking. However, the mechanism by which this A-level process leads to nm-μm sized fragments is poorly enunciated. Through experimental studies on three distinct chemistries, we demonstrate that only polymers with a semicrystalline morphology produce MNPLs under quiescent conditions. In this morphology, comprised of alternate crystalline and amorphous domains, chain scission occurs faster in amorphous regions. Through theoretical arguments, we show that tie molecules and bridging entanglements (connectors), which provide structural integrity to the semicrystalline structure by connecting two adjacent crystals, are preferentially broken. We propose that the cleavage of a threshold amount of connectors (i.e., scission of as little as 1% of chain bonds), leads to the spontaneous release of MNPLs. The resulting fragments comprise highly polydisperse stacks of lamellae, with an individual lamella – tens of nanometers thick - being the building block. Degradation of the crystals occurs over much longer time scales, explaining the environmental persistence of MNPLs, even under non-quiescent conditions. Since ~70 % of polymers are semicrystalline, engineering connectors may represent an effective strategy to reduce MNPL release rates.