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Here, we report on the synthesis and different crystallization behavior of linear-and star-PCL's containing a photocleavable linker (5-hydroxy-2-nitro benzaldehyde), modulated by photochemical switching. Basis is the attachment of a photocleavable moiety close to the star-core of a three-arm star poly(caprolactone), so that the crystallization behavior can be controlled via a photochemical stimulus. The polymerization of e-caprolactone using a trivalent photocleavable initiator and stannous octanoate catalyst resulted in the synthesis of different molecular weights of star-shaped photocleavable polymers. Various techniques like 1 H NMR and ESI-TOF-MS confirmed the successful synthesis of the star-shaped polymers. Complete photocleavage is ensured via GPC, HPLC, and ESI-TOF-MS. DSC studies clearly indicated the enhancement in crystallinity after photocleavage of the star-shaped poly(e-caprolactone)s. Hence, for the first time phototriggered crystallization behavior of PCL polymers is reported, where the confinement exerted by the star architecture is removed by photoirradiation. V C 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 00, 000-000 KEYWORDS: crystallization; photoclevable multivalent initiator; photocleavable polymer; ring-opening polymerization; star poly(e-caprolactone)s INTRODUCTION Crystallization of polymers strongly depends on their molecular architecture, therefore presenting an important factor of structural control. Besides linear polymers or polymer networks, cyclic and star-polymers play an important role in the crystallization of polymers, for example, poly(e-caprolactone)s, where the effect of stararchitectures has been studied intensely by various authors in the past. [1][2][3][4][5][6][7][8][9] The rate of crystallization of star-shaped PCL's is observed to be lower than that of linear polymers, where the melting temperature, the crystallization temperature and the crystallinity X c of the respective s-PCL polymers increases with increasing molecular weight.2 Comparison of the crystallization behavior of different PCl-architectures displaying the same molecular weight showed a significant decrease in the degree of crystallinity in the order linear-, three-arm, four-arm-, and six-arm-PCL's, mostly explainable by disruption of the fold-patterns of the crystals by the free chain ends during crystallization, specifically with short arms.1 For example, a star-shaped resorcinarene centered eight arm poly(e-caprolactone) was reported by Shen et al.