Effects of tetraphenylethylene groups on the crystallization of poly(ʟ-lactide) with different molecular weights

Degradable polymers incorporated with chromophore molecules have great potential in biomedical field. In this paper, hydroxyl functionalized tetraphenylethylene (TPE-2OH) was employed as the initiator to synthesize two-armed poly(ʟ-lactide) (PLLA) with varying molecular weights (TPE-PLLA-L, TPE-PLLA-M and TPE-PLLA-H) by ring-opening polymerization (ROP). Their crystallization behaviors were compared with those of PLLA with similar molecular weights initiated by ethylene glycol (EG-PLLA-L, EG-PLLA-M and EG-PLLA-H). A significant inhibiting effect on crystallization was found in TPE-PLLA-L with low molecular weight (Mw = 6400 g mol-1, ∼40 repeating units per chain) because of the large and rigid TPE cores, which significantly restricted the folding and transportation of the short PLLA chains and resulted the formation of smaller lamellar thickness (lc = 3.9 nm). However, when the chain length of PLLA increased, the proportion of TPE in the TPE-PLLA molecular decreased dramatically, and the nucleation of crystallization was improved because of the aggregation of TPE cores in TPE-PLLA-M (Mw = 13600 g mol-1, ∼90 repeating units per chain) and TPE-PLLA-H (Mw = 19000 g mol-1, ∼125 repeating units per chain) in the molten state. After the crystallization of TPE-PLLA, the aggregation state of TPE groupschanged. This work shed some light on the crystallization behavior of PLA with fluorescence properties.