Internal Plasticization of Poly(Vinyl Chloride) using Thermal Azide-Alkyne Huisgen Cycloaddition and Copper-Mediated Atom Transfer Radical Polymerization
Poly(vinyl chloride) (PVC) is one of the most widely used thermoplastics; uses range from building materials, medical devices, toys, and sports equipment. Pure PVC is rigid and brittle. Typically, small molecule plasticizers are added to modify the flexibility and durability of PVC. The most common external plasticizers are phthalate esters. These small molecules leach out of the PVC matrix into the environment; when inhaled, absorbed, or ingested into the human body, phthalates and their metabolites pose a significant risk to human health. The most efficient way to prevent leaching of plasticizers is to covalently attach them to PVC. This is referred to as “internal plasticization.”
Two strategies have been used to achieve internal plasticization of PVC in this thesis. In the first strategy, thermal azide-alkyne Huisgen cycloaddition was utilized to attach electron-poor acetylenediamides using a branched glutamic acid linker to azidized PVC, incorporating four plasticizing moieties per attachment point. A systematic study incorporating either alkyl or triethylene glycol esters provided materials with varying degrees of plasticization, with depressed glass transition temperature (Tg) values ranging from −1 °C to 62 °C. Tg values of these internally plasticized PVC samples were shown to decrease with increasing chain length of the plasticizing ester. A branched internal plasticizer bearing a triethylene glycol ester had lower Tg values compared to that with a same length linear alkyl ester. Thermogravimetric analysis of PVC bearing internal plasticizers revealed that these branched internal plasticizers bearing alkyl ester chains are more thermally stable than similarity branched plasticizers bearing ethylene glycol esters. These internal tetra-plasticizers were synthesized and attached to PVC-azide in three simple synthetic steps.
In the second strategy, internal plasticization of PVC was achieved in one step using copper-mediated atom transfer radical polymerization (ATRP) to graft random n-butyl acrylate (BA) and 2-2-(2-ethoxyethoxy)ethyl acrylate (2EEA) copolymers from defect sites on the PVC chain. Five graft copolymers were made with different ratios of PBA and P2EEA; Tg values of these functionalized PVC polymers ranged from -28 °C to -50 °C. Single Tg values were observed for all polymers, indicating good compatibility between PVC and the grafted chains, with no evidence of microphase separation. Plasticization efficiency is higher for polyether P2EEA moieties compared with PBA components. The resultant PVC graft copolymers were thermally more stable compared to unmodified PVC. Increasing the reaction scale from 2 g to 14 g produced consistent and reproducible results, suggesting this method could be applicable on an industrial scale. Further optimizations of the ATRP conditions were carried out shortening the reaction time and varying the acrylate monomer to VC unit ratios. Nine different internally plasticized PVC graft copolymers with different weight percents of plasticizer spanning from 24% to 75% were prepared. A wide range of Tg values (-54 °C to 54 °C) were achieved, with Tg values below zero for samples with weight percent of plasticizer more than 50%. In summary, highly effective internal plasticization of PVC was accomplished by Cu-mediated ATRP in only one step. Whereas the azide-alkyne approach may be suffered from the potential danger in handling azides on large scale, the ATRP graft copolymerization approach is expected to be very attractive to industry, to afford internally plasticized PVC products with reliable and durable physical properties.