Macromolecules ARTICLE Figure 1. Gels are formed by the oxidation of thiols on a four-armed PEG with 1% H2O2 and 1 mM NaI and subsequent degradation by photogenerated radicals (In). Note the degradation products include initiator fragment-terminated chains as well as thiyl radical-terminated chains. Figure 2. Photorheometry of the normalized elastic modulus as a function of degradation time with varying initiator concentrations. The original shear elastic modulus was 7.2 ( 0.3 kPa. initiator fragments react with the sulfur atoms in disulﬁde bonds. Gels infused with varying concentrations of the LAP initiator exhibit concentration dependent degradation behavior as ex- pected. According to the theory of rubber elasticity, the elastic modulus of a material is proportional to the concentration of elastically active cross-links; in this case these would be disulﬁde bonds that connect one segment of the network to another segment of the network. As seen from the degradation of the hydrogel infused with 1.7 and 3.4 mM of initiator, doubling the concentration of radical generators (and presumably doubling the ultimate number of radicals generated) results in approxi- mately twice the decrease of modulus, from 30% to 70%. To better understand the degradation behavior and mechan- isms, one can ﬁrst calculate the ﬁnal concentration, after polym- erization and swelling, of sulfur atoms in the hydrogels, which is ∼19 mM. Using the Flory Stockmayer equation,24,25 the critical conversion at gelation is estimated as 34%, which suggest that 66% of the disulﬁde bonds must be cleaved to observe reverse gelation, the transition from a viscoelastic solid to liquid. Since thiyl radicals may rapidly recombine to form new cross-links, 66% or 12.5 mM of the sulfur must be bound to initiator fragments for complete degradation of the gel. As seen in Figure 2, initiator concentrations more than half of 12.5 mM result in total dissolution of the network, while those less than half of 12.5 mM do not, suggesting that both radical fragments, resulting from photoinitiator cleavage, participate in the disulﬁde exchange reaction. This outcome is further supported by 1H NMR and 31P NMR spectra of the degraded products. Emergence of a triplet proton peaks at 2.7 and 2.5 ppm in the 1H NMR spectrum of a completely degraded hydrogel is con- sistent with the formation of a trimethylbenzyl thioester (structure shown in Figure 3a) and a phosphonothioic acid ester (Figure 3b), the peaks associated with the protons alpha to the sulfur. Additionally, 31P NMR of the same sample indicated three distinct phosphorus-containing species at 34, 13.4, and 12.7 ppm (Figure 3). The peak at 13.4 ppm precisely matches the peak of unreacted LAP, while the peak at 34 ppm is indicative of a phosphonothioic acid ester. The peak at 12.7 ppm is consistent with a hypophosphonate which would result from the radical combination of two phosphinate radicals. The mechanism by which this degradation occurs is shown for the phosphinate fragment of the initiator (Figure 4a). Radical initiator fragments attack disulﬁdes, eﬀectively capping one sulfur while the liberated thiyl radical may undergo additional reactions, including further fragmentation with another disulﬁde (Figure 4b), termination with another radical initiator fragment, or termina- tion with another thiyl radical to re-form a disulﬁde (Figure 4c). The process by which the thiyl radical reacts with other disulﬁdes (Figure 4b) is analogous to the RAFT-mediated polymeriza- tion26,27 as well as the addition fragmentation process that has been shown to occur in allyl sulﬁde-containing polymer net- works.21,22 This latter process, by which a large number of network strands are broken and replaced with otherwise identical strands, allows the network to adapt to stresses, causing photo- induced plasticity and enabling a range of valuble property and shape changes. 2446 dx.doi.org/10.1021/ma200202w |Macromolecules 2011, 44, 2444–2450