Gelation in Photoinduced ATRP with Tuned Dispersity of the Primary Chains

Abstract

We investigated gelation in photoinduced atom transfer radical polymerization (ATRP) as a function of Cu catalyst loading and thus primary chain dispersity. Using parallel polymerizations of methyl acrylate with and without the addition of a divinyl crosslinker (1,6-hexanediol diacrylate), the approximate values of molecular weights and dispersities of the primary chains at incipient gelation were obtained. In accordance with the Flory–Stockmayer theory, experimental gelation occurred at gradually lower conversions when the dispersity of the primary chains increased while maintaining a constant monomer/initiator/crosslinker ratio. Theoretical gel points were then calculated using the measured experimental values of dispersity and initiation efficiency. An empirical modification to the Flory–Stockmayer equation for ATRP was implemented, resulting in more accurate predictions of the gel point. Increasing the dispersity of the primary chains was found not to affect the distance between the theoretical and experimental gel points and hence the extent of intramolecular cyclization. Furthermore, the mechanical properties of the networks, such as equilibrium swelling ratio and shear storage modulus showed little variation with catalyst loading and depended primarily on the crosslinking density.

Controlling primary chain dispersity in network polymers: elucidating the effect of dispersity on degradation

“…We attribute this difference to the closer proximity of the propagating polymer chains within the network, which could result in a higher probability of termination through intramolecular combination reactions and also the reduced efficiency of deactivation by chain transfer due to the highly viscous nature of the synthesis. 59,74 This is evidenced by a small amount of high molecular weight shouldering in the molar mass distributions of the various cleaved polymer networks (Fig. 3b) and demonstrates that analogous linear and network polymer syntheses do not yield identical materials.…”

“…However, to the best of our knowledge, there are only limited examples where dispersity-controlled networks have been prepared on variation of the catalyst concentration in ATRP. 59,60 Matyjaszewski and coworkers demonstrated that decreasing copper concentration in activator regenerated by electron transfer (ARGET) ATRP resulted in a higher primary chain dispersity and an earlier gel point. 60 However, the primary chain dispersity of the resulting networks was not measured.…”

“…Most recently, Kopec and coworkers reported the primary chain dispersity of networks by using dispersity values obtained from analogous linear polymerization reaction conditions. 59 There is therefore an ongoing challenge in terms of both the synthesis and characterization of these materials, so any influence of primary chain dispersity on many properties has yet to be ascertained. This includes the effect on degradability, which is arguably one of the most useful and important properties that can be incorporated into networks, with precise tuning of network degradation essential for numerous applications including drug delivery, tissue engineering, wound healing, pollution control, agriculture (water and nutrient retention in soil) and personal care (cosmetics and skin care).…”

Network Polymer Properties Engineered Through Polymer Backbone Dispersity and Structure

“…This can be defined as strongest materials or materials most capable of swelling with solvent, which has been connected to overall network uniformity [14,58] . Indeed, this is an unusual result, because in general RDRP methods lead to more uniform networks than their FRP counterparts due to the control over the primary chain structure [14,59–62] . This highlights the importance of control over the primary chain dispersity when designing polymer materials.…”

Are polymer gels synthesized by free radical polymerization with cleavable crosslinkers really degradable?

“…where vc is the weight-average number of crosslinks per primary chain (equal to 1 at incipient gelation) and ρc is the fraction of crosslinked units. 22,51,52 Hence, only very few crosslinked units (ρc ≥ 1/DPw) are necessary to reach the gel point for the typical DPw values.…”