Mechanophores activation

Silberstein MN, Cremar LD, Beiermann BA, Kramer SB, Martinez TJ, White SR, Sottos NR (2014) Modeling mechanophore activation within a viscous rubbery network. J Mech Phys Solids 63 141-153... 

Compared with flow fields, ultrasound irradiation creates both solvodynamic and thermal effects. The latter may contribute to polymer degradation and mechanophore activation. Although, ideally, hot spots are quenched faster than the diffusion time of the polymer chain (less than 1 ps [110]), an effort to preclude thermal effects should be implemented in the experiment. 

Brown CL, Craig SL (2015) Molecular engineering of mechanophore activity for stress-responsive polymeric materials. Chem Sci 6 2158-2165... 

Larsen MB, Boydston AJ (2013) Flex-activated mechanophores using polymer mechanochemistry to direct bond bending activation. J Am Chem Soc 135 8189... 

Kean ZS, Ramirez ALB, Craig SL (2012) High mechanophore content polyester-aiaylate ABA block copolymers synthesis and sonochcanical activation. J Polym Sci Part A Polym Chem 50 3481... 

Kryger MJ, Munaretto AM, Moore JS (2011) Structure-mechanochemical activity relationships for cyclobutane mechanophores. J Am Chem Soc 133 18992... 

Mechanical Degradation and Activation of Mechanophore in Star-Shaped... 

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

Fig. 2 Schematics of a mechanophore embedded in a polymer chain [15]. (a) The mechanophore is in the middle of the chain, (b) Control polymer having a mechanophore at the chain terminus or (c) as a pendent group. The red stick in the mechanophore represents the active bond. Possible responses to force for a mechanophore in the middle of the chain (d) selective scission (e) release of small molecule (f) isomerization...

This review is organized on the basis of polymer architecture and highlights its effect on polymer mechanochemistry. The topic is restricted mostly to CST, chain degradation, and activation of mechanophores in dilute solution because more experimental and theoretical literature is available than that relating to the solid state. Solution or solid phenomena in which no CST or mechanochemical reaction occurs are therefore excluded. 

From (5) and (6), the shorter of the polymer chains, the higher value of kc is required to observe CST. As k exceeds a seccmd critical value ep, the hydrodynamic force is sufficient to overcome the binding strength of the covalent bonds ( 7 nN, depending on timescale [128]) and the chain scission occurs. The force required to activate the mechanophore may be weaker or close to that to break the covalent bond. The related critical strain rate a is above kc and close to ep (Fig. 9). Below Miim, CST, activation of mechanophore, and chain scission cannot be observed for the given experimental setup (maximum strain rate attainable). 

In QSSF [27, 28, 97-102, 129], FTF [89, 105-113], and turbulent flow [114, 130], polymer chains are broken at the midpoint along the backbone. Similar results are observed in activation of mechanophores under ultrasonic irradiation, where the mechanochemical transduction is only effective at the midpoint of the chain (Fig. 2a) [15, 67, 119, 120]. 

So far, activation of mechanophores in the cyclic chain has not been reported. One important issue is the location of the mechanophore in the cyclic polymer. If only one mechanophore is incorporated into a ring chain (Fig. 18a), it is unlikely to experience the maximum hydrodynamic force (red dots) because the ring has no definitive midpoint in the flow field. Even if the ring chain breaks, the positimi of the mechanophore is unlikely to locate just at the midpoint of the linear product To improve the chance of activation, it is better to have multiple mechanophores incorporated into the cyclic chain, such as random, alternative, or block cyclic copolymers. For example, in Fig. 18b the mechanophores are randomly dispersed into a cyclic macromolecule to increase the activation probability. If the cyclic chain breaks, the mechanophores still have the chance to be located near the midpoint of the linear chain. The linear fragment then undergoes CST and activates the mechanophores. 

Fig. 18 Activation of mechanophore in cyclic polymers, (a) A mechanophore is incorporated into a ring chain, (b) The mechanophores form a block of the ring chain. Red dots represent probable positions of maximum hydrodynamic drag force. Blue and green ovals denotes mechanophores...

Bulk mechanochemistry. Unlike linear polymers, the activation of mechanophore in nonlinear macromolecules in bulk is almost blank. Recently, May found that polymers with branched architectures activate more slowly than linear counterparts in solution, yet more quickly in solid-state tensile experiments [198]. In the bulk, more factors take part in the chain degradation event, including but not limited to chain entanglements, phase separation, crystallization, and supramolecular interactions. Inspections in this direction can aid the design of mechanoresponsive materials in the solid state. 

Chen Y, Zhang H, Fang X, Lin Y, Xu Y, Weng W (2014) Mechanical activation of mechanophore enhanced by strong hydrogen bonding interactions. ACS Macro Lett 3 141-145... 

Recent efforts to report and to repair mechanical damage with mechanochemical reactions form the subject of the current chapter. We start with a brief discussion of the use of the spiropyran unit as a mechanophore for reporting strain. Spiropyran mechanochemistry inspired the development of another stress probe, the highly sensitive mechanoluminescent dioxetane, whose application as scission reporter in several types of polymeric materials is discussed. The chapter continues with a description of recent efforts to develop productive mechanochemistry, where initial scission leads to the formation of new bonds. Bond formation is either induced by the scission of covalent bonds, e.g. by the opening of rings, or bonds are formed under the action of a latent catalyst when it is activated by mechanochemical dissociation of a Lewis acid-base pair. These examples of productive mechanochemistry offer exciting possibilities to develop new modes of self-healing in... 

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