Thermoresponsive Systems, Dendrons, and Dendrimers

It should be noted that although no distinction is made between the endo and exo stereoisomers (Fig. 12.25) of furan-maleimide adduct (assuming that their thermal behavior is at least similar if not equal), differences in thermal reversibility (rDA, DA) can have an important in uence on the material properties, especially in the case of thermoremendable polymers and networks. For instance, by breaking only the covalent bonds of the endo DA-isomers inside the network imder mild conditions and reforming them in a less stressed state, stresses can be relaxed. Important material properties such as durability, adhesion to other materials, appearance of micro-voids and micro-cracks, etc., can be improved (Canadell et al., 2010). 

The reversibihty of the furan-maleimide DA reaction is responsible for its extensive use in a wide variety of thermoremendable polymers and networks, ranging from self-healing (Chen et al., 2003), nonlinear optical (Haller et al., 2004 Kim et al., 2006), hydrogel (Chujo et al., 1990), and 

To obtain segment block dendiimers, Kose et al. (2008) utilized Frechet type dendrons appended with electron rich furan at the focal point and a complementary set of polyester dendrons containing the electron de cient maleimide group at the focal point Combination of these two complimentary dendrons was accomphshed by heating in benzene to obtain thermoresponsive segment block dendrimers in good yields (Hg. 12.27). The dendiimers fell apart upon heating them to 110°C. 

Since anthracene and maleimide form high yielding stable addncts (with cycloreversion tem-peratnre over 200°C), this combination of diene and dienophile was ntilized for the synthesis of dendronized polymers via DA reaction (Tonga et al., 2010). Thns, as shown schematically in Fig. 12.28, polymer appended with anthracene gronps was reacted with dendrons containing fnran 

Problem 12.13 The combination of RAFT chemistry and the FIDA cydoadditicm provides a simple and synthetically nondemanding pathway to well-de ned macromolecular star-shaped architectures. In support of this contention describe the synthesis of polystyrene (PSt) star polymers with up to 4 arms using the aforesaid two different RAFT end groups, namely, diethoxy-phosphoryldithioformate and pyridin-2-yldithioformate, and HDA coupling reactions. 

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