## 2 Covalent Interactions
### CNT-Based Nanocomposites  
#### 2.1.1 Grafting-from through Surface-Initiated Polymerization  
The \"grafting-from\" polymerization approach can produce a core-shell structure nanocomposite. This method can be used to build optoelectronic devices where obtaining an ordered interface between CNTs and the conjugated polymer is particularly hard to design but constitutes a precondition for its efficiency. Surface-initiated polymerization is the most common way to fabricate core-shell structures. The key to this process is the covalent immobilization of the initiator of the CNTs [19]. When the monomers are introduced, they can therefore grow directly from the CNT surfaces to form these structures. [PERSON] et al. [20] carried out a surface-initiated Kumada catalyst-transfer polycondensation (SI-KCTP) of the monomer 2-bromo-5-chloromagnesio-3-hexylthiophene in the presence of the initiator 1,2-bis(diphenylphosphino)propane (DPPP) and multi-wall carbon nanotubes (MWCNTs), resulting in the formation of MWCNT-g-P3 HT nanocomposites in which well-defined chains of poly(3-hexylthiophene) (P3 HT) grew from the surfaces of MWCNT while maintaining ordered heterojunctions. As seen in Figure 1, the grafting-from polymerization yielded a core-shell structure in which cylinder-shaped heterojunctions were formed leading to highly-ordered P3 HT aggregates. The SI-KCTP enabled the formation of regioregular P3 HT (as demonstrated by the observation of a peak in the nuclear magnetic resonance (NMR) spectroscopy attributed to the head- to- tail arrangement of the P3 HT repeat units) which could be harnessed to control the polymer chain lengths (the linear increase in the grafted P3 HT as a function of the monomer to initiator). The nanocomposites also displayed red-shifted and well-resolved absorption maxima, thus confirming the formation of highly ordered aggregates covalently attached to the surface of the MWCNT. The key step was found to be the anchoring of the initiator DPPP on the surface of the MWCNTs, because accurate control over the initial monomer to initiator ratio during the polymerization made it possible to adjust the length of the polymer chain length. The photoresponse capabilities of MWCNT-g-P3 HT were evaluated by charge separation under illumination. [PERSON] et al., reported that the core-shell structure acted efficiently as a donor/acceptor system because a photoinduced electron transfer was detected from the lowest unoccupied molecular orbital (LUMO) of the P3 HT (electron donor) to the Fermi level (E\({}_{\text{F}}\)) of the MWCNTs (electron acceptor) (Figure 2). They suggested that this was based on a two-step mechanism; namely, the rapid diffusion of the excitons in highly ordered P3 HT layers followed by a dissociation of these excitons within the heterojunctions between the MWCNTs and P3 HT. Figure 1: Core-shell structure of multi-wall carbon nanotube-g-poly(3-hexylthiophene) (MWCNT-g-P3 HT) nanocomposites prepared by the “grafting-from” method (Reprinted (adapted) with permission from [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; et al. Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation. _ACS Nano_**2016**, _10_, 5189–5198 [20]. Copyright (2020) American Chemical Society.). Figure 2: Energy-band diagrams of P3 HT and MWNTs (Reprinted (adapted) with permission from [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON], [PERSON]; [PERSON]; et al. Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation. _ACS Nano_**2016**, _10_, 5189–5198 [20]. Copyright (2020) American Chemical Society.). In another study, MWCNTs were functionalized by brominated groups from the initiator 2-hydroxyethyl bromosisobutyrate (HEBB) under acidic treatment assisted by ultrasonication [5].