# Microsphere Polymers in Molecular Imprinting: Current and Future Perspectives
## 3 Preparation of Molecularly Imprinted Microspheres
### Controlled/Living' Radical Precipitation Polymerisation (CRPP)
#### 3.2.3 Reversible Addition-Fragmentation Chain Transfer Precipitation Polymerisation (RAFTPP)  
Among all the CRPs that have been developed, reversible addition-fragmentation chain transfer (RAFT) polymerisation has become one of the most widely used CRP methods because of its effectiveness, applicability with a very broad range of monomers and its ability to perform at a mild reaction temperature [134]. The principle of RAFT polymerisation involves an appropriate chain transfer agent (CTA or RAFT agent, usually a thiocarboylthio compound such as dithiesters, dithiocarbonates, xanthates, dithiocarbates, etc. ), resulting in a product of chain transfer that also acts as a chain transfer agent with identical characteristics to its precursor transfer agent but differing in molecular weight (degenerate or degenerative chain transfer) [135]. This process also results in fast equilibrium among the active species (propagating radicals) and dormant species (thiocarbonylthio-terminated chains), creating an equal probability of forming narrowly polydisperse polymers (Figure 4a). When the process is done, most of chains will maintain the thiocarboulthio end group so it can be isolated as a stable material [5]. First introduced by [PERSON] et al. [5], reversible addition-fragmentation chain transfer precipitation polymerisation (RAFTPP) has been widely used by combining RAFT with precipitation polymerisation in MIM preparation. Merging RAFT polymerisation with precipitation polymerisation is as simple as introducing a RAFT agent into the precipitation polymerisation, producing a MIM with reactive thiocarboylthio groups on its surface, thus creating favourable properties for further surface modification (Figure 4b). [PERSON] et al. [5] demonstrated RAFTTPP using 2,4-dichlorophenoxyayacetic acid (2,4-D as the template with 4-VP as the functional monomer, EGDMA as the crosslinker, AIBN as the initiator, a methanol and water mixture (4:1 in volume, \(\geq\)98% of the total reaction volumes) as the solvent and cumyl dithiobenzoate (CDB) as the CTA and compared it with TRPP under similar conditions without adding CDB. All reactions were performed at 60 \({}^{\circ}\)C for 24 h. The template was then removed from the MIPs via centrifugation. In the end, white MIP or non-molecularly imprinted polymer (NIP, a MIP without a template) particles were obtained via TRPP, while RAFTTPP produced light pink MIP/NIP particles, demonstrating the successful incorporation of dithiester groups into the system. TRPP produced only irregular MIPs, while uniform non-molecularly imprinted microspheres (NIMs) were produced under similar conditions. This result suggests that combining RAFT into precipitation polymerisation might have a significant impact on the morphology and particle sizes of MIM/NIM. RAFTTPP produced MIM with enhanced binding capacity, selectivity for a specific target, a high binding constant, possible maximum number for high-affinity sites and a greater high-affinity binding site density that those prepared by TRPP [5]. Since then, researchers have developed RAFTTPP into various templates [57; 69; 116]. However, these MIPs are generally only compatible with organic solvents and predominantly fail to undergo specific binding in a pure aqueous environment, thus limiting their application in various fields. In response to this drawback, some studies [8; 71; 72; 102; 103; 106; 105] developed an advanced approach that effectively formed a pure water-compatible MIM via surface-initiated RAFTTPP. The procedure differed in the use of a hydrophilic comonomer (such as HEMA, \(N\)-isopropylacrylamide [NIPAAm], acrylic acid, azobenzene [azo]) as the grafted polymer. According to [PERSON] et al. [102], MIM/NIM particles grafted with polyNIPAAm (PNIPAAm) brushes showed improved dispersion stability in a pure water environment at ambient temperature, making it not only pure water-compatible, but also stimuli-responsive (mimicking biological receptors, and highly responsive to external stimuli such as temperature).