## 4 Thermally Initiated Graft Chain Growth  
The use of thermally initiated graft polymerization was first reported for monolithic columns in 1997. [PERSON] _et al_. produced a monolithic column with thermally grafted polymer chains, which exhibited thermal responsive behavior [38]. Two types of thermal responsive composites could be formed: a thermal gate wherein all flow was blocked through the column or a thermal valve which can control flow rate. With the expansion and contraction of the polymer at different temperatures, the resulting change in functionality could be exploited. By controlling temperature, the thermal responsive polymer could be used to selectively elute hydrophilic analytes at high temperatures and to elute hydrophobic analytes at a lower temperature. [PERSON] _et al_. grafted the charged monomer AMPS, to an otherwise neutral poly(GMA-_co_-EDMA) monolithic column [94]. The polymerization was terminated by cooling the column and extensive washing. The column was used for the separation of proteins (myoglobin, chymotrypsinogen, lysozyme) with an efficiency of 6,700 N/m for lysozyme.  
Poly(trimethylolpropane trimethacrylate) (poly(TRIM)) monoliths were thermally grafted with the zwitterionic monomer MEDSA [95], which retained basic proteins when loaded from pure water.  
Other thermally initiated polymerizations used in post-polymerization modification have been employed to introduce new surface functionality and to improve the fine control of porous properties in polymer monolithic columns. A polymerization that can restart chain growth upon the addition of a new monomer is called \"living\". During the polymerization process the stable free radicals are combined with the growing polymer chains. A functional monomer can then be added to the structure, and with the activation of latent dormant radicals, surface localized grafting can proceed upon the application of temperature [96]. This may provide a method for controlling the phase separation, resulting in a more homogeneous monolithic column [17].  
There are numerous types of living polymerization approaches such as nitroxide mediated polymerization [96, 97], ring opening metathesis polymerization (ROMP) [98, 99], atom transfer radical polymerization (ATRP) [100, 101], and reversible addition-fragmentation chain transfer polymerization (RAFT) [102, 103]. Some of these methods have been used in the fabrication of cross-linked gels [100], polymer beads [101], and polymer monoliths [96, 104]. Temperatures above 100 \({}^{\circ}\)C are generally used, while termination of the growing polymer is mediated by decreasing the temperature [104]. [PERSON] _et al_. used 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) facilitated polymerizations for the subsequent grafting of the resulting polymer [105]. Either HEMA or VBC were grafted to the column in the presence of initiators such as benzoyl peroxide (BPO) and TEMPO.  
[PERSON] _et al_. prepared poly(STY-_co_-DVB) monolithic columns [96]. Using a simplified grafting reaction, wherein the monomer and solvent (cyclohexanol) were only present in the column, the surface latent initiators facilitated the reaction. The graft polymerization proceeded at an elevated temperature of 110 \({}^{\circ}\)C. [PERSON] _et al_. prepared poly(STY-_co_-DVB) monolithic columns using BPO and a stable free radical [104]. The hydrophobic poly(STY-_co_-DVB) column was grafted with a hydrophilic monomer (HEMA) using the latent surface radicals. SPM was also grafted to the column using a solution of monomer in dimethyl sulfoxide. The hyrdophilized column (HEMA graft) showed a significantly reduced retention and a decrease in pore volume of 11.5% following grafting. The SPM grafted monolith was used in the cation exchange of proteins, namely myoglobin, cytochrome c, and lysozyme.