## 1 Introduction  
Organic polymer monolithic columns were first devised by Hjerten in the late 1980s [1] and further developed throughout the early 1990s [2, 3, 4, 5]. Porous polymer rods have come to include sophisticated column designs for columns and microfluidic devices, which contain not only multifunctional surface chemistries [6, 7, 8, 9], but also differing concentrations of surface functionality [10, 11, 12, 13], separate to the column morphology. Many reviews regarding monolith fabrication have been written since the inception of polymeric monoliths, with focused reviews on preparation [14, 15, 16, 17, 18], modification [19, 20, 21, 22], and with special consideration of particular applications such as ion exchange [19, 20], solid phase extraction [22, 23], biomolecule immobilization [24, 22, 25, 26], and the role of nano-particles in biomolecule immobilization [27]. This review is aimed at highlighting categories of post-polymerization modifications available for polymeric monolithic columns, prepared by free radical polymerization via thermal or UV initiated methods. In recent years, the use of post-polymerization modification has become a commonplace technique in the preparation of monolithic separation media. The goal of post-polymerization modifications is to separate the control of morphology from the selectivity, modification, and tailoring of the surface chemistry [28]. This type of modification can result in the use of monoliths with readily optimized morphologies, as well as increased surface density of desired functional groups leading to increased efficiency [7, 28]. The most technically straightforward method to incorporate the desired surface functionality is to co-polymerize a desired monomer with a cross-linker, and/or a second functional monomer [4, 5, 11, 28, 29]. Monolithic columns prepared directly from a monomer of desired functionality and a cross-linker also have been reported [30, 31, 32, 33, 34, 35]. Co-polymerization is a well-established method for the preparation of monolithic columns expressing the desired chemistry [28], however, as polymeric monoliths express a high population of large through-pores, they have limited surface areas, and as such can express a low surface availability of functional groups [2]. The available surface functionality can be limited, as monomers with specific functionality may not always be commercially available, and synthesis may not be a viable option [15]. To generate these specific surface chemistries, columns have been subsequently modified following preparation of a \"generic\" base monolith [36, 37, 38]. Using a number of techniques, such as reactions with pendant surface groups, grafted chain growth, hypercross-linking, or indeed with the introduction of nano-structures to the formed monolith, the desired surface functionality can be tailored. Post-polymerization modification can be defined as those reactions limited to the surface of the monolithic column, using (a) reaction chemistry (e.g., glycolyl methacrylate (GMA), N-acryloxysuccinimide (NAS), vinyl azlactone (VAL))from a functional group incorporated into the monolith structure; (b) grafting of additional polymer layers by means of thermal or photo-initiated polymerizations; or (c) the addition of nano-architectures to aid in selectivity or to enhance surface area. A reactive monomer such as GMA [2; 3], VAL [5; 6; 11; 39], or NAS [40; 41; 42; 43], can be incorporated into the monolithic matrix, which can be further modified to express a preferred surface chemistry. Since the introduction of rigid monolithic columns [2], GMA has been used as a co-monomer in monolithic column preparations with varying modification reactions performed _in situ_. The reactive GMA group is generally used to convert monolithic columns to ion exchange selectivity [2; 28; 44; 45; 46; 47; 48; 49; 50; 51], affinity selectivity [52; 53; 54; 55; 56] or, for the immobilization of zwitterionic molecules, for other protein separations [57; 58; 59].