The "grafting to" approach is now very common in the application of polymeric biomaterials; for example, polymer brushes can be grafted onto different types of polymeric substrates to modify the biomaterial or mechanical properties of the substrate. Alfa Chemistry uses a "graft to" approach in which terminally functionalized polymer chains can be grafted onto a solid substrate to form a thin polymer brush layer on the solid surface, which determines the surface properties. Here are some examples of our "grafting to" approach to polymer materials.
|Polymeric Substrates||Grafting chains||Results|
|Polypropylene (PP)||Poly(2-acrylamido-2-methylpropanoesulfonic acid) (PAMPS)||Improve the permeation performance, restrain irreversible fouling, enable protein filtration applications of the membranes|
|Polyethylene terephthalate (PET)||Dermatan sulfate (DS)||Improve biointegration, reduce the foreign body reaction, enhance biocompatibility and hemocompatibility|
|Polyvinylidene fluoride (PVDF)||Quaternary ammonium compounds (QACs)||Impart antibacterial properties, mitigate biofouling, acquire favorable antibacterial properties|
|Poly(methylmethacrylate) (PMMA)||Poly(ethylene glycol) (PEG)||Reduce electroosmotic flow and nonspecific adsorption of proteins on the PMMA surface|
|Nylon 6,6||Poly(acrylic acid) (PAA)||Act as a scaffold to attach other molecules onto the surface to further improve the physical and chemical properties|
|Methoxy poly(ethylene glycol) (mPEG)||Exhibit good resistance toward platelet adhesion, as well as antifouling, and blood compatibility|
A chemical reaction occurs between a functionalized polymer and a complementary reactive group on the substrate's surface in the "grafting to" approach. Graft copolymers are formed by coupling reactions between the functional backbone and the reactive branched chain end groups. Chemical modification of the backbone allows for these coupling processes. Free radical polymerization, anionic polymerization, atom transfer radical polymerization, and reactive polymerization are some of the common reaction processes employed to make these copolymers.
Anionic polymerization processes are commonly used to make "grafting to" copolymers. This approach employs a coupling reaction between the main chain polymer's electrophile group and the anionic reactive polymer's propagation site. Without a main-chain polymer with reactive groups, this approach would not be practicable. This approach has grown in popularity as click chemistry has grown in popularity. Graft polymerization methods rely on a high-yield chemical process known as atom transfer nitrogen-oxygen radical coupling chemistry. Gold, silica, titanium, glass, carbon nanotubes, graphene nanosheets, and polymer substrates are currently the method's principal targets.
Fig 1. Strategies of polymer grafting: "grafting to".
The main advantage of the "grafting to" method over other methods is that the polymer can be thoroughly characterized (prior to grafting) by various chemical and physical methods. This method can be used to prepare polymer brushes with well-defined molecular weight and molecular weight distribution that can be grafted onto a surface. In addition, this method is faster and simpler than other chemical surface modification methods because it does not involve too many complex and complicated synthetic processes or procedures. On the other hand, as the reaction proceeds, the spatial site resistance effect makes it difficult for subsequent chains to continue grafting. Therefore the surface grafting rate of the modified material is very little and it is difficult to obtain thick or dense grafted layers. This is its main drawback.
However, it has been shown that grafting methods can also provide graft layers with high graft density. Nebhani et al. quantified the grafting density obtained by surface modification of divinylbenzene (DVB)-based microspheres using this "graft-to-" approach through a combination of reversible addition-rupture chain transfer (RAFT) polymerization and fast heterogeneous Diels-Alder (HDA) chemistry. As demonstrated in this experiment, the "graft-to-" method can achieve grafting densities that in some sense exceed those obtained from grafting methods by efficient modular chemistry.
Fig 2. The functionalization proceeds via the double bonds inherently available on the microspheres, which are reacted with poly(isobornyl acrylate) chains carrying a highly dienophilic thiocarbonyl functionality. (Nebhani L, et al. 2010)