Core crosslinking monomers

Castell P, Galia M, Sena A (2001) Synthesis of new epoxy liquid-crystalline monomers with azo groups in the central mesogenic core. Crosslinking with amines. Macromol Chem Phys 202 1649-1657... 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

In copolycondensation for example, the more reactive monomer is expected to become exhausted more rapidly than the less reactive one. If the functionalities of the polyfunctional crosslinker are more reactive, short chains are formed in the beginning of the reaction and long chains in the end. If we assume equilibrium conditions throughout the reaction, the unreacted functionalities of the crosslinker on different growing trees, with short links in the beginning, are expected to react more likely with each other and as a result a part of the final network may be more crosslinked than the other part. This may eventually lead to phase separation. If the reaction is diffusion-controlled (177), cores with higher crosslinking density may be formed. 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Fig. 10 Stages of preparation of a copolymer envelope a adsorption of homopolymer chain on a colloidal particle b coloring of the polymer chain (blue corresponds to chemically modified monomer units and red to adsorbed units) and introduction of crosslinks (shown as green sticks) to stabilize the hollow-spherical structure c elimination of the core particle. Adapted from [57]...

The MI method utilizes initiator-functionalized linear chains, which initiate the polymerization and crosslinking of a difunctional monomer (e.g., divinylbenzene). The active chain ends also attack the neighboring linear chains ends, and a core with crosslinked microgel is formed. In the meanwhile, certain numbers of linear chains are attached to the core. However, it is always difficult to obtain star polymers with narrow distribution of arm numbers. Quite often, many linear polymers are not attached to the core, which leads to problems in the course of the purification and for finally applications. By using multifunctional coupling agents, it is possible to get stars with uniform arm numbers. But the purification process is always unavoidable and difficult. 

Many monomers can be grafted by redox polymerization to the magnetic core [magnetic material embedded in poly(vinyl alcohol) matrix nominally 100% crosslinked with glutaraldehyde] to get whisker-type resins211). Acrylic acid and acryl-... 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

Another interesting three-component LIPN consists of crosslinked polyorganosiloxane (tet-raethoxysilane as crosslinker) as the first stage, swollen by butyl acrylate monomer (allyl methacrylate as crosslinker) and then polymerized to form the second component, and finally, poly(methyl methacrylate) was grafted onto the IPN core as the shell layer [Isao et al, 1992], Such kind of LIPNs are found to be a good impact modifiers in thermoplastics i.e., they are toughened by the addition of finely divided low phase. 

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