Copolymerization gradient

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

Analysis of the Copolymerizabilities of Monomers The composition of the copolymers formed was determined by measuring the relative amounts of each monomer, NIPAAM and AAM, that remained in solution after a copolymerization. Copolymerizations were terminated by addition of 1 ml of reaction mix to 9 ml of 0.1% phosphoric acid at 50 C, followed by centrifugation of a 0.4 ml aliquot at 6,500 x g for 5 minutes in an Eppendorf microfuge. After 100 fold dilution of an aliquot of the supernate, 200 pi of this was injected onto an IBM reversed phase Cig HPLC column pre-equilibrated with 2% acetonitrile in 0.1% aqueous phosphoric acid and the eluent monitored at 214 nm. The monomers were eluted using a 0.1% aqueous phosphoric acid (solvent A) acetonitrile (solvent B) gradient as follows for 5 minutes the solvent was 98% solvent A and 2% solvent B, followed by a linear gradient to 80% A and 20% B over 10 minutes. After 5 more minutes at 80% A and 20% B, the solvent was returned to 98% A and 2% B. 

The synthesis of a gradient copolymer is sometimes desirable in a living polymerization to produce a copolymer with a variation in composition along the polymer chain. This is achieved either by carrying out the copolymerization without addition of monomer to maintain a constant feed composition or addition of monomer to deliberately change the feed composition from its original state. 

In the mid-1980s, Belenkii et al. comprehensively studied the separation of proteins on particle-packed columns of variational length and dimension under gradient conditions [20]. They concluded that a short distance of stationary phase is sufficient to enable protein separation with an acceptable resolution. With respect to that, Tennikova et al. came up with the concept of short monolithic separation beds, realized by copolymerization of glycidyl methacrylate as monomer and high amounts of... 

Fig. 26 Schematic diagram of the preparation of a gradient surface varying unidirection-ally in thickness of the photograft-copolymerized layer by using the combination of two types of photomasks and the X-Y step motor-controlled stage...

Fig. 27 AFM sampling locations of the grafted region (1) and surface topological featiu-es of photograft-copolymerized surfaces as observed by AFM (2). AFM images of the gradient surface regions at 200 jjim (position e in 1) (2A) and 1100 jjim (position b in 1) (2B), which correspond to irradiation times of 13.5 and 6.75 min, respectively. Line scan spectra for selected regions corresponding to those in part 1 of the gradient film show the measured film thickness...

The monomer-selective living copolymerization of /-butyl acrylate (/-BuA) and ethyl methacrylate (EMA) was studied on a 750 MHz spectrometer with an H inverse-geometry LC-NMR probe with pulsed-field gradient coils [10]. The detection volume of the flow cell was ca. 60 pi The measurements were performed in chloroform-di, with a flow rate of 0.2ml/min, at 296 K. The copolymers were obtained using bis (2,6-di-/-butylphenoxy) methylaluminium... 

A set of nonamphoteric buffers that are derivatives of acrylamide were synthesized specifically for casting IPG gels.2,33 They are blended with the monomer mixtures and copolymerized into polyacrylamide gel matrices. In properly oriented electric fields, IPG gels develop pH gradients that increase in pH value from the anode to the cathode. 

Copolymerizations of an equimolar mixture of MMA and MA (R-10) or nBA (R-11) with the nickel catalysts led to simultaneous consumption of the two monomers and gave their random copolymers with controlled molecular weights and relatively narrow MWDs (MwIMn 1.5).135 The copper-catalyzed systems also induced controlled random copolymerizations of MMA and nBA in organic solvents and in emulsion (MJMn 1.2).254-267 However, methacrylate/ acrylate copolymerization may result in gradient structures rather than random structures (section III.F). 

Semicontinuous addition methods These are very useful for copolymerizations requiring control of homogeneous compositions at the level of the chain and the particle. Variable composition gradient processes permit the introduction of the functional monomer to a suitable conversion, favoring incorporation on the surface or inside the particles. 

Gradient copolymers of St and AN were also prepared [115,116]. Since St and AN have reactivity ratios that are both significantly less than 1, the copolymerization has an alternating tendency along the backbone with an enrichment in AN at high conversions. Alternatively, a gradient along the backbone can be... 

RAFT has also been used to prepare copolymers. The copolymerization of MMA with nBA in the presence of cumyl dithiobenzoate as the transfer agent resulted in a polymer with a gradient of composition along the backbone, well-defined molecular weights, and low polydispersities [53]. Several copolymers were made by degenerative transfer with alkyl iodides [133]. 

Simultaneous copolymerization of MMA with MMA terminated po-ly(dimethylsiloxane) macromonomers results in the formation of a gradient of composition of PDMS units along the backbone due to a lower macromonomer reactivity which additionally decreases at lower temperatures [139]. 

In conventional emulsion polymerization, the disappearance of the VAc/BA droplets at ca. 25% conversion results from the transfer of monomer from monomer droplets to the locus of polymerization (monomer-swollen polymer particles). The presence of HD in the minidroplets reduces the free energy of mixing of the constituent monomers in the droplets. Therefore, the difference in the free energy of mixing between the monomer and polymer (particles) in mini-emulsion copolymerization is less than that in conventional emulsion polymerization. As a consequence, a smaller flux of monomer from the monomer droplets to polymer particles is achieved during mini-emulsion polymerization. In addition, HD cannot be transported from the droplets to particles because of its extremely low water solubility. Thus, the HD concentration in the droplets is greater than that in the particles, and monomer is retained in the droplets to minimize the HD concentration gradient. 

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