Subject block copolymerization

Moreover, free radical block copolymerization has been performed by means of low-molecular initiators containing two azo groups of different thermal reactivity. The first thermal treatment at a relatively low temperature in the presence of a monomer A results in a polymeric azo initiator. The more stable azo functions being situated at the end of A blocks can be subjected to a second thermal treatment at a higher temperature in the presence of monomer B. 

Although, in the past the ROP of cyclic carbamates has been rarely mentioned, it has been the subject of more detailed study more recently [56]. The six-mem-bered trimethylene carbamate was the first cychc carbamate to be successfuUy ring-opened by employing trifluoromethane sulfonate as the catalyst. Only the unsubshtuted six- and seven-membered cyclic carbamates (Scheme 5.13) can be ring-opening polymerized [68-70]. The cationic ROP has subsequently been used in block copolymerizations starting from tehahydrofuran (THF) to yield either AB- or ABA-type polymers. 

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. 

Methacrylate monoliths have been fabricated by free radical polymerization of a number of different methacrylate monomers and cross-linkers [107,141-163], whose combination allowed the creation of monolithic columns with different chemical properties (RP [149-154], HIC [158], and HILIC [163]) and functionalities (lEX [141-153,161,162], IMAC [143], and bioreactors [159,160]). Unlike the fabrication of styrene monoliths, the copolymerization of methacrylate building blocks can be accomplished by thermal [141-148], photochemical [149-151,155,156], as well as chemical [154] initiation. In addition to HPLC, monolithic methacrylate supports have been subjected to numerous CEC applications [146-148,151]. Acrylate monoliths have been prepared by free radical polymerization of various acrylate monomers and cross-linkers [164-172]. Comparable to monolithic methacrylate supports, chemical [170], photochemical [164,169], as well as thermal [165-168,171,172] initiation techniques have been employed for fabrication. The application of acrylate polymer columns, however, is more focused on CEC than HPLC. 

Oxazolines undergo polymerization upon exposure to a variety of cationic initiators such as strong Lewis acids or strong protic acids. Copolymerization between different oxazolines of defined composition can be carried out in a random manner or in a controlled fashion resulting in block polymers. Alternatively, oxazolines can also be grafted onto other types of polymers. It is beyond the scope of this chapter to review in detail this enormous and important subject. Instead, the... 

The copolymerization of butadiene-styrene with this initiator was carried out heterogeneously in hexane. The data in Table VI show that the styrene content is highly dependent on the percent of conversion. It appears that a block styrene would be obtained if 100% conversion were reached. A detailed discussion regarding this subject will be mentioned later. 

Polystyrene is one of the most widely used thermoplastic materials ranking behind polyolefins and PVC. Owing to their special property profile, styrene polymers are placed between commodity and speciality polymers. Since its commercial introduction in the 1930s until the present day, polystyrene has been subjected to numerous improvements. The main development directions were aimed at copolymerization of styrene with polar comonomers such as acrylonitrile, (meth)acrylates or maleic anhydride, at impact modification with different rubbers or styrene-butadiene block copolymers and at blending with other polymers such as polyphenylene ether (PPE) or polyolefins. 

Studies of ethylene-vinyl aromatic monomer polymerizations continue to be published. Chung and Lu reported the synthesis of copolymers of ethylene and P-methylstyrene [28] and the same group extended these studies to produce and characterize elastomeric terpolymers which further include propylene and 1-octene as the additional monomers [29,30]. Returning to the subject of alternative molecular architectures for copolymers, Hou et al. [31] has reported the ability of samarium (II) complexes to copolymerize ethylene and styrene into block copolymers. 

By sequential copolymerization of styrene and propylene using a modified Ziegler-Natta catalyst, MgCl2/TiCl4/NdClc(OR) //Al(iBu)3, which was developed in our laboratory, a styrene-propylene block copolymer is obtained. After fractionation by successive solvent extraction with suitable solvents, the copolymer was subjected to extensive molecular and morphological characterization using 13C-NMR, DSC, DMTA, and TEM. The results indicate that the copolymer is a crystalline diblock copolymer of iPS and iPP (iPS-fo-iPP). The diblock copolymer contains 40% iPS as determined by Fourier transform infrared spectroscopy and elemental analysis. 

The copolymers which are the subject of this section are formed by the copolymerization of a prepolymer, which will ultimately form the soft block or segments, with one or more monomers which will form the hard blocks... 

This approach was shown to yield block copolymers but its efficiency was not quantified, although it was apparent that it was subject to side reactions which left a significant quantity of the starting polymeric material as the homopolymer. The reaction could possibly involve disproportionation between two molecules of the transient silver adduct (reaction 6) in competition with the uni-molecular decomposition into radicals as shown above. It is difficult to distinguish between the last two alternatives kinetically since both are bimolecular, and both would be reduced if the rate of reaction between the silver salt and the polystyrene lead adduct were retarded, with a consequent increase in copolymerization efficiency. 

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