Polymeric macro-radicals

Hence the Norrish II process leads to formation of an alkene [3(a)] or chain scission [3(b)]. The fate of the polymeric macro-radical depends on the sequence distribution of the copolymer. Consider the macroradical ... 

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

Polyfvinyl chloride) (PVC) is produced by mass, suspension, and emulsion processes. Mass polymerization is an exatiiple of a heterogeneous bulk system. PVC is virtually insoluble in vinyl chloride because the polymer is about 35% more dense than the monomer under normal polymerization conditions. Vinyl chloride, however, is quite soluble in polymer. The two phases in PVC polymerizations are pure monomer and monomer-swollen polymer. Polymerization proceeds in both phases, but it is very much faster in the polymer-rich phase because the mobility of macro radicals and mutual termination reactions are. severely restricted (cf. Section 6.13.2). 

For revealing of influence of conformational state of macro-radicals on kinetics of radical polymerization of acrylate- and methacrylate-guanidines in water mediums with the help of viscosimetry method the values of macroscopic viscosities in solutions modeling reaction mixtures at low conversion degrees were measured and obtained data were compared with kinetic ones. 

Let s compare of this value D with the diffiision coefficients of the macro-radicals in polymeric matrixes TGM-3, TGM-3-GMA and GMA which estimated experimentally [24] based on the kinetics of macroradicals decay, which under the given temperature consist of 10 + 10 mVs. 

Highly controlled ultrasound-induced polymer scission of 0.1 wt% PMMA in MMA in the presence of the radical scavenger i,i-diphenyl-2-piciylhydrazyl (DPPH) to prevent further polymerization of the formed macro-radicals. 

The reducing of the polymerization rate at the stage of gel effect can be result of the chain transfer to the additive or the interaction with the growing macro radicals (Scheme 3). 

Radical Processes.— The major disadvantage of free-radically initiated polymerizations as a means of producing block copolymers is the ease with which macro-radicals can indulge in transfer and termination steps both of which might reduce the efficiency of the process. However, new processes have been described in recent years which show considerable promise for the production of block polymer structures via radical intermediates. 

Bulk and suspension polymerization follow the same principles when all the system components are soluble in the monomer phase (initiators, macro radical, chain transfer agents). The rate of polymerization is not influenced by particle size and stabilizer type. 

The contribution of the homolytical substitution to the total sum of reactions occuning in free-radical polymerization in the presence of anthracene of its derivatives depends on the nature of the monomer and the anthracene-containing compound and on the polymerization conditions. It has been shown that the macro-radicals of the methaorylic esters do not interact with anthracene under usual conditions On the other hand, in the polymerization of styrene, substitution with the formation of terminal LM of type II (Scheme 1) proceeds only if one or both meso-positions of the anthracene ring are free ... 

Monomer polymerization can be initiated by macro radicals generated by mechanical stresses (resulting in block copolymers) or by free radicals obtained by intennolecular transfer (leading to graft polymers), such as free radicals obtained by intennolecular transfer ... 

The formation of macro-radicals created by non-thermally stable structures is observed prior to statistical chain scission. The thermal behavior of PMMA at temperatures below major degradation depends on many factors that have to be considered, in particular during certain stabilizer-free polymerization processes with high levels of radical generation. 

The terminal carbon atom with unpaired electron in the growing macro-radical molecule exhibits probably a flat sp hybridization. For the monomer CH=CXY (Y = H, X or R) polymerization to occur the molecule must approach the end carbon atom of the macroradical and set in the mirror position at which the same substituents are on the same side (scheme a ) or in nonmirror position at which the same substituents are on opposite side (scheme b ) as shown below ... 

The mechanism depicted in Eqs. (45) and (46) involves the production of one proton and of oxygen-centered radicals, which initiate vinyl polymerization in the presence of the monomer. As a result, a polymer with one central azo bond is formed. When heated in the presence of a second monomer, this macro-initiator is split at the azo side, giving rise to two initiating macro-radicals [Eq. (46)]. The final result is tailor-made multiblock copolymers. Other hydroxyl-groups containing high-molecular-weight compounds used in conjunction with Ce(IV) salts include methyl cellulose and methyl hydroxypropyl llulose [137]. 

This type of chemistry towards well-defined star-shaped (polymer)j,Qo is closely linked to progress made during this last decade in radical polymerization [3]. Indeed, the various controlled radical polymerizations such as nitroxide-mediated polymerization (NMP) or atom transfer radical polymerization (ATRP) allow us to produce polymer chains with tailored chain length and low polydisper-sities. Furthermore, the as-prepared chains bear at their end a function (stable radical or C—halogen bond) that can be converted under the right conditions into a macro-radical. If this latter is produced in the presence of Cgo, several chains should add to the fullerene. 

At low and moderate conversions, overall is adequately represented by i-e. K = k Y>- Since the chemical reaction between two (macro)radical functionalities is extremely fast (associated with a high value of A cr), termination is dilfusion controlled from the initial phase of polymerization on. Termination usually depends on the rate-determining diffusion step, denoted by SD- or TD-eontrolled k. ... 

Radical polymerizations of macromonomers are greatly influenced by the diffusion control effect [44]. Segmental diffusivity and translational diffusivity of the growing chains of macromonomers are strongly affected by the feed concentration and the molecular weight of the macromonomers. Furthermore, there is little difference in the degree of polymerization between macro-... 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

The controlled radical polymerization techniques opened up a new era in polymer synthesis, and further growth and developments are certain. However, the control of the molecular characteristics and the variety of macro-molecular architectures reported by these methods cannot be compared with those obtained by other living polymerization techniques such as anionic polymerization. 

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