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The dialkyldiazene or azo-compound initiators, which include azonitriles such as azobis(isobutyronitrile) (AIBN), are one of the most important classes of initiator being widely used in both conventional and reversible-deactivation radical polymerization (RDRP). This paper briefly reviews the mechanism of radical generation from dialkyldiazenes and provides a critical assessment and recommended values for the decomposition rate coefficients (kd) and efficiencies (f) of those that are commercially available. A critique of the methods that have been used for the determination of these kinetic parameters is also provided. In this review, a focus is placed on initiation of radical polymerization of the more reactive monomers such as methyl methacrylate (MMA) and styrene at low monomer conversions; where addition of the initiator-derived radicals to monomer is not rate determining (i.e., ki>kp) and, thus, the efficiencies for radical generation (fg) and for initiation of polymerization (fi) are similar. However, the dependence of the kinetic parameters on such factors as monomer type, monomer conversion, reaction medium, temperature, photo-irradiation, microwave irradiation and magnetic fields is also discussed. We additionally provide comment on the use of dialkyldiazenes in heterogeneous polymerization (emulsion or dispersion polymerization) and in RDRP. Dialkyldiazenes are the most used initiators in RAFT (reversible addition-fragmentation chain-transfer) polymerization and other RDRP based on degenerative chain transfer, which include iodine transfer polymerization (ITP) and tellurium-mediated polymerization (TERP). They also find significant use in various forms of stable-radical-mediated radical polymerization (SRMP), in particular nitroxide-mediated polymerization (NMP) and cobalt-mediated radical polymerization (CMRP), and have an integral role in certain atom-transfer radical polymerization (ATRP) processes, such as reverse and ICAR (initiators for continuous activator regeneration) ATRP and variations on these procedures. In each of these RDRP methods, knowledge of the parameters that characterize the kinetics and efficiency of dialkyldiazene decomposition and the mechanism of initiation is beneficial to understanding and for optimizing control over the process.

Poly(vinyl chloride) (PVC) is one of the more consumed polymers worldwide due to its general versatility and low cost. It is currently used in various applications ranging from packaging to construction. Currently PVC is only being prepared on an industrial scale by free-radical polymerization (FRP). However, the several intrinsic limitations of FRP triggered interest in synthesizing this polymer by reversible deactivation radical polymerization (RDRP) methods. At the present time, single electron transfer living radical polymerization (SET-LRP), single electron transfer degenerative chain transfer living radical polymerization (SET-DTLRP), cobalt-mediated radical polymerization (CMRP), supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP), reversible addition-fragmentation chain transfer polymerization (RAFT) and nitroxide-mediated polymerization (NMP) are RDRP methods that have been reported for vinyl chloride (VC) (co)polymerization. Here, these RDRP methods are critically discussed from three major standpoints. These are degree of control over polymerization, limitations and potential applications on an industrial scale.

How to achieve the toughening of hydrogels has always been a problem that researchers are committed to solving. Recently, many strategies have been proposed to improve the toughness of hydrogels. However, most of them involve complicated processes, which severely restrict the practical application of hydrogels. Herein, we provide a straightforward method to toughen polyacrylamide (PAM) hydrogels via a degenerative chain transfer effect of allylamine (XBA). The produced hydrogels crosslinked by a traditional crosslinker N,N-methylenebisacrylamide (MBA) possess a relatively uniform network with the existence of entangled pendant chains. The homogeneous network distributes stress evenly, while the disentanglement of the entangled pendant chains dissipates energy and the chemical crosslinks maintain elasticity. As a result, the breaking strain and toughness of the hydrogels can reach 1370% and 1.41 MJ/m3, which are 1.9 and 1.6 folds higher than those of pure PAM hydrogels, respectively. More importantly, the toughening mechanism based on the degenerative chain transfer effect is generalizable to other allyl monomers and other hydrogel systems. Overall, this research offers a facile strategy for designing tough hydrogels and may pave the way for their practical application in industrialized production.

Acid-cleavable branched polymers composed of a methacrylate and two acrylates were prepared through reversible addition-fragmentation chain transfer (RAFT) polymerization. The molecular weight of primary chains and the degree of branching of the polymers were controlled by the feed ratio of monomer/chain transfer agent (CTA) and brancher/CTA, respectively. Glass transition temperatures of the polymers increased as the degree of branching grew due to an increase in molecular weight. When the branched polymers were hydrolyzed, molecular weight and polydispersity index value were similar to those of linear polymer prepared by RAFT polymerization with the same molar feed ratio of monomer/CTA.

The controlled synthesis of narrowly distributed low molecular weight polymers with functionalization possibilities is of great industrial interests. Although living polymerization allows for control over polymer architecture, the production of low molecular weight polymers with low polydispersities via living polymerization systems is challenged by the use of large amounts of catalysts and broadening in molecular weight distribution. This review addresses the synthesis of narrowly distributed, functional, low molecular weight polyethylene and polyethylene mimics. The review is structured for quick identification of relevant systems for the production of specific polymer architectures with specific cost, efficiency, and safety concerns.

The reversible addition–fragmentation chain transfer (RAFT) polymerization is one of living radical polymerizations. In this study, four different 9 H -fluoren-9-yl benzodithiolates (FBDTs) were synthesized, and used along with azobis(isobutyronitrile) (AIBN), a radical initiator, in polymerization of styrene (ST) at the molar ratio of 3:1. This new transfer agent exhibited the typical characteristic living free radical polymerization behaviors such as good control of molecule weight and narrow molecule weight distribution. It was concluded that the FBDTs can be used as the RAFT agents in free radical polymerization of vinyl monomers.

This work reports the preparation and study of poly(vinyl chloride) (PVC) with medium and high molecular weight using a Living Radical Polymerization (LRP) method in aqueous systems. The synthesis of ultra thermal stable PVC that falls outside the typical values known for the PVC industry is presented and PVC thermal stability is evaluated using a standard industrial method. The results presented in this manuscript suggest that the single electron transfer–degenerative transfer mediated living radical polymerization (SET–DTLRP) mechanism is the main factor influencing the thermal stability, since the polymerization temperature has no detectable effect on this property. In contrast to the Free Radical Polymerization (FRP), where low temperatures lead to PVC with high thermal stability that corresponds to a high molecular weight polymer, the polymerization temperature in SET–DTLRP (range 21–42 °C) has no detectable effect on the thermal stability of PVC. The final molecular weight is defined by the [monomer] 0 /[initiator] 0 ratio (degree of polymerization). The PVC synthesis reported in this manuscript was carried out in a 150 L pilot reactor, which represents a reliable approach in terms of hydrodynamic conditions, when compared with those at the industrial scale.

The aim of this work is to evaluate PVC resins prepared by Single Electron Transfer Degenerative Chain Transfer Living Radical Polymerisation (SET-DTLRP) in a 150 L reactor under industrial conditions. The product features were analysed considering the standard resin features of PVC prepared by the conventional free radical polymerisation (FRP) method. It was proved that the FRP and SET-DTLRP have different particle growing mechanism and led to grains with completely different features (particle size, particle size distribution and morphology). It is critical for the PVC producers the preparation of PVC with a predetermined grain shape, particle size distribution and porosity which have an indubitable influence on polymer processing. It is presented a detailed study on the particle formation of the PVC synthesised by SET-DTLRP providing first insights into PVC features associated with a new polymerisation technique taking into account the normal requirements of the PVC industry. A possible mechanism of the SET-DTLRP particle formation is proposed.

This review traces the development of addition–fragmentation chain transfer agents and related ring-opening monomers highlighting recent innovation in these areas. The major part of this review deals with reagents that give reversible addition–fragmentation chain transfer (RAFT). These reagents include dithioesters, trithiocarbonates, dithiocarbamates and xanthates. The RAFT process is a versatile method for conferring living characteristics on radical polymerizations providing unprecedented control over molecular weight, molecular weight distribution, composition and architecture. It is suitable for most monomers polymerizable by radical polymerization and is robust under a wide range of reaction conditions. It provides a route to functional polymers, cyclopolymers, gradient copolymers, block polymers and star polymers.

This work reports the synthesis of several copolymers of poly(vinyl chloride)-b-poly(n-butyl acrylate)-b-Poly(vinyl chloride) prepared by single electron transfer/degenerative chain transfer mediated living radical polymerization (SET-DTLRP) in a two step process: first, a bifunctional macroinitiator of α,ω-di(iodo)poly(butyl acrylate) [α,ω-di(iodo)PBA] was synthesized by SET-DTLRP in water at 30 °C. The obtained macroinitiator was further reinitiated also by SET-DTLRP leading to the formation of the desired product. Several copolymers were synthesized in a 5L pilot reactor with different molecular weights and relative amounts of PBA and PVC. The possibility of synthesizing flexible materials made of PVC without using normal free plasticizes is extremely important for the industry. After processing the materials in a two-roll mill laboratorial equipment, the block copolymers were characterized concerning thermal and mechanical. The materials characterized in this study were prepared in a 5L pilot reactor under similar conditions to be used in industrial scale.

1-alkyl-3-methylimidazolium hexfluorophosphate ([C-x][PF6], where x = 4, 6-8) is used as solvent for the polymerisation of methyl methacrylate, methyl acrylate and styrene by the reversible addition-fragmentation chain transfer process. In the case of styrene, the insolubility of the polymer in ionic liquid stops the polymerization at an early stage. The acrylate and methacrylate polymerizations lead to products with molecular weights close to the theoretical ones and polydispersity indexes lower than 1.3. The polymerizations are shown to be living by chain extension of the products formed in ionic liquid. In the case of the methyl methacrylate, the kinetics of the polymerizations are followed and the molecular weight of the polymer is shown to increase linearly with the conversion, as expected for a living polymerization. (C) 2002 Elsevier Science Ltd. All rights reserved.

The effect of kinetic chain length and chain transfer on the polymerization kinetics and network structure in polymerizations of loosely crosslinked 2-hydroxyethyl methacrylate/di(ethylene glycol) dimethacrylate mixtures was explored. Polymerization behavior of the monomer mixture in the presence and absence of a chain transfer agent was monitored at various initiation rates and chain transfer agent concentration levels. Dependence of the polymerization rate on the initiation rate was found to deviate from the classical square-root relationship because of chain-length-dependent termination. This effect was further confirmed by addition of a chain transfer agent. The presence of a chain transfer agent led to the formation of shorter kinetic chains, which enhanced termination and slowed the polymerization. Investigation of the polymerization kinetics after cessation of irradiation yielded kt/kp[M] values for both systems. Prior to the onset of reaction diffusion-controlled termination, the system that included a chain transfer agent exhibited much higher kt/kp[M] values than the polymerization system without added chain transfer agent. In addition, the onset of reaction diffusion-controlled termination was delayed to higher conversions in the system containing chain transfer agent. The impact of a chain transfer agent on the polymerization behavior and kinetics demonstrates that the chain-length-dependent termination phenomenon is indeed important and must be considered in kinetic modeling of loosely crosslinked systems.

Evidence is presented in support of the existence of chain length dependent termination in cross-linking free radical polymerizations. Photopolymerizations of multifunctional methacrylates and acrylates were examined, and the difference in the inherent chain transfer characteristics of these systems provided a means for studying the effect of kinetic chain length on the polymerization behavior. The kinetic chain length was varied in each system by changing the initiation rate ( R i ) and via the addition of a chain transfer agent (1-dodecanethiol). Changing R i in the methacrylate systems led to a non-classical dependence of the rate of polymerization on R i similar to that observed in linear polymerizations that exhibit chain length dependent termination. Additionally, incorporation of a chain transfer agent into the polymerization system, even at very low concentrations (0.1 wt%), had a dramatic effect on the methacrylate polymerization kinetics. As expected, because of their predisposition to chain transfer, the acrylate systems were minimally affected by changes in R i and the addition of chain transfer agent.

Alkylation of the phosphorus coordination of the diphenyl(dithioformato)phosphine ligand in [W(CO) 5 (PPh 2 CS 2 )]NEt 4 ( 1 ) at the S atom results in the formation of the novel RAFT agent S═C[W(CO) 5 PPh 2 ]S–R ( 2a , R = CH 2 Ph; 2b , R = CH 2 CH═CH 2 ). These compounds have been shown to be highly effective in reversible addition-fragmentation chain transfer (RAFT) polymerization to produce polymers (homopolymer and diblock copolymer) of predetermined molecular weight and narrow polydispersity (

A modelistic study is conducted for an emulsion polymerization that involves chain transfer to the polymer, by focusing our attention to the effect of very small reaction volume on the formed molecular weight distribution. In emulsion polymerization, a polymer radical that causes the polymer transfer reaction must choose the partner only within the same particle, which makes the expected size of polymer molecule to be chosen smaller compared with the corresponding bulk polymerization that involves an infinitely large number of polymeric species. The usual assumption for homogeneous polymerization that the rate of chain transfer to a particular polymer molecule is proportional to its chain length cannot be used, except when branching frequency is low and particle size is large enough. This fact invalidates the direct use of models developed for homogeneous non-linear polymerizations to emulsion polymerizations.

Molecular weight averages and molecular weight distribution (MWD) of polymers formed via nonterminating polymerization in a series of continuous stirred tank reactors (CSTRs) under conditions of chain transfer reactions are considered theoretically. For chain transfer to solvent, the temperature regime is defined which provides the highest Pn at the outlet of the series. Chain transfer to polymer is shown to be the possible reason for gel formation in the first reactor. Critical conditions for gel formation are derived.

The authors have calculated the form of the MD, the mean degrees of polymerization Pn, PwandPz of the global polymer and also separately the primary (formed on the initial initiator) and secondary (resulting from chain transfer) fractions for the case of non-terminating polymerization with chain transfer to the solvent using polyfunctional initiators. The dependences of these characteristics on the degree of functionality of the initial initiator and the intensity of chain transfer have been determined. The limits of the desirability of using polyfunctional initiators in real technological processes of synthesis of high molecular weight polymers are evaluated.

Theoretical studies were carried out of the possibility of using the kinetics of variation of molecular weight distribution (MWD) and the distribution of unsaturated end groups in macromolecules during homophase radical polymerization of vinyl monomers, accompanied by the addition of the macroradical to an unsaturated end group of the polymer molecule, to determine the kinetic parameters of the system and the distribution of branching centres along macromolecules. Equations were derived to determine the rate of formation of branching centres and some parameters characterizing distribution along macromolecules using the experimental results obtained.

The action of reaction heat on MMD characteristics in radical polymerization has been examined theoretically. Attention was paid mainly to the ratios of degree of polymerization and breadth of MMD to the Semenov parameter (degree of non-isothermality of the process). The influence of the chain transfer reaction, gel-effect and thermal initiation on the character of the MMD has been studied.

A study was made of changes in the molecular weight of polyoxymethylene during the polymerization of monomeric formaldehyde in benzene at 40° with BF3 etherate. It was found that the increased concentration of the macromolecules with time is due to chain transfer to water that is initially present in the solvent, and is also fed into the system continuously in the course of polymerization along with the gaseous monomer. The chain transfer constant to water (ktr = 2000 l./mole·min) was determined on the basis of a mechanism involving spontaneous initiation, linear decay of active centres and a chain transfer reaction.

A study has been made of the molecular weight and distribution changes that take place during the polymerization of cyclopentene initiated by the non-living metathesis catalyst WCl 6 AliBu 3 . The system appears to be characterized by two distinct phases. In the first phase, the molecular weight distribution, which is bimodal, would appear to be kinetically controlled and the bimodal distribution to result from the presence of two kinetically independent metathesis species. The initial molecular weight distribution would, therefore, appear to be dependent upon the relative concentrations of these active species. The second phase of the polymerization is a slow conversion of the molecular weight distribution to that of an equilibrium distribution after a long reaction time.

A study was made of the effect of reactions taking place during cationic polymerization of cyclic ethers on the variation of molecular weight distribution of polymers during polymerization. It was shown that this variation may be a means for establishing the type of reaction taking place. Ratios were derived to determine rate constants of the reactions studied using experimental results.

A method is suggested for derivation of the statistical molecular weight distribution function of macromolecules, allowing for intermolecular chain transfer to polymer in radical polymerization processes. In the derivation the polymer macromolecule is represented as a “tree”. Expressions have been derived for calculation of MWDs of branched polymers. In the case of termination solely due to disproportionation the distribution function has been obtained in an explicit form in terms of standard functions. MWDs of linear and branched polymers have been found to be interrelated.

A theoretical study was made of formation kinetics of branching centres in homophase radical polymerization accompanied by kinetic chain transfer to the polymer by abstraction of the hydrogen atom or addition of the macroradical at the unsaturated double bond of the polymer chain. Relationships were derived to determine some characteristics of distribution of branching centres along macromolecules using experimental results available. A study was made by polymerization with constant concentrations of reagents and a constant rate of radical formation of the dependence of parameters of MWD and distribution of branching centres along macromolecules during the reaction on the ration of various process rates.

The process of radical polymerization in a periodic ideal mixing reactor is considered. A numerical analysis has been made of the infuence on the MWD characteristics of the reaction of chain transfer to the polymer, the conditions of the heat exchange and slowing of the reaction of termination as a result of increase in viscosity (kinetic gel effect). It is shown that the effect of chain transfer to the polymer fundamentally differs for chain termination by disproportioning and recombination. The influence of the gel effect on the MWD for the two variants of termination is similar. The distinctive temperature regimes of the process have been investigated with heating in the stage of completion of the reaction the onset of which is associated with worsening of heat exchange with growth in viscosity in the course of polymerization. It is shown that heating in the course of the reaction leads to weakening of the influence of chain transfer to the polymer and the gel effect on the MWD paarticularly marked in the high temperature regime.