Homopolymerization retardants

Brominated Styrene. Dibromostyrene [31780-26 ] is used commercially as a flame retardant in ABS (57). Tribromostyrene [61368-34-1] (TBS) has been proposed as a reactive flame retardant for incorporation either during polymerization or during compounding. In the latter case, the TBS could graft onto the host polymer or homopolymerize to form poly(tribromostyrene) in situ (58). 

Several radical copolymerizations of vinyl 2-furoate with well-known monomers (50 50) were also studied. Complete inhibition was obtained with vinyl acetate, very strong retardation with styrene, vinyl chloride and acrylonitrile methyl methacrylate homopolymerized without appreciable decrease in rate. It is evident that the degree of retardation that vinyl 2-furoate imposes upon the other monomer depends on the stability of the latter s free radical. With styrene and vinyl chloride the small amounts of fairly low molecular-weight products contained units from vinyl 2-furoate which had entered the chain both through the vinyl bond and through the ring (infrared band at 1640 cm-1). 

Each set of experiments was carried out under the same reaction condition except using different comonomers, i.e. p-methylstyrene, o-methylstyrene, m-methylstyrene and styrene, respectively. The compositions of copolymers were determined by H NMR spectra, and the thermal properties (melting point and crystallinity) were obtained by DSC measurements. Overall, all comonomers show no retardation to the catalyst activity. In fact, the significantly higher catalyst activities were observed in all copolymerization reactions (runs 2-5), comparing with that of ethylene homopolymerization (run 1). Within each set (runs 2-5 and 6-9) of comparative experiments, p-methylstyrene consistently shows better incorporation than the rest of comonomers, i.e. o-methylstyrene, m-methylstyrene and styrene. Both catalysts with constrained mono- and di-cyclopentadienyl ligands are very effective to incorporate p-methylstyrene into polyethylene backbone. In runs 2 and 6, more than 80 % of p-methylstyrene were converted to copolymer within one hour under constant (- 45 psi) ethylene pressure. On the other hand, only less than half of styrenes (runs 5 and 9) were incorporated into ethylene copolymers under the same reaction conditions. The significantly... 

Sodium bisulfite-china clay proved to be an efficient initiator for homopolymerization and graft polymerization of methyl methacrylate onto cellulose. Grafting reactions using ceric ammonium sulfate, sodium bisulfite-soda lime glass or -china clay are inhibited or retarded on adding soda lignin to the grafting medium. 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerization is usually initiated by alkali hydroxides, especially potassium hydroxide. In the base-catalyzed polymerization of propylene oxide, some rearrangement occurs to give allyl alcohol. Further reaction of allyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly difunctional. By using zinc hexacyano cobaltate as catalyst, a more difunctional polyol is obtained (20). Olin has introduced the difunctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalyzed homopolymerization of tetrahydrofuran. Copolymers derived from tetrahydrofiiran and ethylene oxide are also produced. 

These methods suggested in the present form by Caunt83) rely on inhibition (retardation) effects of strong catalyst poisons on polymerization. Typical poisons potentially usable for this purpose are carbon oxides, carbonyl sulfide, carbon disulfide, acetylenes and dienes. All these substances exhibit a strong unsaturation they have either two double bonds or one triple bond. Most of the works devoted to application of the poisons to determination of active centers 10,63 83 102 1O7) confirm a complicated nature of their interaction with the catalytic systems. To determine the active centers correctly, it is necessary to recognize and — as much as practicable — suppress side processes, such as physical adsorption and chemisorption on non-propagative species, interaction with a cocatalyst, oligomerization and homopolymerization of the poison and its copolymerization with the main chain monomer. 

Steric influences may retard some radical polymerizations and copolymerizations. Double bonds between substituted carbon atoms are relatively inert (unless the substituents are F atoms) and 1,2-substituted ethylenes do not homopolymerize in normal radical reactions. Where there is some tendency of such monomers to enter into polymers, the trans isomer is more reactive. When consideration is restricted to monomers that are doubly substituted on one carbon atom, it is usually assumed that steric effects can be neglected and that the influence of the two substituents is additive. Thus vinylidene chloride is generally more reactive in copolymerizations than is vinyl chloride. 

It has long been known that the rate of silane homopolymerization is increased by pH or metal salt catalysis and decreased by increased concentration and higher temperature. Most silanes are hydrolyzed most rapidly at pH between 3 and 5. Solution stability depends on the rate of homopolymerization to siloxane polymer. This is affected by pH, the presence of soluble salts of lead, zinc, iron, etc., and silane concentration. A pH in the range of 4 to 5 generally favors the monomeric form and retards polymerization. The formation of homopolymer can be detected as silane loses solubility and forms a gel which is not active in the coupling process. It is, then, desirable to retain silane in the monomeric or dimeric form. In the next two steps a bond is formed with the substrate (e.g., filler). 

Introduction of the hydrogen-donating accelerators N,N,N, N -tetramethylethylenediamine and triethanolamine (0.05-0.5%) had little effect on the rate at which polymer was photografted onto wool (16). The extent of homopolymerization, however, was dramatically lowered, and these compounds can be considered homopol3nnerization retardants. 

The degree of copolymerization of the carboxylated lignin with propylene oxide, and the degree of homopolymerization of propylene oxide, can be expected to vary with the reactor pressure at the end of the oxyalkylation reaction. A typical pressure-temperature diagram of this reaction is presented in Figure 2. Polymerization usually commences between 165 and 185°C as is indicated by a retardation of the temperature rise and a distinct peaking of the reactor pressure. The entire reaction is completed after three to four hours, when the final pressure declines to around 180 psi. 

DRM does not homopolymerize under the same conditions as that for polymerization of DRF, but it can homopolymerize in the presence of a radical initiator and morpholine as an isomerization catalyst via a monomer-isomerization radical polymerization mechanism [3,29]. However, since morpholine also serves as a retarder, the polymerization reactivities do not exceed those for DRF in the absence of morpholine. 

Copolymerization systems involving the combination of a furan monomer and a conventional counterpart, confirmed this general state of affairs in that only the use of polymerizable furan monomers yielded the expected copolymers. Conversely, comonomers like 2-vinyl furoate either retarded the homopolymerization of well-stabilized standard monomers like methacrylates, or actually inhibited the polymerization of poorly stabilized conventional monomers like vinyl acetate [4d, 7]. 

The morphologies of both ABA and ANA homopolymers and 73/27 ABA/ANA copol5nner prepared by thin-film polymerization show that crystallization occurs in the homopolymerization systems and the liquid crystal state remains stable in the copolymerization system (39-44), clearly indicating that the random copolymerization is an effective way to retard the crystallization. Transmission electron microscopy revealed that the microstructures of homopol5nners of ABA and ANA had more obvious lamellar texture (41-44). 

Allylic compounds (CH2MDHCH2X) are usually reluctant to undergo homopolymerization, because of the activation of the allylic hydrogen atom toward abstraction. The generated allylic radical is highly stabilized both by the substituent X and by delocalization of the free electrons into the double bond. These radicals add to monomer very slowly and perform side reactions that in turn lead to retardation. 

However, the situation was quite different when we examined [5] a 16-mer consisting only of the A units, and its interaction with a 16-mer of T units. Now there was a hypochromic effect in the UV, there was an additional CD when the strands were mixed, and there was retardation in the gel electrophoresis. We pointed out that these contrasting results did not really make sense in terms of a double helix, and suggested that a triple or higher-order helix must be involved. Our subsequent collaborative studies confirmed this the homopolymeric DNA isomeric strands form a triple helix only, with one poly A and two polyT strands [6]. 

Different valence states are also a fairly widespread type of unit variability. By analogy with macromolecular complexes (Section II), it may be expected that homopolymerization and copolymerization of metal-containing monomers would prevent or retard redox processes involving participation of metal ions. Experimental data confirm the fact that a polymeric matrix stabilizes complexes of metals in low oxidation states (e.g., Pd" ). Moreover, the stability of the Cu+ state during polymerization (including thermal polymerization) of copper acrylate controls the use of this method for the preparation of coordination compounds of Cu". The polymeric framework plays a stabilizing role, whereas the metal ions that are localized on the surface layer are oxidized to Cu +. However, polymerization of monomers that contain metal ions in high oxidation states is often accompanied by their reduction Fe + ->Fe +, and Mo + ->Mo" (scheme 14). For example, polymerization of Cu " and Fe + acrylates may be accompanied by intramolecular chain termination. This may be attributed to the relatively low standard reduction potentials of these metal ions (7io(Cu + Cu+) = 0.15, o(Fe ->Fe ) = 0.77 V). 

In spite of MA versatility, it was commonly believed and taught until the early 1960s that this monomer would not homopolymerize. In fact, the reluctance of MA to polymerize was so well known that it was often used as an example to reinforce the concept that stearic hindrance, polar effects, etc., retard or prohibit the homopolymerizability of 1,2-disubstituted olefins. This long established thought was shattered in 1961 when it was reported that MA had been homopolymerized. " Subsequent studies confirmed the discovery and additional studies showed that MA can be polymerized with both y and UV radiation, in the presence of free-radical initiators, with various pyridine-type bases, electrochemically, and under shock waves. 

A wide variety of copolymers have been synthesized by RAFT polymerization and many examples are provided in the tables (Section 3.07.3.2). RAFT copolymerization can be successful (provide molecular weight control and narrow molecular weight distributions) even when one of the monomers is not amenable to direct homopolymerization using a particular RAFT agent. For example, severe retardation is observed for NVP polymerization in the presence of trithiocar-bonate RAFT agents (e.g with 129 °), yet copolymerization of NVP with an acrylate provides good control and litde retardation (e.g., NVP/octadecyl acrylate (ODA) with 12.6, Scheme 33). 

Existing knowledge of free radical polymerizations would suggest that macromonomers of type 1 should be less reactive than those of type 2 or 3 owing to greater steric hindrance in the former. It is well known, for example, that a-substituents larger than methyl in acrylates retard both homopolymerization and copolymerization (2). 

Non-homopolymerizing y-butyrolactone (BLO) behaves at 150 °C in a totally different way it is a slow activator, and is not incorporated into the PA 6 chains. Rather, higher concentrations of BLO retard the polymerization [38]. 

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