Addition polymerization initiation

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Plasma-induced species act as initiator of polymerization. Polymerization characteristics and properties of polymers formed by plasma-induced polymerization strongly resemble those of the thermal polymerization of the corresponding monomer [2-12]. Results indicate that plasma-induced polymerization is a free radical addition polymerization initiated by difunctional free radicals created by plasma. The molecular weight of polymer increases with the polymerization time, which is distinctively different from the initiator-initiated free radical addition polymerization. 

Table 3.4). This is because of the correlation between the mechanical properties of each polymer and those of the crazes. A similar parallel behaviour between the microhardness values of the bulk and the craze fibrils was not observed for PVC. This discrepancy is probably due to the presence of additives (polymerization initiators, stabilizers, etc.).

A typical free-radical polymerization involves all of the steps in addition polymerization initiation, propagation, chain transfer, and termination. In contrast to anionic... 

PBS and PBSA are synthesized from 1,4-butane-diol and succinic (and/or adipic) acid. The reaction at 215-220°C under high vacuum is Sn-catalyzed. The resulting M = 40 kg/mol is not adequate, thus a small amount of unsamrated carboxylic acid is added and the addition polymerization (initiated by peroxides) increases M to the desired level, viz. M = 220 kg/mol [Takiyama and Fujimaki, 1994 Yoshikawa et a/., 1996]. 

A second class of silicones cures by addition polymerization initiated by a catalyst Polymerization occurs by a free-radical mechanism involving a vinyl, allyl, or other unsaturated group of a silicone monomer (Figure 3.9). Homopolymers can be formed... 

The -Si-O-Si-O- backbone of silicones is referred to as siloxane. The silicon atoms may be linked to a wide variety of aliphatic or aromatic groups, as shown in Fig. 3.6 where the R groups are commonly methyl (-CH3), phenyl (C6H3-), allyl (-CH2-CH=CH2), or vinyl (-CH=CH2). Silicones used in electronic assembly and packaging may be either room-temperature vulcanizing (RTV) that cure by condensation polymerization or vinyl types that cure by addition polymerization initiated by a catalyst. Table 3.4 lists some formulations for each type. 

We studied a behavior of 5-trimethylsilyl-2-norbomene under the conditions of addition polymerization initiated by some Ni- and Pd-containing catalytic systems. The catalysts of this type have already demonstrated high activity in addition polymerization of norbomene and its alkyl derivatives [25,26]. The known Pd-containing catalytic systems (Ti -allyl)Pd(SbF6) [27] and a,7t-bicyclic complex [NB(OMe)PdCl]2 [28] turned out to be practically inactive in polymerization of 5-trimethylsilyl-2-norbomene. On the contrary, Ni-based complexes displayed a real activity in respect to this monomer. As a result saturated cyclolinear polymers were formed according to Scheme 6 of addition polymerization. The absence of any unsaturation in these polymers was confirmed by both infrared (IR) (no bands in 1620-1680 cm region) and H NMR spectroscopy (no signals at 5-6 ppm). 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

In the next three sections we consider initiation, termination, and propagation steps in the free-radical mechanism for addition polymerization. One should bear in mind that two additional steps, inhibition and chain transfer, are being ignored at this point. We shall take up these latter topics in Sec. 6.8. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Tetrahydrofurfuryl alcohol is used in elastomer production. As a solvent for the polymerization initiator, it finds appHcation in the manufacture of chlorohydrin mbber. Additionally, tetrahydrofurfuryl alcohol is used as a catalyst solvent-activator and reactive diluent in epoxy formulations for a variety of apphcations. Where exceptional moisture resistance is needed, as for outdoor appHcations, furfuryl alcohol is used jointly with tetrahydrofurfuryl alcohol in epoxy adhesive formulations. 

In addition to the monomers, the polymerization ingredients include an emulsifier, a polymerization initiator, and usually a chain-transfer agent for molecular weight control. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

Anionic Polymerization. Addition polymerization may also be initiated and propagated by anions (23—26), eg, in the polymerization of styrene with -butyUithium. The LL gegen ion, held electrostatically in... 

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

Butadiene is also known to form mbbery polymers caused by polymerization initiators like free radicals or oxygen. Addition of antioxidants like TBC and the use of lower storage temperatures can substantially reduce fouling caused by these polymers. Butadiene and other olefins, such as isoprene, styrene, and chloroprene, also form so-called popcorn polymers (250). These popcorn polymers are hard, opaque, and porous. They have been reported to... 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Polymerization methods [I], [II], and [III] (Fig. 1) indicate, respectively, the dropwise addition of VAc and initiator the dropwise addition of VAc and the stepwise addition of initiator the batch method, in which all ingredients of water, VAc, PVA, and initiator were put into the reaction vessel before starting polymerization. In method [I], when the temperature of the PVA solution in the flask attained 70°C, dropwise additions of 20 g of an aqueous solution containing initiator and 250 g of VAc were started. In method [II], the process was similar to method [I], except the initiator was added stepwise. When the temperature of the contents in the flask was raised to 70°C, 24 g of an aqueous solution containing half the prescribed amount of initiator was first added. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

Thus, even if /-amyloxy radicals (101) show similar specificity for addition 1 5-abstraction to /-butoxy radicals, abstraction will be of lesser importance.42"42 The reason is that most /-amyloxy radicals do not react directly with monomer. They undergo [3-scission and initiation is mainly by ethyl radicals. Ethyl radicals are much more selective and give addition rather than abstraction. This behavior has led to /-amyl peroxides and peroxyesters being promoted as superior to the corresponding /-butyl derivatives as polymerization initiators.423... 

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M"1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

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