Monomer homopolymerization

It is then easy to determine Taa and Tba the reactivity ratios corresponding to the most homopolymerizing monomer. This simplification explains that the influence of remote units on the reactivity of the growing end radical has been first evidenced which such systems. 

It should be pointed out that the presented picture applies to homopolymerization. Monomers that do not undergo homopolymerization at particular conditions may still be able to undergo copolymerization. It should also be remembered that thermodynamic parameters of polymerization are valid for the transformation of monomer into relatively high-molecular-weight polymer. Therefore, even at the conditions when monomer does not polymerize due to the thermodynamic restrictions, some low-molecular-weight oligomers may still be formed. 

Polymerization takes place, in the following manner in the presence of suitable peroxide catalyst these compounds polymerize with themselves (homopolymerizatiOn) in aqueous emulsion. When the reaction is complete, the emulsified polymer may be used directly or the emulsion coagulated to yield the solid polymer (312). A typical polymerization mixture is total monomer (2-vinylthiazole), 100 sodium stearate, 5 potassium persulfate, 0.3 laurylmercaptan, 0.4 to 0.7 and water, 200 parts. 

In this chapter we deal exclusively with homopolymers. The important case of copolymers formed by the chain mechanism is taken up in the next chapter. The case of copolymerization offers an excellent framework for the comparison of chemical reactivities between different monomer molecules. Accordingly, we defer this topic until Chap. 7, although it is also pertinent to the differences in the homopolymerization reactions of different monomers. 

Although Table 7.1 is rather arbitrarily assembled, note that it contains no systems for which rj and r2 are both greater than unity. Indeed, such systems are very rare. We can understand this by recognizing that, at least in the extreme case of very large r s, these monomers would tend to simultaneously homopolymerize. Because of this preference toward homopolymerization, any copolymer that does form in systems with rj and r2 both greater than unity will... 

The tendency toward alternation is not the only pattern in terms of which copolymerization can be discussed. The activities of radicals and monomers may also be examined as a source of insight into copolymer formation. The reactivity of radical 1 copolymerizing with monomer 2 is measured by the rate constant kj2. The absolute value of this constant can be determined from copolymerization data (rj) and studies yielding absolute homopolymerization constants (ku) ... 

Write structural formulas for maleic anhydride (M ) and stilbene (M2). Neither of these monomers homopolymerize to any significant extent, presumably owing to steric effects. These monomers form a copolymer,... 

The use of monomers that do not homopolymerize, eg, maleic anhydride and dialkyl maleates, reduces the shock sensitivity of tert-huty peroxyesters and other organic peroxides, presumably by acting as radical scavengers, that prevent self-accelerating, induced decomposition (246). 

DADC HomopolymeriZation. Bulk polymerization of CR-39 monomer gives clear, colorless, abrasion-resistant polymer castings that offer advantages over glass and acryHc plastics in optical appHcations. Free-radical initiators are required for thermal or photochemical polymerization. 

Addition of dialkyl fumarates to DAP accelerates polymerization maximum rates are obtained for 1 1 molar feeds (41). Methyl aUyl fumarate [74856-71-6] (MAF), CgH QO, homopolymerizes much faster than methyl aUyl maleate [51304-28-0] (MAM) and gelation occurs at low conversion more cyclization occurs with MAM. The greater reactivity of the fumarate double bond is shown in copolymerization of MAF with styrene in bulk. The maximum rate of copolymerization occurs from monomer ratios, almost 1 1 molar, but no maximum is observed from MAM and styrene. Styrene hinders cyclization of both MAF and MAM. 

DiaUyl fumarate polymerizes much more rapidly than diaUyl maleate. Because of its moderate reactivity, DAM is favored as a cross-linking and branching agent with some vinyl-type monomers (1). Cyclization from homopolymerizations in different concentrations in benzene has been investigated (91). DiaUyl itaconate and several other polyfunctional aUyl—vinyl monomers are available. 

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Bismaleimides are best defined as low molecular weight, at least diftinctional monomers or prepolymers, or mixtures thereof, that carry maleimide terminations (Eig. 3). Such maleimide end groups can undergo homopolymerization and a wide range of copolymerizations to form a highly cross-linked network. These cure reactions can be effected by the appHcation of heat and, if required, ia the presence of a suitable catalyst. The first patent for cross-linked resias obtained through the homopolymerization or copolymerization of BMI was granted to Rhc ne Poulenc, Erance, ia 1968 (13). Shordy after, a series of patents was issued on poly(amino bismaleimides) (14), which are synthesized from bismaleimide and aromatic diamines. 

The effects of increasing the concentration of initiator (i.e., increased conversion, decreased M , and broader PDi) and of reducing the reaction temperature (i.e., decreased conversion, increased M , and narrower PDi) for the polymerizations in ambient-temperature ionic liquids are the same as observed in conventional solvents. May et al. have reported similar results and in addition used NMR to investigate the stereochemistry of the PMMA produced in [BMIM][PFgj. They found that the stereochemistry was almost identical to that for PMMA produced by free radical polymerization in conventional solvents [43]. The homopolymerization and copolymerization of several other monomers were also reported. Similarly to the findings of Noda and Watanabe, the polymer was in many cases not soluble in the ionic liquid and thus phase-separated [43, 44]. 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

During mutual graft copolymerization, homopolymerization always occurs. This is one of the most important problems associated with this technique. When this technique is applied to radiation-sensitive monomers such as acrylic acid, methacrylic acid, polyfunctional acrylates, and their esters, homopolymer is formed more rapidly than the graft. With the low-molecular weight acrylate esters, particularly ethyl acrylate, the homopolymer problem is evidenced not so much by high yields as by erratic and irreproducible grafting. 

To avoid homopolymer formation, it is necessary to ensure true molecular contact between the monomer and the polymer. Even if this is initially established, it needs to be maintained during the radiation treatment while the monomer is undergoing conversion. Several methods are used for minimizing the homopolymer formation. These include the addition of metal cations, such as Cu(II) and Fe(II). However, by this metal ion technique, both grafting and homopolymerization are suppressed to a great extent, thus permitting reasonable yield of graft with little homopolymer contamination by the proper selection of the optimum concentration of the inhibitor [83,90,91]. 

Homopolymerization of macroazoinimers and co-polymerization of macroinimers with a vinyl monomer yield crosslinked polyethyleneglycol or polyethyleneglycol-vinyl polymer-crosslinked block copolymer, respectively. The homopolymers and block copolymers having PEG units with molecular weights of 1000 and 1500 still showed crystallinity of the PEG units in the network structure [48] and the second heating thermograms of polymers having PEG-1000 and PEG-1500 units showed that the recrystallization rates were very fast (Fig. 3). 

Currently, graft post-polymerization of monomers in the gaseous phase (2) is the more widely used process because it has at least two basic advantages. First, side processes of homopolymerization are minimized which reduces the consumption of monomers and makes unnecessary additional treatment of the modified materials with solvents. Second, this method is universal and allows the grafting to the surfaces (such as silica) to be carried out with low radiation yields of active sites as compared to the monomers. 

All this evidence suggests that the radical produced from 2-vinylfuran is a rather strongly stabilized entity, compared with those of more common monomers, and is therefore, not very active in homopolymerization. On the other hand, because of its relative stability, it does not add easily to monomers like styrene, vinylidene chloride or butadiene, and thus the copolymerization rates are also low. Aso and Tanaka86) calculated the values of Q and e as 2.0 and 0.0, respectively. 

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). 

Since Ce4+ salts are capable of causing the homopolymerization of vinyl monomers starting after a certain induction period, the grafting process is carried out during a time period shorter than the period of induction so as to synthesize graft PAN copolymers without any homopolymer being formed68). 

The investigations have shown, however, that graft copolymerization carried out according to this method is accompanied with a simultaneous reaction of monomer homopolymerization which, naturally, reduces the effectiveness of the method. This is explained by the presence of hydroxyl radicals in the reaction medium, which are formed as formulated in the above scheme. 

This section describes polymerizations of monomer(s) where the initiating radicals are formed from the monomer(s) by a purely thermal reaction (/.e. no other reagents are involved). The adjectives, thermal, self-initialed and spontaneous, are used interchangeably to describe these polymerizations which have been reported for many monomers and monomer combinations. While homopolymerizations of this class typically require above ambient temperatures, copolymerizations involving certain electron-acceptor-electron-donor monomer pairs can occur at or below ambient temperature. 

Aspects of thermal initiation have been reviewed by Moad et al., w Pryor and Laswell, 10 Kurbatov/" and Hall.312 It is often difficult to establish whether initiation is actually a process involving only the monomer. Trace impurities in the monomers or the reaction vessel may prove to be the actual initiators. Purely thermal homopolymerizations to high molecular weight polymers have only been demonstrated unequivocally for S and its derivatives and MMA. For these and other systems, the identity of the initiating radicals and the mechanisms by which they are formed remain subjects of controversy. 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

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