Grafting homopolymerization

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). 

Graft or block copolymers are expected to be formed via process (28) (i) and (ii), respectively, depending on whether the polymer Ap is of a crosslinking type or a degrading type. No homopolymerization occurs in... 

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]. 

The OH radicals may initiate grafting via abstraction of the hydrogen atom from cellulose or may also initiate homopolymerization. 

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]. 

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. 

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. 

A major issue is the control of the side reactions that accompany grafting. These reactions include radical-induced degradation of the substrate by cross-linking and/or chain scission and homopolymerization of the graflee monomer. 

A A, and less often MAA or itaconic acid33 5 have been successfully grafted onto polyolefins. In the case of AA, grafting is often accompanied by homopolymerization.333... 

A1 Malaika et a/.35" 754 have reported on the grafting of antioxidant moieties onto PP as mono- (e.g. 33) or bis-(meth)acrylic derivatives (34). Moderate grafting yields (10—40%) and some homopolymerization was observed in the case of the monoacrylate. However, with the bis-acrylate (34) close to 100% grafting yield was reported. 

The vinylsilanes (e.g. 40, 41) do not readily homopolymerize. Forsyth et al.Mj explored the mechanism of grafting these monomers using dodecane as a model for PE. Their work suggests that multiple monomer units are attached through a sequence of addition and intramolecular hydrogen atom transfer steps by a mechanism analogous to that shown in Scheme 7.33 on page 394. 

All grafted samples are extracted in water for 24h before they are analyzed and most homopolymer is then washed off. It is concluded from these experiments that graft-co-polymerization occurs predominantly during the early stages of the reaction. In the later stages (here after about 20 min.) we have mainly homopolymerization. 

Many vinyl monomers were reported to have been grafted onto fluoropolymers, such as (meth)acrylic acid and (meth)acrylates, acrylamide, acrylonitryl, styrene, 4-vinyl pyridine, N-vinyl pyrrolidone, and vinyl acetate. Many fluoropolymers have been used as supports, such as PTFE, copolymers of TFE with HFP, PFAVE, VDF and ethylene, PCTFE, PVDF, polyvinyl fluoride, copolymers ofVDF with HFP, vinyl fluoride and chlorotrifluoroethylene (CTFE). The source of irradiation has been primarily y-rays and electron beams. The grafting can be carried out under either direct irradiation or through the use of preliminary irradiated fluoropolymers. Ordinary radical inhibitors can be added to the reaction mixture to avoid homopolymerization of functional monomers. 

The hydroxyl radicals formed may abstract hydrogen from the cellulose fiber substrate which gives grafting sites and subsequently grafted polymer with monomer present. The HO- radicals may also initiate homopolymerization. This means that reaction (17) is not specific for initiation of grafting. Another disadvantage is that the Fe + ions formed - if not carefully removed -may cause discoloration of the resulting product. 

The hydroxyl radicals may also initiate homopolymerization by addition to monomer. In the redox system (20) the hydroxyl radicals are reduced to hydroxyl ions leaving the acetoxy radicals which are more specific for grafting (21)., ... 

In the absence of sensitizer, neither graft nor homopolymerization was induced by irradiation at 366nm. However, irradiation by a low pressure mercury lamp through a quartz plate(the glass plate(c) in Figure 1 was replaced by a quartz plate.) induces slow surface grafting sensitized by acetone. 

Combination of BP with 2-propanol or amines induces homopolymerization alone. The rate constants of BP 3 - isopropylamine and triethylamine are 2.95 10 and 2.42 1()9m-1s-1, respectively(22) whereas that of BP 3 - isooctane as a model of OPP is 1.0 lO M s l (24). Also hydrogen abstraction from 2-propanol(k=1.0 106 M s"1) (25) is much more efficient than that from aliphatic hydrocarbons. Even methanol is more reactive (k=2.8 10% - s - -) (25) than OPP towards BP 3. The aforementioned results and the finding that surface grafting does not occur in methanol are well interpreted by the following elementary reactions. 

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