Polymer initiation

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

While the chemistry of radiation curable hot melt adhesives is the same as that used in liquid (syrup) adhesives and coatings discussed elsewhere in this volume, there is a fundamental difference between the objectives of reaction in the two types of systems. Syrups consist largely or entirely of reactive monomeric and/or oligomeric materials. Radiation is used to initiate the polymerization of virtually the entire mass. In contrast, hot melts generally contain polymers initially, and these polymers are capable of reaction via radiation to produce chain extension and... 

The end group of the polymers, photoinitiated with aromatic amine with or without the presence of carbonyl compound BP, has been detected with absorption spectrophotometry and fluororescence spectrophotometry [90]. The spectra showed the presence of tertiary amino end group in the polymers initiated with secondary amine such as NMA and the presence of secondary amino end group in the polymers initiated with primary amine such as aniline. These results show that the amino radicals, formed through the deprotonation of the aminium radical in the active state of the exciplex from the primary or secondary aromatic amine molecule, are responsible for the initiation of the polymerization. 

The polymers initiated by BP amines were found to contain about one amino end group per molecular chain. It is reasonable to consider that the combination of BP and such polymers will initiate further polymerization of vinyl monomers. We investigated the photopolymerization of MMA with BP-PMMA bearing an anilino end group as the initiation system and found an increase of the molecular weight from GPC and viscometrical measurement [91]. This system can also initiate the photopolymerization of AN to form a block copolymer, which was characterized by GPC, elemental analysis, and IR spectra. The mechanism proposed is as follows ... 

CH2-AcAc)], the grafting reactions of this polymer initiated with Ce(IV) ion will be discussed in Sec. VI.B. Furthermore, the order of the promoting activity of diketone is always greater than that of monoketone. 

Polyacrylics are produced by copolymerizing acrylonitrile with other monomers such as vinyl acetate, vinyl chloride, and acrylamide. Solution polymerization may be used where water is the solvent in the presence of a redox catalyst. Free radical or anionic initiators may also be used. The produced polymer is insoluble in water and precipitates. Precipitation polymerization, whether self nucleation or aggregate nucleation, has been reviewed by Juba. The following equation is for an acrylonitrile polymer initiated by a free radical ... 

It is appropriate to mention here an alternative method for synthesizing monodispersed polymers which was developed by Zimm.67 Emulsion polymerization is initiated by flash photolysis. The second flash terminates the polymers initiated by the first one, starting on a new chain to be terminated by the third flash, and... 

The resulting active end-groups have a covalent structure, rather than ionic, with tetrahedrally bound ligands persisting unaltered during the lifetime of the polymeric chains 46). This persistence of the various initially formed structures accounts for the multimodal distribution of the molecular sizes of polymers initiated by Grignard compounds46). 

Figure 8. Computed drift curves for instantaneous and cumulative CCs of styrene-methyl methacrylate polymers initiated with AIBN...

The di- and monoalkyltin compounds are considered to be effective as stabilizers because they (i) inhibit the onset of the dehydrochlorination reaction by exchanging their anionic groups, X, with the reactive, allylic chlorine atoms in the polymer (ii) react with, and thereby scavenge, the hydrogen chloride that is produced and that would otherwise induce further elimination (jii) produce the compound HX, which may also help to inhibit other undesirable side reactions and iiv) prevent breakdown of the polymer initiated by atmospheric oxidation, i.e., by acting as antioxidants. 

To see if oxidative spreading could take place between different polymers, initiation of stabilized PP using a piece of unstabilized polyamide 6 has been performed. However, polyamide cannot initiate oxidation of PP even if oxidized polyamide is in direct contact with PP [15]. 

The stretching of amorphous but crystallizable materials can greatly increase the rate of crystallization in some cases. Natural rubber and polyethylene terephthalate are examples. The stretching of the polymer initially causes the crystallites to grow so that the chains in the crystallites are oriented parallel to the applied stress. Thus the growth of the crystallites... 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

In the second stage of the reaction, the free radical produced on the backbone of the base polymer initiates polymerisation which results in the formation of graft copolymerisation as under ... 

The thermal decomposition and photolysis of this alkyl have been studied by Buchanan and Creutzberg112. The pyrolysis mechanism is not fully understood. The overall process is first-order and is unaffected by an 8.5-fold increase in surface-to-volume ratio. Based on measurements of pressure increase, the reaction exhibits an induction period ranging from 2-3 minutes at 513 °C to 40 minutes at 466 °C. Short chains are apparently involved. A polymer initially of empirical formula (BCH2) but slowly losing hydrogen to form (BCH) is deposited on the surface. The mechanism probably involves the reactions... 

Now we will try to answer the question why at very large cone angles the cis bidentate ligands give ester formation rather than polymer. Initially, increasing steric bulk leads to faster polymerisation, but at some point it is overtaken by the termination reaction giving propanoates. 

In the presence of a dissolved polymer, the radical polymerization of a monomer by thermal decomposition of an initiator results in a mixture of homopolymerization and graft polymerization [Brydon et al., 1973, 1974 Ludwico and Rosen, 1975, 1976 Pham et al., 2000 Russell, 2002], Polymer radicals (XXX), formed by chain transfer between the propagating radical and polymer, initiate graft polymerization of styrene. The product (XXXI) consists of polystyrene grafts on the 1,4-poly-1,3-butadiene backbone. Polymer radicals are also formed... 

The exact mechanism of polymer initiation is unknown. Initiation of rubber synthesis has been studied in several plants and a common finding is that the end groups found in low molecular weight rubber (such as rubber from goldenrod and H. brasiliensis leaves) are not made up of c/x-isoprene units, unlike the bulk of the rubber [259, 260]. Structural studies [261, 262] have led to the suggestion that the Cl5 FPP may be the most common initiator in vivo, at least in H. brasiliensis. 

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