Intermolecular transfer to polymer

Chain scission. The midchain radical structure formed by intra- or intermolecular transfer to polymer is less reactive than a chain-end radical. Under higher temperature conditions, the radical may undergo -ffagmentation (chain scission) as shown in Scheme 3.10 for BA. As well as lowering polymer MW, sdssion produces an unsaturated chain end that can react further (Scheme 3.7b). Scission is important for acrylate polymerizations at temperatures > 140°C [18,21], is a dominant mechanism in styrene polymerizations at 260-340°C [15], and also occurs during LDPE production [14]. Kinetic treatment is difficult, as scission is coupled with LCB and/or SCB formation. 

Scission events can occur in any system where mid-chain radicals are formed. However, scission is more temperature-activated than H-abstraction and thus becomes important only at elevated temperatures. The reaction is not believed to occur during butyl acrylate polymerization at 75 C [37], but is shown to be important at 140°C [29, 45], Scission is a dominant mechanism in styrene polymerizations at 260-340°C [26], and also occurs during LDPE production [30]. Scission of midchain radicals formed via intermolecular transfer to polymer can have a significant effect on the breadth and the shape of polymer MWD [46]. 

Active radicals (ethylene, acrylate, vinyl acetate) are more likely to abstract from a polymer chain than styrenic or methacrylate radicals, and acrylate and vinyl acetate monomer units on a chain are more likely to have an H-atom abstracted. Thus it is not uncommon for the intermolecular transfer to polymer rate of one pairing (for example, acrylate radical to acrylate monomer unit) to dominate the system, with the overall transfer rate decreasing rapidly with increasing content of the less-reactive monomer. 

Chain transfer to polymer leads to branched polymer chains and thus greatly affects the physical and mechanical properties of a polymer, such as the ability to crystallise. Transfer to polymer does not necessarily lead to a decrease of molecular mass, but the chain length distribution becomes broader. If we consider transfer from a growing polymer chain to another dead chain, this is intermolecular transfer to polymer. If the radical attacks a proton in the same chain we call this intramolecular transfer to polymer, also called backbiting. 

Figure 2.3 Reactions that can occur during a polymerisation reaction with acrylates. Y is an acrylate ester side group (or the acid itself). Intramolecular transfer to polymer leads to a branched chain (2a). Intermolecular transfer to polymer results in a shorter chain (species 1) and reinitiates a dead polymer chain and results in a branched chain (2b). The backbone radical can undergo /S-scission that results in two shorter chains (species 3 and 4). The unsaturated species 3 can undergo copolymerisation resulting in species 5 (after van Herk, 2001).

Intermolecular hydride transfer to polymer probably accounts for the short-chain branching found in the polymerizations of 1-alkenes such as propene. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer... 

Transfer to polymer, causing reactivation of a polymer molecule al some point along its length, leads to the growth of branches. The process can occur intermolecularly and also intramolecularly the latter process is particularly important in the free radical polymerization of ethylene at high pressure where it leads to the production of numerous short branches which considerably affect the properties of the polymer. 

Cyclization results from intramolecular chain transfer to polymer, being also indicative of the extent of intermolecular chain transfer leading to formation of branched (nonreactive in the case of oxetanes) tertiary oxonium ion (cf., Section II.D.3). 

Although the two possible modes of chain transfer to polymer (i.e., intra- and intermolecular reactions) does not have to be directly interrelated (intramolecular reaction for example may be prohibited due to the stiffness of chain), having the same origin, they should in many systems proceed simultaneously. Thus, if one of these processes is detected in any system, this is a strong indication that the other reaction also occurs. 

The intermolecular chain transfer to polymer (scrambling), however, is detrimental for preparation of functional polymers, leading to disproportionation of monofunctional macromolecules, as shown schematically below ... 

Formation of block copolymers in the sequential polymerization may be affected by chain transfer to polymer. As already discussed, in several systems the intramolecular chain transfer to polymer leads to formation of cyclic fraction. Cyclic macromolecules, being neutral, do not participate in further reaction and constitute the homopolymer fraction in resulting copolymer. Intermolecular chain transfer to polymer may lead to disproportionation, i.e., formation of fraction of macromolecules which do not carry active species ... 

Intermolecular chain transfer to polymer leads also to the exchange of segments between macromolecules (scrambling). This may effectively preclude the isolation of block copolymers. This phenomenon is especially pronounced in the polymerization of cyclic acetals. 

Transfer to polymer may occur by addition of the growing radical end to a double bond of the polymer chain, or it may occur by II atom abstraction from an active H atom of the chain. H atoms in the a position to a double bond (C=C or C=0) or an ether linkage are easily abstractable and thus lend themselves to polymer transfer reactions. The transfer reaction to polymer may be intramolecular or it may be intermolecular. Both have been demonstrated. In the case of intermolecular transfer the demonstration has been made of the productioTi of graft polymers. The method is to polymerize a monomer in the presence of inert polymer of a different composition. " The final product will contain the inert polymer with the new polymer grafted onto it. 

Chain transfer to polymer in the polymerization of THF is discussed separately, because THF is the model mononKr for which the polymerization is best understood. This transfer leads to the transformation of cyclic, strained tertiary oxonium ions into non-strained oxonium ions, linear or macrocyclic, e.g. (for the intermolecular transfer resulting in a linear structure) ... 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer to cationic propagating center can occur either by electrophilic aromatic substitution (as in the polymerization of styrene as well as other aromatic monomers) or hydride transfer. Short chain branching found in the polymerizations of 1-alkenes such as propylene may be attributed to intermolecular hydride transfer to polynier. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer ... 

In polymerizations of 1-aDcenes such as propylene, intermolecular hydride transfer to polymer can occur giving rise to short chain branches. Such transfer is explained by the fact that propagating carbocations being reactive secondary car-bocations can abstract tertiary hydrogens from the polymer (Plesch, 1953 Odian, 1991). For example, reaction (8.119) produces a relatively stable tertiary carbocation from a more reactive propagating secondary carbocation. 

The MCRs can also be formed by intermolecular chain transfer to polymer (leading to long-chain branches), but its contribution is small. 

Intermolecular chain transfer to polymer results in long-chain branches and proceeds via abstraction of either a backbone tertiary hydrogen atom (in repeat units from vinyl monomers) or an atom from a substituent group. An example... 

Thus, intermolecular chain transfer to polymer leads to premature termination of the growth of one propagating chain and the reactivation of a dead chain which then grows a long-chain branch. As a consequence, the molar mass distribution of the polymer broadens. The changes in skeletal structure and molar mass distribution inevitably have major effects upon bulk polymer properties. 

Since chain transfer to polymer does not change the numbo- of polymo molecules formed, it has no effect upon the number-average molar mass, M . Furthermore, although it results in the formation of branched polymer molecules, only in the case of intermolecular diain transfer to polymer is thrae an effect upon molar mass distribution (see Section 1.3.4) which broadens, leading to increases... 

Scheme 3.7 LCB formation by (a) intermolecular chain transfer to polymer and (b) addition to a terminally unsaturated polymer chain.

Complexities also emerge when chain transfer to polymer mechanisms are examined. The rate of chain transfer to polymer is dependent on both the reactivity of the radical and the abstractability of the hydrogen atom on the repeat unit in the polymer chain. For the case of intermolecular chain transfer (LCB), this is represented by ... 

Non-linear polymers are frequently characterized by their solubility in a given solvent. The insoluble part, which corresponds to high molecular weight heavily branched polymer and polymer networks, is called gel. In the polymerization of monofunctional monomers that form gel by intermolecular chain transfer to polymer followed by termination by combination (e.g., butyl acrylate), the addition of CTA may reduce the gel content to nil, whereas the sol MWD remains essentially unaffected [89]. 

In other polymers, such as poly(vinyl acetate), the branching that results from transfer to polymer is predominantly intermolecular ... 

Quenchers. When a polymer absorbs UV energy, it may be able to dispose of it harmlessly by intermolecular transfer to certain additives that can then carry the energy away and dispose of it harmlessly. These additives are referred to as energy quenchers. Or-... 

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