Reactive Chains

In the typical ring-opening polymerization, reactive hydroxyl groups are automatically formed at the ends of the chains. Substitution reactions 

A pair of vinyl or other unsaturated groups can also be linked by their direct reactions with free radicals. Similar end groups can be placed on siloxane chains by the use of an end blocker during polymerization. Reactive groups such as vinyl units can be introduced as side chains by random copolymerization involving, for example, methylvinylsiloxane trimers or tetramers.  

One of the most important uses of end-functionalized polymers is the preparation of block copolymers. ° The reactions are identical to the chain extensions already mentioned, except that the sequences being joined are chemically different. In the case of the—OSiR R Y chain end R is typically and Y can be NH, OH, COOH, CH = CH, and so on. 

The siloxane sequences containing these ends have been joined to other polymeric sequences such as carbonates, ureas, urethanes, amides, and imides. Other functional groups include amines and sulfonic acids, ° ° ammonium groups, epoxides, and chloroalkyl groups.  

It is also possible to prepare polysiloxane ionomers. For example, PDMS with carboxyl side groups has been prepared with a controlled number of 

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The initial sulfur copolymer that is formed is often high conversion and gelled. Molecular weight is reduced to the required level by cleaving some of the polysulfide Linkages, usually with tetraethylthiuram disulfide. An alkaU metal or ammonium salt (30) of the dithiocarbamate, an alkaU metal salt of mercaptobensothiasole (31), and a secondary amine (32) have all been used as catalysts. The peptization reaction results in reactive chain ends. Polymer peptized with diphenyl tetrasulfide was reported to have improved viscosity stabiUty (33). 

Chain transfer is kinetically equivalent to copolymerization. The Q-e and Patterns of Reactivity schemes used to predict reactivity ratios in copolymerization (Section 7.3.4) can also be used to predict reactivities (chain transfer constants) in chain transfer and the same limitations apply. Tabulations of the appropriate parameters can be found in the Polymer Handbook 3 ... 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

An extremely favorable consequence of both strategies is the presence of significant amounts of covalent, or inactive, chain ends. This substantially lowers the overall concentration of reactive chain ends which results in a decrease in the occurrence of unwanted side reactions such as termination, disproportionation, or combination. This enables the polymer chain to grow in a controlled fashion, exhibiting many of the attributes typically associated with a living polymerization. However, it should be pointed out that the occurrence of these side reactions is not eliminated and in the strictest sense, the polymerizations are not truly living. 

Note 2 A reaction of a reactive chain end of a linear macromolecule with an internal reactive site of another linear macromolecule results in the formation of a branch point, but is not regarded as a crosslinking reaction. 

The most critical point of all CRP techniques is to gain absolute control over the activation and deactivation of the reactive chain end. This can be simply controlled by altering the polymerization temperature or increasing the deactivator concentration. Thus, additional stable free-nitroxide compounds can be added to the... 

Initiation involves a loss of two electrons and one proton from aniline to form a nitrenium ion (Eq. 2-223), which subsequently attacks aniline by electrophilic substitution (Eq. 2-224). Propagation proceeds in a similar manner by oxidation of the primary amine end of a growing polymer chain (Eq. 2-225) followed by electrophilic substitution (Eq. 2-226). The process has been referred to as reactivation chain polymerization to highlight the fact that the chain end formed after each addition of aniline must be reactivated to the nitrenium... 

To the first category belong the homo- and copolymerization of macromonomers. For this purpose, macromolecules with only one polymerizable end group are needed. Such macromonomers are made, for example, by anionic polymerization where the reactive chain end is modified with a reactive vinyl monomer. Also methacrylic acid esters of long-chain aliphatic alcohols or monofunctional polyethylene oxides or polytetrahydrofurane belong to the class of macromonomers. 

Chain-transfer reaction to a reactive chain-transfer agent... 

In light of the higher imidization temperatures associated with poly(amic alkyl esters), this approach could be extended to yield improved coating formulations [82]. Since it was now possible to balance the relative reactivity of the chain-extender with the imidization temperature of an oligofamic alkyl ester), here R = methyl, ethyl, etc., less reactive chain-extenders could be utilized. The lower reactivity of the chain-extender would be reflected in improved solution stability and shelf-life of the formulation without sacrificing the mechanical properties of the final polyimide, see Table 10 where EGX, TFE,... 

It is established that more reactive monomer forms less reactive chain end (23). Therefore, Eq. (29) implies that the kp/kp value becomes dose to unity as growing ends become more reactive. Simultaneously, this is the case as attacking monomers become more reactive. 

The terms X and t/i in Equation 17 must be determined by an experimentally accessible expression. This is possible by calculating the distribution of the sequential lengths of the reactive chain ends. For monomer Mi we can postulate the following equations... 

The last terms in Equation 18 express the fact that new reactive chain end —(Mi) .i — M. is formed by the reversible alternating polymerization step... 

The model with different end-groups is not realistic in all cases isotactic polypropylene and syndiotactic polypropylene are chiral, or more precisely, their structure is cryptochiral. This model is to be chosen when examining oligomers, and especially when studying the polymerisation mechanism where the structure of the reactive chain end is of extreme importance [16]. 

Control of Molecular Weight. Studies have been conducted on techniques for controlling the molecular weight of poly-p-xylylenes produced from di-p-xylylenes by the vacuum pyrolysis route. Earlier work by Szwarc (17), Errede (3), and Auspos (I) indicated that very reactive chain transfer agents were required to achieve a significant effect in the polymerization of p-xylylene derived from p-xylene. This general picture was confirmed in the present study. 

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