End groups stability

An aqueous slurry of the salt is heated with acetic acid end-group stabilizer... 

Poly(2,6-dimethyl-l,4-phenylene oxide), commonly called poly(phenylene oxide) or PPO, was introduced commercially in 1964. PPO is manufactured by oxidation of 2,6-dimethyl phenol in solution using cuprous chloride and pyridine as catalyst. The monomer is obtained by the alkylation of phenol with methanol. End-group stabilization with acetic anhydride improves the oxidation resistance of PPO. 

An alternative way to obtain end-group stabilized homopolymers is the etherification or total acetalization of the semi-acetal end groups. Alkylating agents, such as dimethyl sulfate or ortho-formic acid ester, epoxy alkanes, such as ethylene oxide or propylene oxide, and acetals in the presence of Lewis acids, or cation exchange resins are suitable. End-group etherified homopolymers have the additional advantage of alkali stability over esterified polymers, known also in copolymers blocked by comonomer components. 

EMMA, EMMAQUA 173 End group stabilization 305 Energy dose 538... 

Apart from fatty acids, straight-chain molecules containing other hydrophilic end groups have been employed in numerous studies. In order to stabilize LB films chemical entities such as tlie alcohol group and tlie metliyl ester group have been introduced, botli of which are less hydrophilic tlian carboxylic acids and are largely unaffected by tlie pH of tlie subphase. 

Howardt describes a model system used to test the molecular weight distribution of a condensation polymer The polymer sample was an acetic acid-stabilized equilibrium nylon-6,6. Analysis showed it to have the following end group composition (in equivalents per 10 g) acetyl = 28.9,... 

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

The stability of the copolymers may be enhanced by alkaline hydrolysis following polymerisation to remove oxymethyl end-groups and replace them with the more stable oxyethyl groups. 

Commercial polymers are made so that they possess terminal end groups for enhanced stability. Differences in the nature of these end groups have been claimed to be the main reason for the higher hot water and alkali resistance of the coplymers over the homopolymers (see section 19.3.3). 

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

BPO is commonly used as an initiator for S polymerizations and copolymerizations and it has been reported that its use can lead to yellowing and impaired stability in PS.13 1 1 The initiation and termination pathways observed for S polymerization when BPO is used as initiator have been discussed in Sections 3.2 and 3.4.2.2.1. These give rise to benzoyloxy and phenyl end-groups as follows (Scheme 8.1). 

NMR studies15 1 1 on polymers prepared with, 3C-labeled BPO have shown that the primary benzoyloxy and phenyl end groups formed by tail addition to monomer are thermally stable under conditions where the polymer degrades. They persist to > 50% weight loss at 300°C under nitrogen. Thus, these groups are unlikely to be directly responsible for the poor thermal stability of PS prepared with BPO as initiator. On the other hand, the secondary benzoate end groups, formed by head addition or transfer to initiator, appear extremely labile under these conditions. Their half life at 300°C is <5 min. 

These examples show how initiator selection can be critical in determining the properties of PS prepared by radical polymerization. If thermal stability were of importance, then, since some initiator-derived ends cannot be avoided, a preferred initiator would be one which gives rise to end groups that do not readily eliminate or dissociate. End groups formed with AIBN initiator appear stable with respect to the polymer backbone,19 Many other systems remain to be studied. 

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

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