Ester end group

Acetal resins are generally stable in mildly alkaline environments. However, bases can catalyse hydrolysis of ester end groups, resulting in less thermally stable polymer. 

In general, aliphatic diacyl peroxide initiators should be considered as sources of alkyl, rather than of aeyloxy radicals. With few exceptions, aliphatic acyloxy radicals have a transient existence at best. For certain diacyl peroxides (36) where R is a secondary or tertiary alkyl group there is controversy as to whether loss of carbon dioxide occurs in concert with 0-0 bond cleavage. Thus, ester end groups observed in polymers prepared with aliphatic diaeyl peroxides are unlikely to arise directly from initiation, but rather from transfer to initiator (see 3.3,2.1.4),... 

The choice of the alcohol permits manipulation of the structure of the polymer. Water and monohydric alcohols afford linear chains with carboxylic acid and ester end groups, respectively. Polyhydroxy initiators afford a route to ester end-blocked star and comb polymers (Fig. 4) (47). 

Figure 15 shows this same expanded region when di-methyl terephthalate is used as the monomer. The only difference is the sharp single peak labelled g, which arises from residual methyl ester end groups. This resonance sits on top of the triplet arising from the -CH2OH (f) end (at 400 MHz) and, therefore, deconvolution software is used to measure these residual methyl ester ends. 

Methanolysis can occur via inter- or intramolecular nucleophilic attacks by MeOH at the carbon atom of a Pd-acyl moiety and produces an ester end group and a Pd - H+ species. Again, the hydride may deprotonate with formation of less active Pd(0) and dimeric species (Fig. 3) [47,57]. 

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

We note, however, that even if methane comes from the photolysis of the methyl ester, this does not necessarily mean that all of it arises as a result of first breaking down the PET itself by photolysis. Both the PET of early reports and also the PECT were undoubtedly produced using dimethyl terephthalate (DMT) instead of terephthalic acid and so have methyl ester end groups owing to the well-known incomplete reaction of the DMT. There may be only a small level of these ester groups, but the amount of methane produced was small as well. This, at the very least, causes some potential confusion about the source of all the methane should some of it come from methyl ester photolysis. No such confusion would exist for the ethane should that be coming from photolysis of an ethyl ester. 

The styrene/CO polymers formed with palladium complexes of diimine ligands indeed contain ester and alkene end groups [65,66,67], Slightly more ester end groups than alkene groups are formed, showing that in addition to P-hydride elimination some termination via methanolysis of acylpalladium chain ends occurs. 

Amongst the first studies presenting the use of dendritic polymers for thermoset applications was the work of Hult et al. [62]. They modified hyperbranched hydroxy functional polyesters with various ratios of maleate-allyl ether/alkyl ester end groups. Dependent on this ratio, resins with different vis-... 

Figure 2 MALDI-TOF mass spectrum of a ST-PHPMA-COOCH3. Peak series a represent polymer chains with initiator (IBN) end groups peak series b represent polymer chains with methyl ester end groups.

The percentage of diester, keto-ester and diketone end groups in the copolymers depends on the relative rates of alcoholysis and protonolysis. For example, when these two termination processes occur at a comparable rate, the diester, keto-ester and diketone end groups are in a 1 2 1 ratio, respectively [11], The use of oxidant co-reagents may increase the amount of ester end groups [lb], while the chain-transfer mechanism controls the nature of the termination metal product (Pd-OR, Pd-OH and Pd-H). 

The chain transfer by alcoholysis involves the attack of ROH on a propagating Pd-acyl to give a free chain, bearing at least an ester-end group, and a Pd-hydride species, which re-initiates the chain growth by insertion of ethene (Scheme 7.15a). 

Thus the alkyl ester end groups of convergently grown poly(benzyl ether) dendrimers can be subjected to post-synthetic modification by hydrolysis [28], reduction [29], transesterification/amidation [28] in a variety of ways. Subsequent modification of poly(benzyl ether) dendrons bearing p-bromobenzyl end groups by palladium-catalysed coupling reactions permitted preparation of dendrons with phenyl, pyridinyl, or thiophenyl end groups [30]. 

Fig. 40 Intramolecular (a) and intermolecular (b) transesterification reactions with phe-noxyl ester end groups of poly(e-caprolactone)...

Poly(lactone-co-trimethylene carbonates) containing photocurable coumarin ester end groups have been prepared which are crosslinkable upon irradiation with ultraviolet light by a [2 -I- 2] cycloaddition. These materials are useful in the preparation of in vivo implants. 

Either halide or ester end groups L could be used, and then functionalized further as outlined in Eq. (102). 

Some of the carboxylic acid groups were esterified to methyl ester end groups when methanol was present. 

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