Degradation of Homo- and Copolymers

Hakkarainen, M., Albertson, A. and Karlsson, S., Weight loss and molecular changes correlated with the evolution of hydroxyacids in simulated in vivo degradation of homo and copolymers of PLA and PGA. Polymer Degradation and Stability 52, 283-291 (1996). 

Estimated onset of weight loss during in vivo degradation of homo and copolymers in dependence on initial intrinsic viscosity. 

The generation of homo- and copolymers prepared by Friedel-Crafts reactions of 2,7-bis [(2-ferroceneyl)methylene]cycloheptanone with various diacid chlorides has been reported.222 These polyketones were insoluble in most common organic solvents, but were quite soluble in concentrated H2S04. The homopolymers were thermally stable with two decompositions between 220 and 520°C, and the copolyketones showed decomposition at temperatures between 250 and 600°C. In both cases the first weight loss was determined to depend on the nature of the polymers and occurred at a faster rate than the second degradation. 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

Homo and copolymers of tetrahydrofuran (THF) and alkylene oxides (AO) are used as soft segment glycols in the preparation of polyurethanes. In the process of producing the THF-AO homo and copolymers certain oligomeric cyclic ethers are also produced and can comprise 7-15 % of the polymer. These cyclic ethers are undesirable because when the polymers are used to prepare polyurethane the cyclic ethers tend to degrade the polyurethane s properties. 

Several excellent reviews have recently been published that describe the broad field of degradable biomedical polymers [9,10]. Well-known hydrolytically degradable polymers developed or being developed for biomedical used include homo- and copolymers of polyamides (usually derived from amino adds), polyesters, polyanhydrides, poly(ortho ester)s, poly(anfido amines), and poly(P-amino esters). This chapter is focused on polyacetals, which... 

O Driscoll reports that several polymer-monomer combinations were used in the controlled preparation of homo- and block polymers by ultrasonic radiation(108). Similarly, Fujiwara reports the preparation of poly(styrene-b-methyl methacrylate) by ultrasonic degradation of polystyrene(109). The utility of ultrasonic irradiation is limited since yields are usually low, and the block copolymer is often contaminated with relatively large amounts of the homopolymers. 

The two aromatic units were chosen so that the electroactivity of the preformed polymer was not degraded through an overoxidation process in the course of the electroformation of the second polymer. Generally this condition was fullfilled when the oxidation potentials of these two units were relatively close. The effect of cations on the electrochemical behavior of the homo- and copolymer resulting from 10 has been investigated in the presence of alkali cations [259]. 

Zhu et al carried out an in vivo degradation study of several dl-LA/G A homo-and copolymers and found that all corresponding threads were completely absorbed by 60 days [115]. Ogawa et al, studied in vivo degradation of 1 x 10 x 10 mm plates made of dl-LA/G A polymers and found that both the lag time and the half-life decreased with the increase in GA content [116]. Amarpreet and Hubbell prepared a series of 66 terpolymers of DL-lactide, glycolide and e-caprolactone [117]. The half-lives of these polymers were found to vary from a few weeks to several months. 

POM homo- and copolymers represent a special class in terms of degradation behavior, because they exhibit degradation reactions that either do not occur in other polymers, or are of secondary significance [86]. 

The main difference between homo- and copolymers is the fact that copolymers exhibit higher thermal-oxidative resistance. Figure 5.196. Thermal-oxidative degradation in polyoxymethylene leads to the formation of unstable end groups that initiate depolymerization under the formation of formaldehyde. In homopolymers, this results in total decomposition. In copolymers, depolymerization proceeds only to the next comonomer unit (mostly polyethylene, comonomer content approx. 0.5 to 5 wt.%). That widens the processing window for copolymers and reduces the risk of mold fouling [771]. 

Strictly thermal cleavage of the C-O-C bond in polyoxymethylene can be observed above 270 °C under complete exclusion of oxygen. Because the maximum service temperatures for POM-homo- and copolymers range between 100 and 140 °C, thermal degradation does not occur at operating temperatures [86]. 

Catiker, E., Gumusderelioglu, M., and Gimer, A. 2000. Degradation of pla, plga homo- and copolymers in the presence of serum albumin A spectroscopic investigation. Polym Int 49 728-34. 

Schwamborn, M. (1998) Chemical Synthesis of polyaspartates a biodegradable alternative to currently used polycarboxylate homo- and copolymers. Polym. Degrad. Stab., 59 (1), 39-45. 

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