Water-soluble, Degradable Polymers

Two types of water-soluble, degradable polymers are well known in the polymer literature semidegradable or completely degradable. 

Semidegradable polymers degrade to smaller polymers, which must be under the renal clearance limit [21, 22] (including supramolecular structures, e.g., nanogels [23] and micelles [24]). However, it should be noted here that lysosomal accumulation is also a risk for polymers with a below the renal clearance limit and hence where possible, polymers used should degrade to small molecules [15]. 

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The desizing of water-soluble size polymers can be summarised as follows. Batchwise or continuous methods can be used in both cases an adequate supply of hot water is needed during the washing-off. Hot water and detergent are needed to remove polyfvinyl alcohol) or carboxymethylcellulose. The addition of alkali may be beneficial with carboxymethylcellulose. Alkali is essential with modified starches and acrylic acid copolymers. Polyfvinyl alcohol) can be degraded effectively by alkaline oxidation. 

Polyvinyl alcohol is a vinyl polymer with a carbon-carbon backbone similar to other polymers such as polyethylene and polystyrene and is widely used as a water-soluble biodegradable polymer in the manufacture of delivery systems for fertilizers, pesticides and herbicides and is also used to manufacture containers and films. To date, only bacteria have been described as causing biodegradation of polyvinyl alcohols with some causing complete degradation of the polymer (see Shimao, 2001 for review). 

After the fermentation is complete, extracted water-soluble degradation products lose much of their water solubility if taken to dryness in air or if heated. Freeze-drying overcomes this problem, but molecular weight distributions should be determined before freeze-drying because polymer chains can be fractured by growing solvent crystals (8). 

Interest has centred on the possible use of water-soluble phosphazene polymers (Section 12.15) as carrier molecules for drugs, enzymes and other bioactive agents. Attachment of a chemotherapeutic ageut to a suitable polymer may achieve targeting of the drug ou a specific site within the body, and secure controlled release. Such a carrier molecule should (a) be water soluble at physiological pH 7.0, aud (b) be degradable into small non-toxic molecules which can subsequently be eliminated from the body [30,31]. 

Poly(aspartic acid) (PAA) is another promising biomaterial synthesized from aspartic acid. It is a highly water-soluble ionic polymer which is readily degraded by lysosomal enzymes. It can be readily prepared as a hydrogel for various biomedical applications, and it has also been copolymerized with PLA, PCL, and PEG to improve its mechanical performance and reduce its hydrophilicity and rate of degradation. 

Figure 2.5 Degradation profiles for the water-soluble PYRP (polymer 1) and PCPP (polymer 4). PYRP poly[ 2s(2-(2-oxo-l-pyrrolidinyl)ethoxy phosphazene]. Combinations of the two side groups produce polymers 2 and 3 with intermediate degradation rates. Conditions pH 7.4 at 55 °C. Reproduced with permission from A.K. Andrianov, A. Marin and P. Peterson, Macromolecules, 2005, 38,19, 7972. 2005, American Chemical Society [41]...

Yoshioka et al, examined the influence of basic additives on HB/HV polymer degradation [134, 167]. They found that the degradation of PHB and HB/HV copolymers could be accelerated by incorporating basic compounds and that the rate could be controlled by changing the loading amount and the basicity of basic compounds. On the other hand, the hydrolysis rate was governed by water solubility and polymer-water partition of incorporated bases, and corresponded with the diffusion rate of water in films. 

The presence of water can influence both the polymer itself and any filler materials contained in it. This aspect has to be considered not only for parts in direct contact with water. Water plays a special role in the form of humidity or precipitation, especially in atmospheric aging, because water soluble degradation products capable of catalyzing further degradation are washed off. On the other hand, water also extracts water soluble additives. In addition, water can act as a plasticizer and be responsible for swelling and deterioration of plastics. The presence of water also tends to accelerate oxidation reactions. 

Kawai F (1997), Microbial aspects of the degradation of water-soluble synthetic polymers , Macromol Symp, 123, 177-187. 

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