Applied Polymer Science - American Chemical Society

7 New Developments and Trends in Free-Radical Polymerization G E O R G E E. H A M

Downloaded by PENNSYLVANIA STATE UNIV on March 14, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch007

G. E. Ham Associates, 284 Pine Road, Briarcliff Manor, NY 10510

Catalysts Telechelic Polymers Photopolymers Polyolefins and Modified P o l y o l e f i n s Weather-Resistant Grafts of V i n y l Monomers to EPDM and Polyacrylates Poly(phenylene oxide) Grafts and Blends S t y r e n e - A c r y l o n i t r i l e - M a l e i c Anhydride Terpolymers Modifiers for P o l y ( v i n y l chloride) Nitrile Barrier Resins Polymerization in Air Emulsion Polymerization Powders Diallyldimethylammonium Chloride Charge-Transfer Polymerization Template Polymerization Incorporation of Polymer Additives in Chains B i o l o g i c a l l y Active Polymers

P o l y m e r i z a t i o n of vinyl monomers by free-radical mechanisms i s perhaps the most w i d e l y encountered and best understood mode of vinyl polymerization. The popularity of f r e e - r a d i c a l polymerization is due in substantial part to the many advantages that t h i s route to polymers o f f e r s to i n d u s t r y . The p o l y m e r i z a t i o n process i s noteworthy for its ease, convenience, and relative insensitivity to impurities, such as w a t e r and o x y g e n , t h a t p l a g u e ionic p o l y m e r i z a t i o n s . Indeed, it is common to c a r r y out free-radical polymerizations i n water as a suspending medium, as in emulsion and suspension p o l y m e r i z a t i o n . Another advantage of free-radical polymerization is that it offers a convenient approach toward the design and synthesis of myriad s p e c i a l t y polymers for use in almost every area. 0097 6156/ 85/ 0285 0151 $06.00/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

152

A P P L I E D P O L Y M E R SCIENCE


Catalysts A g r e a t impetus has been f u r n i s h e d toward the syntheses of new specialty polymers as well as toward improved processes f o r the s y n t h e s i s of commodity polymers by the i n c r e a s e d availability of n o v e l l o w - and high-temperature and more versatile initiators. These c a t a l y s t s belong to the c l a s s e s of peroxides, percarbonates, peroxyphosphates, and o t h e r s . These new and improved initiators a l l o w tailoring of the c a t a l y s t system to the s p e c i f i c requirements of the monomer and polymer system. For example, the polymerization of e t h y l e n e at h i g h p r e s s u r e s has been r e v o l u t i o n i z e d by the a v a i l a b i l i t y of selected hydrocarbon-soluble c a t a l y s t s such as t e r t b u t y l p e r p i v a l a t e and b i s ( 2 - e t h y l h e x y l ) percarbonate. In a d d i t i o n , a new c l a s s of a s y m m e t r i c a l azo c a t a l y s t s now o f f e r e d i s c h a r a c t e r i z e d by an a - c y a n o i s o a l k y l group on one s i d e and a tertb u t y l group on t h e o t h e r s i d e . Such components a v o i d t h e d e l e t e r i o u s o x i d a t i v e c h a r a c t e r i s t i c s of p e r o x i d e s and o f f e r improved s o l u b i l i t y . They can be t a i l o r e d to decompose i n the a p p r o p r i a t e ranges f o r s e l e c t e d v i n y l monomers. Another n o v e l c a t a l y s t c l a s s based on l a b i l e carbon compounds may a l l o w the a v o i d a n c e of some of the problems of c o n v e n t i o n a l f r e e - r a d i c a l i n i t i a t o r s , such as d e g r a d a t i v e c h a i n t r a n s f e r and l o s s of unused catalyst. T e l e c h e l i c Polymers Interest i n f r e e - r a d i c a l c a t a l y s t s containing organic f u n c t i o n a l i t y , such as hydroxyl groups, c a r b o x y l i c acid groups, and amide groups i s i n c r e a s i n g . For example, butadiene has been p o l y m e r i z e d i n the presence of 4,4'-azobis (j3-hydroxy i s o b u t y r o n i t r i l e ) t o produce a s o c a l l e d t e l e c h e l i c p o l y b u t a d i e n e c o n t a i n i n g t e r m i n a l h y d r o x y l groups. Of c o u r s e , i n f r e e - r a d i c a l p o l y m e r i z a t i o n one encounters not o n l y b i m o l e c u l a r c o u p l i n g but d i s p r o p o r t i o n a t i o n as w e l l . Accordingly, under normal c i r c u m s t a n c e s , i t i s i m p o s s i b l e t o produce 100% polymer w i t h the d e s i r e d f u n c t i o n a l i t y on both ends. I n the case of b u t a d i e ne p o l y m e r i z a t i o n , s u f f i c i e n t c h a i n t r a n s f e r w i t h the polymer takes p l a c e so t h a t almost every polymer m o l e c u l e w i l l have a t l e a s t dual functionality. When the polymer has s e v e r a l branches, some c h a i n ends may not have the d e s i r e d f u n c t i o n a l i t y . However, f o r many purposes i t i s s u f f i c i e n t t h a t each m o l e c u l e have m e r e l y , two f u n c t i o n a l groups.

A p p r o p r i a t e t e l e c h e l i c polymers produced from a v a r i e t y of monomers may be incorporated as r e a c t i v e components i n a v a r i e t y of a p p l i c a t i o n s such as sealants, elastomers, foams, and f i b e r s . Such t e l e c h e l i c polymers may impart almost any d e s i r e d c h a r a c t e r i s t i c such as h y d r o p h i l i c properties, elastomeric properties, d y e a b i l i t y , and s o l v e n t r e s i s t a n c e . A d i h y d r o x y t e l e c h e l i c polymer may be r e a c t e d w i t h a t e l e c h e l i c polymer c o n t a i n i n g two c a r b o x y l i c a c i d groups to produce a condensed p o l y e s t e r . A c c o r d i n g l y , from these t e l e c h e l i c e n t i t i e s one may produce block copolymers that w i l l have a l t e r n a t i n g a d d i t i o n polymer r e s i d u e s of l i k e or u n l i k e repeat units. I n a d d i t i o n , b l o c k c o p o l y m e r s can be p r o d u c e d by m o d i f i c a t i o n s of t h i s procedure i n which a d d i t i o n polymers are alternated with condensation polymer u n i t s .


7. H A M

Free-Radical Polymerization

153


Photopolymers These m a t e r i a l s , a l t h o u g h known and used f o r many years i n s m a l l quantities i n demanding a p p l i c a t i o n s , have only recently burgeoned i n t o a major o u t l e t f o r s p e c i a l t y monomers and polymers. Two important modes of use of these m a t e r i a l s s h o u l d be noted: photopolymerization and photocross-linking and chain extension. In photopolymerization, l i g h t - s e n s i t i v e a c r y l i c monomers, a c r y l a t e d polyurethanes, polyethers, polyesters, epoxies, and other compounds are exposed to l i g h t i n a c o n t r o l l e d manner f o r r a p i d c u r i n g to products of i n d u s t r i a l importance. In general, the materials are converted on curing from s t i c k y or l o w - v i s c o s i t y materials to tough, durable f i n i s h e s . In photocross-linking and chain extension, pree x i s t i n g o l i g o m e r s or polymers, g e n e r a l l y i n f i l m form, are p h o t o c r o s s - l i n k e d i n the presence of p h o t o s e n s i t i z e r s . These products i n c l u d e e n e - t h i o l - t y p e products, e p o x i e s , unsaturated polyesters, or unmodified polymers exposed to high r a d i a t i o n . Areas of a p p l i c a t i o n of photopolymerized or p h o t o c r o s s - l i n k e d products i n c l u d e a s b e s t o s - v i n y l f l o o r - t i l e c o a t i n g s , n e g a t i v e photoresists, tough protective coatings, p r i n t i n g inks, lithography, p r i n t i n g plates, "plaster" casts, can coatings, coatings for p l a s t i c containers, coatings for wood, paper, p l a s t i c s , m i c r o e l e c t r o n i c s , communications, a d h e s i v e s , chemical s p e c i a l t i e s , f o l d i n g cartons, g r a p h i c a r t s , m e d i c a l and d e n t a l uses, and pigments. Suitable p h o t o s e n s i t i z e r s f o r the c o n v e r s i o n of the photopolymers include benzoin ethers, acetophenone, halogenated ketones, and halogenated aromatics. The market for radiation-curable materials, c u r r e n t l y at a l e v e l of about 100 m i l l i o n l b and v a l u e d at $450 m i l l i o n , i s expected to grow to over 200 m i l l i o n l b by 1990. Polyolefins and Modified P o l y o l e f i n s In the i n d u s t r i a l area, polyethylene i s now being produced at entry pressures as high as 45 000 to 47,000 l b / i n . . One of the important consequences of the use of higher pressures and temperatures i s to g a i n homogeneous (one phase) r e a c t i o n c o n d i t i o n s . In t h i s way d e l e t e r i o u s branching and poor r h e o l o g i c a l c h a r a c t e r i s t i c s i n fabricated products from polyethylenes may be avoided. In a d d i t i o n , improved f i l m r e s i n c l a r i t y may r e s u l t . The use of homogeneous reaction conditions a l l o w s one to carry out the polymerization of ethylene i n a way to l i m i t the important g r a f t i n g r e a c t i o n t h a t l e a d s to l o n g c h a i n branching i n h i g h pressure p o l y e t h y l e n e . This grafting reaction i s e s p e c i a l l y pronounced on polymers that are swollen rather than d i s s o l v e d i n the r e a c t i o n phase. In a r e l a t e d development, m o d i f i c a t i o n of polyethylene with s m a l l concentrations of v i n y l acetate has become very p o p u l a r . The presence of v i n y l a c e t a t e monomer and v i n y l a c e t a t e u n i t s i n p o l y e t h y l e n e a l s o i n c r e a s e s the s o l u b i l i t y of p o l y e t h y l e n e i n the r e a c t i o n phase w i t h the d e s i r e d r e d u c t i o n i n d e l e t e r i o u s g r a f t i n g . Such e t h y l e n e copolymers are important i n t h e i r own r i g h t , however, because of improved toughness and c l a r i t y i n comparison with straight polyethylene. Indeed, the next 10 years should see important extension of commercial ethylene copolymers to include not only the widely used e t h y l e n e - v i n y l acetate copolymers 2

?



154


but ethylene-acrylate copolymers, e t h y l e n e - a c r y l i c acid copolymers, e t h y l e n e - c a r b o n monoxide c o p o l y m e r s , and p e r h a p s e t h y l e n e a c r y l o n i t r i l e copolymers. For many y e a r s , polymer chemists have b e l i e v e d t h a t the a v a i l a b i l i t y of amorphous polyethylene that would not c r y s t a l l i z e under ordinary use conditions would be an important addition to the spectrum of p o l y m e r i c m a t e r i a l s . Indeed e t h y l e n e copolymers a c c o m p l i s h t h i s r e s u l t by p r e v e n t i n g the c r y s t a l l i z a t i o n of p o l y e t h y l e n e . Improved toughness, c l a r i t y , and low temperature properties can be effected. Another benefit of copolyerraization of ethylene i s incorporation of appropriate polar groups such as c a r b o x y l i c acid groups or s a l t groups i n the ionoraers. Du Pont i s s t i l l the o n l y producer of the Surlyn-type of polyethylene ionomers. However, s e v e r a l companies produce copolymers of e t h y l e n e w i t h a c r y l i c a c i d (EAA). Such copolymers o f f e r improved thermal p r o p e r t i e s w i t h r e t e n t i o n of c l a r i t y , improved heat-seal strength, and impact strength, as w e l l as improved adhesion to a v a r i e t y of substances, such as aluminum f o i l , g l a s s f i b e r s , and mineral f i b e r s . Weather-Resistant Grafts of V i n y l Monomers to Ethylene-PropyleneDiene-Monomer (EPDM) and Polyacrylates Because of i n c r e a s i n g s a l e s r e s i s t a n c e to c o n v e n t i o n a l a e r y l o n i t r i l e - b u t a d i e n e - s t y r e n e (ABS) materials, manufacturers are l o o k i n g f o r improved a l t e r n a t i v e s . Important and p r o m i s i n g m a t e r i a l s are the g r a f t s of s t y r e n e and/or s t y r e n e - a c r y l o n i t r i l e onto weather-resistant elastomers, such as ethylene propylene-diene terpolymers (EPDM) and p o l y a c r y l a t e s . These products are produced by f r e e - r a d i c a l polymerization of styrene and a c r y l o n i t r i l e i n the presence of the elastomers i n mass, s o l u t i o n , or emulsion processes. Care must be taken to employ elastomers containing s u i t a b l e grafting s i t e s such as, i n the case of EPDM, 1,4-hexadiene or e t h y l i d e n e norbornene. Appropriate conditions of grafting, i n c l u d i n g s u i t a b l e peroxide i n i t i a t o r s , temperature and s o l v e n t s , must be s e l e c t e d . The r e s u l t i n g products possess far superior weather resistance than that of conventional ABS materials. Products have been introduced to the market both i n the U n i t e d S t a t e s and i n Europe. These products include not only the EPDM graft materials but, i n a d d i t i o n , grafts of styrene and methacrylates to elastomeric p o l y a c r y l a t e s . Poly(phenylene oxide) Grafts and Blends Poly(phenylene oxide) (PPO)/polystyrene blends are widely known and used as engineering molding resins. What i s not g e n e r a l l y known i s that c e r t a i n types of these materials are grafts of polystyrene and copolymers onto PPO. T h i s p r a c t i c e a s s u r e s c o m p a t i b i l i t y and homogeneity d u r i n g manufacture and use of the product and i s one avenue toward improved b l e n d c o m p a t i b i l i t y between PPO and polystyrene. In general, grafting techniques are s i m i l a r to those described e a r l i e r . Bulk, s o l u t i o n , and emulsion methods may be used for the grafting of styrene to PPO. Appropriate attention must be g i v e n to the use of s u i t a b l e p e r o x i d e s and c o n t r o l of temperature and other reaction conditions.


7. H A M


155

S t y r e n e - A c r y l o n i t r i l e - M a l e i c Anhydride Terpolymers


Recently, products composed of styrene, a c r y l o n i t r i l e , and maleic anhydride have been introduced as improvements over the older ABS m a t e r i a l s . These products e x h i b i t s u p e r i o r heat r e s i s t a n c e and thermal s t a b i l i t y i n comparison to conventional ABS materials. In a d d i t i o n , the impact r e s i s t a n c e of s t y r e n e - a c r y l o n i t r i l e - m a l e i c anhydride t e r p o l y m e r s can be improved by the a d d i t i o n of or by g r a f t i n g onto a p p r o p r i a t e e l a s t o m e r s . The product a l s o e x h i b i t s desirable r h e o l o g i c a l behavior due to polar i n t e r a c t i o n of anhydride groups during processing and drawing. Modifiers for P o l y ( v i n y l chloride) In a q u i t e d i f f e r e n t but very important i n d u s t r i a l a r e a , f r e e r a d i c a l polymerizations have made great inroads i n the optimization of the desired commercial properties of impact-modified p o l y ( v i n y l c h l o r i d e ) (PVC). In a most sophisticated v a r i a t i o n , grafted impact m o d i f i e r s based on the q u a t e r p o l y m e r i z a t i o n of a c r y l i c e s t e r s , butadiene, styrene, and a c r y l o n i t r i l e have been produced and almost p r e c i s e l y match the r e f r a c t i v e index of PVC. The b l e n d i n g of the impact modifier with PVC y i e l d s a completely c l e a r polymer s u i t a b l e for shampoo b o t t l e s and food containers. In addition to e x c e l l e n t c l a r i t y these polymers have extremely good impact strength combined with improved f a b r i c a b i l i t y by flow molding equipment. N i t r i l e B a r r i e r Resins Another p o t e n t i a l l y important development i n the f r e e - r a d i c a l polymerization area i s the n i t r i l e b a r r i e r resins. These products g e n e r a l l y are graft polymers containing a glassy phase comprised of 80% a c r y l o n i t r i l e and 20% s t y r e n e or other v i n y l monomer g r a f t e d onto a rubber substrate. The commercialization of these materials has been i n t e r r u p t e d by an adverse r u l i n g by the Food and Drug A d m i n i s t r a t i o n (FDA) i n c o n n e c t i o n w i t h e x t r a c t i o n of t r a c e s of a c r y l o n i t r i l e i n food-related a p p l i c a t i o n s . The amounts i n v o l v e d are very s m a l l . A new a t t i t u d e under development i n the FDA may lead to the eventual clearance of these materials. These resins may be blow molded to y i e l d e x c e l l e n t containers f o r carbonated beverages. In a d d i t i o n , they show a d e s i r a b l e low l e v e l of transmission of carbon dioxide, oxygen, and water vapor i n comparison with other p l a s t i c s a v a i l a b l e today. Polymerization i n A i r In another commercial a p p l i c a t i o n of f r e e - r a d i c a l polymerization, p o l y m e r i z a t i o n s may be c a r r i e d out i n i n d u s t r i a l c o a t i n g s i n the presence of a i r to y i e l d a v a r i e t y of c o a t i n g s and s t r u c t u r e s of commercial import. This development i s p o s s i b l e , i n part, because c e r t a i n v i n y l monomers, p a r t i c u l a r l y the a c r y l a t e s , are l e s s s e n s i t i v e to retardation by oxygen compared with other monomers. I t i s therefore p o s s i b l e to produce radiation-cured coatings. UV-cured p r i n t i n g i n k s and the photopolymers are important i n imaging f o r p r i n t i n g , photoresist, and r e l a t e d a p p l i c a t i o n s .


156

A P P L I E D POLYMER SCIENCE


Emulsion Polymerization Perhaps one of the most powerful and yet o l d e s t p o l y m e r i z a t i o n techniques f o r v i n y l monomers i s e m u l s i o n p o l y m e r i z a t i o n . The technique i s h i g h l y v e r s a t i l e and r e a d i l y a p p l i c a b l e to the manufacture of copolymers c o n t a i n i n g two to f i v e monomer c o n s t i t u e n t s . In t h i s way, two or three components may be used f o r the o p t i m i z a t i o n of the g l a s s t r a n s i t i o n temperature or thermal s t a b i l i t y of the r e s u l t i n g product, and the remaining one or two components can impart desirable c h e m i c a l p r o p e r t i e s such as adhes i o n , c r o s s - l i n k a b i l i t y , p h o t o s e n s i t i v i t y , and c o m p a t i b i l i t y . Emulsion p o l y m e r i z a t i o n may be employed for the manufacture of products to be i s o l a t e d and used as molding p e l l e t s , or for end-use polymer emulsions, which can be applied d i r e c t l y or with s u i t a b l e f o r m u l a t i o n as surface c o a t i n g s , a d h e s i v e s , pigment v e h i c l e s , l a c q u e r s , p a i n t s , e t c . E m u l s i o n p o l y m e r i z a t i o n a l s o o f f e r s the v e r s a t i l i t y , by gradual monomer addition during polymerization or s e l e c t i v e a d d i t i o n of monomers at d i f f e r e n t r a t e s , of producing uniform copolymers. For the manufacture of uniform copolymers by other methods, such as mass and s o l u t i o n p o l y m e r i z a t i o n , the manufacturing process must be interrupted at conversions of only 10 or 20% and t h i s r e s u l t s i n higher costs of manufacture. In the case of e m u l s i o n p o l y m e r i z a t i o n , uniform product may be produced at conversions of 85-95+%. Powders Another r e l a t e d development of importance i s powder coatings, which are polymer concentrates i n monomers of low v o l a t i l i t y and may be sprayed as powder onto a s u r f a c e and cured by subsequent h e a t i n g . In a sense, these are nonsolvent-based coatings i n that no s o l v e n t must be subsequently removed from the system. Diallyldimethylammonium Chloride Another f r e e - r a d i c a l polymer of importance i s d i a l l y l d i m e t h y l a m monium c h l o r i d e , which i s employed as a c o a t i n g on paper i n e l e c t r o g r a p h i c r e p r o d u c t i o n f o r the purpose of improving moisture retention of paper. The polymer a l l o w s better d i s s i p a t i o n of s t a t i c e l e c t r i c i t y , an e s s e n t i a l step i n the imaging process. The polymer i s a l s o used i n water p u r i f i c a t i o n as a f l o c c u l a n t . Charge-Transfer Polymerization Charge-transfer polymerization has been regarded by some s c i e n t i s t s as a n o n r a d i c a l type of p o l y m e r i z a t i o n o r , at l e a s t , as an unconventional polymerization. Some viewpoints hold that i n t h i s polymerization the charge-transfer complex i n v o l v i n g , for example, s t y r e n e and a c r y l o n i t r i l e complexed w i t h zinc c h l o r i d e undergoes decomposition with monomers adding to both ends of the nascent chain in a matrix-type polymerization. The evidence of n o n r a d i c a l c h a r a c t e r of these p o l y m e r i z a t i o n s i n c l u d e s , f o r example, i n s e n s i t i v i t y to c h a r g e - t r a n s f e r agents i n r e d u c i n g m o l e c u l a r weight, nonessentiality of f r e e - r a d i c a l c a t a l y s t s , and n o n i n h i b i t i o n by the usual f r e e - r a d i c a l i n h i b i t o r s .


7.

HAM


157


By now most of these p o i n t s have been s u c c e s s f u l l y overcome. P o s s i b l y , the l a s t remaining q u e s t i o n a b l e i s s u e surrounds the n o n e f f i c a c y of c h a i n - t r a n s f e r agents i n l i m i t i n g the m o l e c u l a r weight of the c h a r g e - t r a n s f e r polymers. Recent work i n Great B r i t a i n suggests t h a t the use of more e f f e c t i v e c h a i n - t r a n s f e r agents, such as aromatic amines, l e a d s to the expected m o l e c u l a r weight reduction. C h a r g e - t r a n s f e r p o l y m e r i z a t i o n i s c h a r a c t e r i z e d by a h i g h l y e f f i c i e n t propagation step that tends to exclude other f r e e - r a d i c a l reactions, except those that can compete i n an e f f i c i e n t manner. Template Polymerization S t i l l another development of increasing i n t e r e s t i n v o l v e s attempts to c o n t r o l f r e e - r a d i c a l p o l y m e r i z a t i o n by o f f e r i n g d u r i n g the propagation step a s u i t a b l e surface or template for polymerization. In one example, c h l o r o p r e n e has been s u c c e s s f u l l y p o l y m e r i z e d t o g i v e e x c l u s i v e l y h e a d - t o - t a i l trans polymer by using frozen monomer. In a d d i t i o n , p o l y a c r y l o n i t r i l e , when produced i n the presence of p r e - e x i s t i n g polymer, has an ordered c h a r a c t e r s u g g e s t i v e of template polymerization. V i n y l c h l o r i d e and v i n y l i d e n e c h l o r i d e a l s o show s i m i l a r behavior. S t u d i e s by Wegner of Mainz on conjugated diacetylenes confirm that one may polymerize c r y s t a l l i n e monomer and r e t a i n much of the c r y s t a l l i n e character i n the r e s u l t i n g polymer. I t i s now s e t t l e d as to the mechanism of t h i s p a r t i c u l a r polymerization, but i t seems p r o b a b l e t h a t f r e e - r a d i c a l as w e l l as i o n i c p o l y m e r i z a t i o n s may d i s p l a y such effects i n appropriate cases. Such ordered polymerizations i n any of the categories discussed offer the p o t e n t i a l for polymerization "without end" by minimizing polymer r a d i c a l - t r a n s f e r p o s s i b i l i t i e s . The amount of e x t e r n a l c a t a l y s t required for such polymerizations would be very s m a l l for s i m i l a r reasons. Incorporation of Polymer Additives i n Chains Interesting p o s s i b i l i t i e s f o r r e p l a c i n g polymer a d d i t i v e s e x i s t . For example, p o l y m e r i z a b l e v i n y l monomers c o n t a i n i n g l i g h t s t a b i l i z i n g moieties, such as benzophenone or benzotriazole groups, have been produced. Incorporation of such monomers at a l e v e l of 1 or 2% i n t o a parent polymer p r o t e c t s the polymer a g a i n s t the deleterious effects of s u n l i g h t . B i o l o g i c a l l y Active Polymers F i n a l l y , t h e r e s h o u l d be mention of a development of R i n g s d o r f f (Mainz) on the preparation of b i o l o g i c a l l y a c t i v e polymers (mostly of f r e e - r a d i c a l o r i g i n ) to r e v o l u t i o n i z e the whole a r e a of chemotherapy. Ringsdorff has pointed out the important differences i n drug treatment between the step of taking the drug to the desired s i t e of a p p l i c a t i o n and the release of a s u i t a b l e pharmacological or b i o l o g i c a l function at the s i t e . In h i s research, polymers with a molecular weight of 25,000 were employed because of l i m i t a t i o n s on d i f f u s i v i t y through c e l l w a l l s . Attached to these polymers were a v a r i e t y of b i o l o g i c a l l y a c t i v e f u n c t i o n s h e l d to the backbone by


158



l a b i l e segments t h a t undergo c l e a v a g e i n the a p p r o p r i a t e environments. T h i s approach o f f e r s promise of p r o d u c i n g a n t i c a r c i n o g e n i c , a n t i v i r a l , a n t i r a d i a t i v e , and b i r t h c o n t r o l agents, as w e l l as numerous other drugs or b i o l o g i c a l l y a c t i v e agents of importance.


Applied Polymer Science - American Chemical Society

Recommend Documents