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Graft- copolymer formation in latex

The initiation of polymerization of vinyl monomer in rubber latex can be affected in various ways.

Initiation by polyamine- activated hydroperoxides

Early attempts to polymerize common vinyl monomers in natural rubber were vitiated by the presence of ammonia, and a search was therefore made for an initiating system which would work well with ammonia present and also without requiring de-aeration of the latex1. A successful method employs a hydroperoxide activated by a polyamine. Originally developed by Whitby, this system is utilized in rubber as follows. The hydroperoxide (e. g. t-butyl hydroperoxide) is dissolved in the monomer (typically, methyl methacrylate) and

then added with stirring to diluted latex containing, if need be, an added stabilizer. After dispersion is complete, the polyamine (usually tetraethylene pentamine) is added. Polymerization starts at room temperature and is complete in a few hours. Provided substantial proportions of stabilizing soaps are avoided, polymerization takes place entirely within the rubber particles and no separate polymer particles are formed. It is necessary to allow sometime for the monomer to diffuse into the rubber particles and, after polymerization is completed, the small particles contain a higher proportion of polymer than do the larger particles (in the case of methyl methacrylate). Graft copolymer latices of this type been referred to Heveaplus.

The products of polymerization can be separated into free rubber, graft copolymer and free vinyl polymer. In the case of methyl methacrylate it is found that the pattern of combination2 is almost precisely the same as that found when polymerization takes place in solution. Of the total poly (methyl methacrylate) formed, only about half is grafted to the rubber (i. e. fv is about 0.5).

Initiation by aerated by aerated latex

Sekhar observed that if latex is aerated by rotating a half- filled container for a period of days the product is able to initiate polymerization of a range of monomers without the addition of a hydroperoxide. The procedure is simply to add monomer to aerated latex, followed by a small amount of ferrous and polyamine activator. Centrifuging experiment established that the activity was associated with the rubber component of the latex and not with the serum. Sekhar considers that reactive sites are formed on the rubber during aeration (or much more slowly on storage). These probably are being hydroperoxidic in nature.

Whilst there had been earlier observations that aerated latex can initiate polymerization of added monomers, the importance of Sekhar's work is that he was able to establish that, in the case of methyl methacrylate, the grafting efficiency is very high. Little free polymer is formed, fv being 0,1 or less. This is contract with the conventional method of initiation and is presumably due to the fact that initiation proceeds directly from rubber or rubber peroxy- radicals. This must materially reduce the possibility of simultaneous formation of uncombined polymer.

Activated latex

Since the preparation of graft copolymer latices from aerated latices is a slow process, investigations were undertaken to find an alternative method, which would also give a product with efficiency of grafting. One such method involves pretreatment of latex with hydroperoxide and activator, followed by the addition of monomer after a time interval long enough to ensure that the original hydroperoxide will have decomposed. In a typical procedure, t- butyl hydroperoxide is added to 60% latex, followed by tetraethylene pentamine. This is allowed to react for 24 hours; separate tests (in the absence of latex) show that no hydroperoxide remains after this period. Methyl methacrylate, together with a small amount of stabilizer, is then added. Polymerization takes place quite rapidly at room temperature. To obtain maximum conversion, it is advisable to add ~ 50 ppm of ferrous ion either before or after adding the redox catalyst.

With methyl methacrylate the efficiency of grafting is as high as found the aerated latex method, fv being typically as slow as 0,02.

Notes

1. de-aeration of the latex: sù lo¹i khÝ khái mñ

2. pattern of combination: ®Æc tr­ng cña ph¶n øng

Exercises

I. Answer the following question:

1.Were the early attempts to polymerize vinyl monomers in natural rubber successful?

2. How can one effect the initiation of polymerization of vinyl monomer in latex?

3. What kinds of initiating systems do you know?

4. What phenomenon did Sekhar observe?

5. What latices are referred to as Heveaplus?

6. What fact was established due to the centrifuging experiments?

I. Group the following words into pairs of synonyms:

employed

diluted

presumably

investigation

if

initial

utilized

entirely

probably

provided

watered

totally

original

precisely

procedure

exactly

search

process

III. Group the following words into pairs of antonyms:

original

rapidly

start

insoluble

as high as

final

slowly

complete conventional

in particular

above

as slow as

soluble

unusual

below

in general

IV. Give Vietnamese equivalents:

as slow as 0.02

efficiency of grafting

in contract with long enough

on storage

refer to

V. State the function of the words after and before and translate the sentences:

1. The polyamine is added after the dispersion.

2. After dispersion is complete, the polyamine is added.

3. Before the discovery of vulcanization, the great drawbacks of rubber were its thermoplastic nature and its sensibility to oxidation.

4. Neoprene was used for balloons, before natural rubber was available.

5. Polymerization starts at room temperature and is complete after a few hours.

6. After substantial proportions of stabilizing soaps are avoided, polymerization takes place.

VI. State the functions of the words in bold type:

1. Some scientists observed that aerated latex could initiate polymerization.

2. A plasticizer is a material that increases the plasticity of a mass.

3. Sekhar considers that reactive sites are formed on the rubber during aeration, these probably being hydroperoxidic.

4. The vulcanization of plastic chloroprene polymer differs from that of all other rubbers in requiring the addition of no vulcanizing ingredient.

5. The methods of producing modified latices depend on those of initiation.

6. In the case of methyl methacrylate it is found that the pattern of combination in the same as that found when polymerization takes place in solution.

VII. Find in the text sentences with the absolute participle construction.

VIII. Translate into English:

1. Sù kh¬i mµo cho qu¸ tr×nh polime ho¸ vinyl trong latex cã thÓ ®­îc tiÕn hµnh b»ng nhiÒu ph­¬ng ph¸p.

2. §Ó kh¬i mµo cho ph¶n øng trïng hîp ghÐp trong latex cã thÓ sö dông hÖ kh¬i mµo tan trong dÇu, trong ®ã mét cÊu tö ®­îc hoµ tan trong n­íc, mét cÊu tö kh¸c ®­îc hoµ tan trong dÇu.

3. Sau khi qu¸ tr×nh polime ho¸ kÕt thóc th× nh÷ng h¹t bÐ chøa nhiÒu polime h¬n nh÷ng h¹t lín.

4. §Ó tiÕn hµnh polime ho¸ ng­êi ta sö dông réng r•i ph­¬ng ph¸p dïng hydropeoxit ®­îc ho¹t ho¸ bëi polyamin.

5. Xekhar nhËn thÊy r»ng møc ®é th«ng khÝ rÊt cao trong tr­êng hîp sö dông metyl metacrylat.

6. Sù ®iÒu chÕ copolime tõ latex b•o hoµ kh«ng khÝ s¶y ra chËm do ®ã ng­êi ta cè g¾ng t×m mét ph­¬ng ph¸p míi hiÖu qu¶ h¬n.